Vol. 21, No.2, August 2010A Bulletin of the Indian Laser Association

Special Issue on Frontiers in Laser Spectroscopy

President

Prof. P. K. Gupta

RRCAT, Indore

Vice President

Prof. S. K. Sarkar

BARC, Mumbai

Gen.Secy.I

Prof. V. P. M. Pillai

Univ. of Kerala, Thiruvananthapuram

Gen.Secy.II

Dr. S. K. Majumder

RRCAT, Indore

Treasurer

Mr. P. Saxena

RRCAT, Indore

Regional Representatives

Dr. S. K. Bhadra,

CGCRI, Kolkata

Prof. M. P. Kothiyal,

IIT Madras, Chennai

Prof. D. Narayana Rao,

University of Hyderabad, Hyderabad

Dr. Hema Ramachandran,

Raman Research Institute, Bangalore

Dr. A. K. Razdan,

Laser Science & Technology Centre, Delhi

EditorProf. L.M. Kukreja (RRCAT, Indore)

Guest EditorProf. S. K. Sarkar (BARC, Mumbai)

Associate EditorProf. P.A. Naik (RRCAT, Indore)

Editorial BoardDr. A.K. Gupta (SCTIMST,

Thiruvananthapuram)Dr. A. Maini (LASTEC, New Delhi)Prof. S. Maiti (TIFR, Mumbai)Prof. S.C. Mehendale (RRCAT, Indore)Prof. V.P.N. Nampoori (CUSAT, Kochi)Prof. B.P. Pal (IIT, Delhi)Dr. Reji Phillip (RRI, Bangalore)Prof. Asima Pradhan (IIT, Kanpur)Prof. B.P. Singh (IIT, Bombay)Prof. B.M. Suri (BARC, Mumbai)Prof. C. Vijayan (IIT, Madras)

Editorial Committee (RRCAT, Indore)Dr. C.P. Paul Dr. S. VermaDr. P.K. Mukhophadhyay Dr. P. MisraMr. H.S. Patel Mr. S. Sendhil Raja

ILA Executive Committee Editorial Team of

(a) (b)

(c) (d)

Cover Photo : For the main image on the cover page:

The photograph shows the experimental set up of Molecular Beam-Resonance

Enhanced Multiphoton Ionization-Time-of-Flight (MB-REMPI-TOF), a highly

sophisticated analytical detection system, for investigating the

photodissociation dynamics at Radiation & Photochemistry Division, BARC.

Under the present configuration the resolution is about 400, and detection limit 6 3

is better than 10 species per cm . The other Details are given on page 35

For the image in the inset (top left corner):

The REMPI spectra of Br and Br* produced from photodissociation of CHBr3 .

The (2+1) REMPI transitions of Br and Br* atoms, in the wavelength region of

230-235 nm, are used to probe Br and Br* atoms. The relative quantum yields of

Br and Br* were extracted from the relative integrated signal intensities in the

spectrum. From the integrated areas for the two-photon transitions of Br* and 2 2

Br, the Br* ( P )/Br ( P ) ratio was estimated to be 1.4. More details are given 1/2 3/2

on page 35

A Bulletin of the Indian Laser Association

Contents

Page No.

From the desk of Guest Editor 1

1. Analysis and Manipulation of the Ground State Vibrational Dynamics in 2Large Molecules by Means of Femtosecond Time-Resolved CARSA. MaternySchool of Engineering and Science, Jacobs University Bremen Campus Ring 1, 28759 Bremen, Germany

2. Attempt to control femtosecond two-photon processes in solution 11Amit Nag, Sumit Ashtekar and Debabrata GoswamiDepartment of Chemistry, Indian Institute of Technology Kanpur, Uttar Pradesh - 208016, India

3. Magneto Optical Kerr Effect (MOKE) for study of thin film magnetism 16Dileep Kumar and Ajay GuptaUGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452017, India

4. Spectroscopy in Coherently Prepared Atomic Medium and its Applications 22Y.B. Kale, Niharika Singh, Ayan Ray and B.N. JagatapBhabha Atomic Research Centre, Mumbai-94, India

5. Advances in Laser Photochemical Research at Radiation & 28Photochemistry Division (RPCD), BARCSisir K. SarkarBhabha Atomic Research Centre, Mumbai-94, India

Vol. 21, No. 2, August 2010

1

In the coming decades basic energy science will shift from the study of how quantum systems are organized and assembled to the study of how they work and, ultimately, how to make such systems work for us. Much of the last century has focused on understanding how electrons in matter - their charge, their spin, and their dynamics - determine the properties of materials and how they direct chemical, electrical, or physical processes in materials. We are now on the verge of a new science of quantum control where our tools will go beyond probing what is there, towards the goal of controlling these processes and properties through direct manipulation of the electrons

Lasers can now produce coherent radiation with almost any field strength, frequency, temporal shape, bandwidth, and polarization. Engineering and controlling coherent quantum states have become a reality. Such coherent interactions are poised to open new scientific directions. The intellectual pay-off of this field will be vast because essentially all dynamical events in chemistry and materials science start at the atomic and molecular scale that could be controlled by lasers. While keeping in view all these possibilities around the corner and composing the issue to commemorate the 50 th year of Laser, our endeavour was to bring out the perspectives on some of the frontier areas of spectroscopy – which I believe will be exciting to students as well as young researchers.

The first article describes the application of femtosecond time resolved CARS experiments for the analysis and the manipulation of ground state vibrational dynamics of porphyrin and polydiacetylene systems. These results demonstrate that CARS with fs time resolution and wave number resolved detection is a powerful tool for selective generation and detection of the different vibrational motions of large molecules. In the second article, the authors demonstrate that the phase characteristics of the fs laser pulse play a very important role in modulating the two photon absorption and fluorescence (TPA & TPF) processes of the chromophores in liquid phase. The next article brings out how magneto optical Kerr effect (MOKE) is utilized to study the thin film magnetism which is very important in technologically challenging areas of spintronics and recording media industry. This fast and sensitive method competing with best SQUID can analyze ultrathin films-even in a fraction of monolayer. The fourth article deals with th spectroscopy in coherently prepared atomic medium and its application. The authors focused on electromagnetically induced transparency (EIT) utilized for spectroscopy with sub-natural resolution and for cutting edge technologies like precision atomic clock and ultra sensitive magnetometer. In the final article, we provide some glimpses of laser photochemical research being carried out at Radiation & Photochemistry Division (RPCD) at BARC. During the last two decades, we have built several facilities based on absorption, fluorescence, Raman and REMPI techniques with fast and ultrafast lasers in ns to fs time domain. We have also included the upcoming facility of coherent control of chemical reactions.

Finally, we want to acknowledge the individual and collective contributions of the authors of this issue. They represent an admirable group of busy but unselfish professionals volunteering their limited time tending to the scientific “commons” on which we all depend. It was a special opportunity to have worked with such an outstanding group. I thank my colleague, Dr. Ajay K Singh, for the help provided right from the inception for bringing out the issue. Wishing you all a scientifically rewarding New Year and happy reading.

Sisir K sarkar

From the desk of Guest Editor

Dr. Sisir K. Sarkar is presently the Head, Radiation & Photochemistry Division, BARC. He

is the Vice-President of Indian Laser Association (ILA) and Indian Society for Radiation &

Photochemical Sciences (ISRAPS). He is also Professor at Homi Bhabha National Institute,

Mumbai and Fellow of Maharashtra Academy of Sciences. His research interest includes

Radiation and Photochemistry using lasers and accelerators.

2

CARS has been extended to the study of molecular systems in the gas phase, where it has proven to be a valuable tool in the excitation and probing of coherent

7, 8 nuclear motion in small molecules. In this short review, works are discussed were fs time-resolved CARS spectroscopy was applied to investigate and influence the electronic ground state vibrational dynamics of two large polyatomic molecular systems, porphyrins and polydiacetylenes.

Porphyrins are of great biological relevance, because 9 they form the active site of many important proteins.

Their dynamics in excited electronic states have been extensively studied by means of fs time-resolved spectroscopy in the last couple of years and are well

10 understood. Studying ground state vibrational dynamics in these systems, the difficulty arises, that the high density of normal modes and the large spectral width of the employed fs laser pulses results in a coherent excitation of a multitude of vibrational states, and therefore in a rather complex structure of the transient signal. To overcome this problem, the coherently scattered CARS signal can be detected spectrally dispersed, which leads to a two-dimensional (time and wavenumber) data set. It was shown, that an analysis of this two-dimensional time domain data on the basis of Fourier transform methods enables one to obtain detailed spectral information on the vibrational mode structure in addition to the vibrational dephasing times that are readily deducible from the decay behavior of the transient

11 signals. Moreover, it was demonstrated, that for time resolved CARS measurements it is possible to employ a polarization sensitive setup, in which the polarization direction for the CARS signal analyzer can be freely varied. This provides an effective tool to select the contribution of individual modes to the detected signal and opens up a new method to gain further information.

One topic of femtochemistry that has attracted considerable attention in the last couple of years is the field of coherent control or quantum control of molecular dynamics. The first important step for the experimental realization of this goal is the selective preparation of

Abstract

A short review about the application of femtosecond time-resolved CARS (coherent anti-Stokes Raman scattering) experiments for the analysis and the manipulation of ground state vibrational dynamics of porphyrin systems and polydiacetylenes is given. The presented works are examples for the use of nonlinear spectroscopic techniques as versatile tools in ultrafast spectroscopy. The experiments discussed were performed in a pump and probe fashion. By detecting the spectrum of the transient CARS signal, a detailed mapping of the dynamics initiated by the stimulated Raman pump process is achieved. The method is capable to yield the dephasing behavior and spectral information of the investigated system at the same time. For the polydiacetylene system it is shown, that the excitation of vibrational modes can be influenced by varying the timing and the chirp of the exciting laser pulses. In the measurements on the porphyrin systems, the different contributions to the ground state vibrational dynamics are selected by changing the direction of the CARS signal analyzer in the employed polarization arrangement.

Keywords: femtosecond, CARS, vibrational dynamics, optimal control

Introduction

The rapid progress in the field of femtosecond (fs) time-resolved spectroscopy that has been achieved over the last decade has paved the way for studies on many interesting aspects of molecular dynamics in the ultrafast

1, 2 time-regime. Most of the studies that have been performed deal with the investigation of dynamics of molecular systems in electronically excited states. An experimental technique that is capable to study dynamics in the electronic ground state of molecular systems is the method of fs time-resolved CARS (coherent anti-Stokes

3–8 Raman scattering). The first experiments using this technique have been performed in the condensed phase. Valuable information about the time scales of vibrational relaxation processes as well as on rotational dynamics in

4–6 liquids has been obtained in this way. More recently, fs-

Analysis and Manipulation of the Ground State Vibrational Dynamics in Large Molecules by Means of Femtosecond Time-Resolved CARS

A. MaternySchool of Engineering and Science,

Jacobs University Bremen Campus Ring 1, 28759 Bremen, Germany

3

which can be written as a sum of the contributions from 4, 18 individual vibrational modes,

(2)

with the angular frequencies ω and phase factors φ. Q (t) j j j

denotes the coherent amplitude of a vibrational state, which for the stimulated Raman excitation by the pump

4 and the Stokes pulses is determined by

where T denotes the dephasing time of the vibrational 2j

0 0mode ν, and ∆ω is defined as ∆ω = ω -ω -ω . The first j j j Pu St j

part of the function within the time integral consists of the product of the amplitudes and the temporal envelopes of the fields of the pump and the Stokes pulses, which is proportional to what is generally considered as the

driving force, F(τ) ∝E (τ)E (τ). In the present case it is Pu St

centered at time zero, and the overall function is therefore non-zero only around τ = 0. Since it is interesting to obtain the vibrational amplitude when the interaction of the laser pulses with the sample is over, i.e. at t >> ∆t, the

upper integration limit can be extended to +∞. In the experiment it is furthermore reasonable to assume that T 2j

is considerably longer than the pulse duration ∆t. One then obtains an integral, which is essentially the Fourier transformation of the driving force F(τ) with respect to ∆ω . In other words, the excitation profile in the j

wavenumber domain is given by the Fourier transformation of the driving force, with a bandwidth that is equal to the sum of the bandwidths of the two exciting laser pulses. For sufficiently short pulses and a high density of the normal modes of large molecules, this results in a coherent excitation of several vibrational states.

The time-resolved CARS experiment is performed in a pump and probe fashion. The coherently excited molecular vibrations are monitored by a delayed probe

0pulse of center angular frequency ω at a variable time Pr

delay T. Assuming that the phase matching condition

(4)

is fulfilled and neglecting non-Raman resonant contributions to the signal (see below), the generated

4anti-Stokes light field can be written as

(5)

The time-integrated spectrum of the CARS signal is

coherent vibrational motion. Gerdy et al. have succeeded in population control of the wave packet in gaseous iodine molecules on the bound B state by changing the

12 delay time of two pump pulses. Tannor and Rice have discussed a so-called pump-dump scheme to generate vibrational excitation by the stimulated Raman effect

13 with a pair of time-delayed femtosecond laser pulses.An example for an experimental realization of this scheme on the potassium dimer was recently given by

14 Pausch et al. For multidimensional potential energy surfaces (PES) of large polyatomic molecules fast vibrational relaxation processes become relevant. If one generates vibrational motion of a molecule by a stimulated Raman process using two ultrashort laser pulses, the driving forces are determined by the time-dependent phases of the two laser pulses (see below). In this review, an example is given for the application of fs-CARS to selectively excite ground state vibrations in polydiacetylenes. The control is achieved by varying the timing and phase (chirp) of the pump and Stokes laser pulses preparing the vibrational modes. Newest results on an application of optimal control techniques based on a self-learning loop approach will not be discussed here (for this topic, see e.g. refs. 15, 16, and 17).

Theory of Spectrally Dispersed Femtosecond-CARS

The basic theory of time-resolved CARS has been treated 3, 4, 6, 8, 18 before in a number of reports. In the following, a

theoretical description of the fs-CARS measurements discussed in the following is shortly outlined. Here, an approach has been adopted that has been introduced by

18 Laubereau and Kaiser. The specific situation the presented experiments is described, where the anti-Stokes signal is recorded two-dimensionally, i.e. as a function of delay time and signal wavenumber.

The CARS process involves altogether four photons. The pump and Stokes pulses with center angular frequencies

0 0ω and ω , respectively, interact with the sample and Pu St

coherently excite several ground state vibrational modes j with angular frequency ω by stimulated Raman j

scattering. All polarization directions are neglected and the incident light fields are treated as plane waves and Gaussian shaped pulses with equal pulse durations ∆t, e.g.

The coherent vibrational excitation is determined by the expectation value of the vibrational amplitude,

4

the energy space Q(ω),

(10)

where

is the resonance angular frequency. Γ is the bandwidth,

which is a function of pulse duration ∆t as well as φ"(0)

and c is a function of φ"(0). The relative amplitude Q of the vibrational mode with angular frequency ω is j

maximal when ω = ω . It becomes obvious that even in 0 j

this simple model a clear dependence of the excitation process on both the delay time τ and the time dependent 1

phase exists. Therefore, the control of the phase shape of pump and/or Stokes laser pulses will influence the excitation process. Using a pulse-shaper as suggested by

20Weiner et al. , this can be realized experimentally in a 15–17 defined way. In the experiments on polydiacetylenes

presented in the following, τ and the chirp of the lasers 1

are varied separately. It is shown that the dependence of the excitation on both parameters is substantial.

3 Experimental

The experimental apparatus used for the discussed 7, 21 experiments has been described in detail elsewhere.

For the measurements on the porphrins, pulses of about 90 fs duration at 800 nm from a Ti:Sapphire oscillator (Mira, Coherent Inc.) are amplified in a regenerative amplifier (CPA-1000, Clark MXR) with a repetition rate

of 1 kHz, yielding output pulse energies of ≈ 1 mJ. The amplified pulses are used to pump two optical parametric amplifiers (OPA) (TOPAS, Light Conversion Ltd.) that produce two independently tunable pulse trains with

pulse durations of ≈ 70 fs.

For the measurement on the polydiacetylenes, the experimental setup is described in detail in Ref. 22. The laser system consists of a Ti:Sapphire femtosecond laser (Mira, Coherent Inc.), a regenerative amplifier (REGA, Coherent Inc.), and two white-light optical parametric amplifiers (OPA, Coherent Inc.). The output from one OPA was split into two beams which were used as pump and probe lasers. The red-shifted beam from the other OPA served as Stokes laser. The pulse length was about 80 fs (FWHM). The polarizations of all the laser pulses were chosen to be parallel to the b-axis of the monoclinic diacetylene crystal.

In all experiments, for the CARS process the so-called folded BOXCARS configuration has been employed. The phase-matching condition (Eqn. 4) is fulfilled. A

detected at delay time T, which is proportional to the absolute square of the Fourier transformation of the coherently scattered anti-Stokes field,

(6)

According to Eqn. 5 every individual coherently excited molecular vibration contributes to the signal with a bandwidth equal to the bandwidth of the probe pulse. Furthermore, for a large number of coherently excited vibrations, the signal at a given wavenumber position

ν will not decay simply exponentially. Since the CARS

coherent signal is proportional to the absolute square of a sum over contributions from several coherently excited vibrational modes (Eqn. 6), oscillating contributions to the signal can be expected with angular frequencies that are determined by the differences ∆ω = ω - ω of the jk j k

individual vibrations.

In the discussion above, the vibrational motion in the electronic ground state is excited by the temporally coincident and chirp free pump and Stokes pulses. If one introduces chirped Stokes pulses in the pump process the classical driving forces applied to the investigated molecules can be written as:

(7)

where the time delay τ between the pump and Stokes 1

laser pulses and the time dependent phase function φ(t) of the Stokes pulses have been introduced.

The equation of vibrational motion can be expressed assuming a driven harmonic oscillator for the coherent-

19phonon displacement amplitude Q:

(8)

where µ is the reduced mass. ω is the angular frequency j

of the vibrational mode j. γ is the damping constant of this j

mode, where γ =1/T . T is the transverse relaxation time j 2j 2j

of the vibrational motion.

Using Taylor’s expansion for φ(t) to the second order, one obtains

(9)

Fourier transforming Eqn. 3 results in the amplitude in

5

0 0 signal are set to +60 and -60 polarization, respectively (see inset of Fig. 1).

The spectrally dispersed transient CARS signal from MgOEP dissolved in Cl CH is shown in Fig. 2 as a 2 2

function of the probe pulse delay time T and the CARS

wavenumber, ν= ω /2πc. In order to ensure, that the CARS AS

almost instantaneous response from residual non-Raman resonant scattering contributions, e.g. from the influence of the optical Kerr effect on the polarization of the light fields, is ignored in the data analysis, the obtained data are shown and further analyzed only for delay times T longer than 300 fs.

The CARS signal of MgOEP is centered at about 1400 -1cm , corresponding to the wavenumber difference of the

pump and the Stokes lasers, ν∼- ν∼, and extends from Pu St

-1about 1000 to 1800 cm . In the vibrational spectrum of MgOEP this is the interesting region where predominantly modes of the porphyrin macro-cycle are

23 located. The signal lasts for about 4 ps and is strongly modulated, both in the time and the wavenumber axis direction. Although it seems, that the latter might be an indication for a poor signal to noise ratio and/or a low reproducibility, the signal is very robust and for equal experimental parameters it is highly reproducible. This is indicated in Fig. 3, where the transient CARS signal is plotted for three different detection wavenumbers, with every transient being modulated by oscillations with

-1 definite frequencies. At 1780 cm the signal has a single oscillatory component with a periodicity of approx. 670

-1 fs, whereas at 1500 cm an additional weaker oscillatory component of about 260 fs periodicity is apparent. At

-1 1270 cm the oscillatory structure is dominated by a fast oscillating component of about 185 fs with a superimposed slower modulation.

To further investigate the wavenumber spectrum of the

monochromator has been used to spectrally disperse the CARS signal, which was then detected by a CCD camera.

As a solvent for the porphyrin sample dichloromethane was chosen, because it possesses only a weak vibrational structure in the excitation region. The CARS measurements have been performed using a flow-type rectangular glass capillary. The TS6 and FBS diacetylene single crystals were cooled to approximately 10 K by means of a closed-cycle helium cryostat. The monomer crystals contained less than 1% polymer.

4 Results and Discussion

4.1 Femtosecond-CARS on Porphyrin Systems

4.1.1 Spectrally dispersed femtosecond-CARS on MgOEP:

In this section, spectrally dispersed fs-CARS measurements on Magnesiumoctaethylporphyrine (MgOEP) are presented. In Fig. 1, the wavelength arrangement of the laser beams together with the absorption spectrum of MgOEP in dichloromethane is displayed. The pump and the probe pulse wavelength of 581 nm roughly coincide with the Q absorption band 00

maximum at 580 nm. The Stokes pulse is tuned to 632 nm, resulting in an anti-Stokes signal centered at 538 nm, in the vicinity of the Q -band maximum. The overall 01

wavelength arrangement leads to a strong resonance enhancement of the CARS process. In order to suppress non-Raman resonant scattering contributions from the solvent to the CARS signal around T = 0, a magic angle polarization geometry for the four beams has been

6 applied in these measurements. The temporally overlapping pump and Stokes pulses are kept parallel

0polarized (0 ), and the probe pulse and the anti-Stokes

Figure 2: Spectrally dispersed transient CARS signal of MgOEP.

Figure 1: fs-CARS wavelength arrangement and absorption spectrum of MgOEP dissolved in Cl CH .2 2

6

decaying component of the original signal has been subtracted, retaining only oscillating components, before subjecting it to the FFT procedure. The PSD elucidates the quantum beat dynamics of the generated vibrational coherence in the wavenumber domain. It is now obvious, that the complex oscillatory pattern of the time domain signal is mainly attributable to four peaks (Table 1). As pointed out above, the peak location along the FFT wavenumber axis corresponds to the wavenumber differences of two coherently excited vibrational modes of MgOEP, and the location along the CARS wavenumber axis is approximately given by the arithmetic mean wavenumber position of the two beating modes.

It is now appropriate to compare the obtained results to data that are available from spectrally resolved

23 techniques on the same molecule. In order to assign the peaks in the PSD to the wavenumber difference of two vibrational modes ν and ν , results from ns-CARS k j

spectroscopy and resonance Raman measurements are taken into account (see Table 1).

4.1.2 Selection of the detected vibrational dynamics:

Because of the well defined polarization directions of the CARS signal contributions from individual vibrational modes, the quantum beat structure of the detected signal is influenced by the magic angle polarization geometry of

23 the laser beams in the experimental setup. Contrary to ns-CARS (time domain) though, where all three incident laser beams are temporally overlapped and fixed, in fs-CARS the signal can be analyzed at delay times when the non-Raman resonant scattering contribution is already decayed. Therefore, when the pulse duration of the laser pulses ∆t is considerably shorter than the vibrational dephasing time T, the non-Raman resonant background does not obscure the signal at T> 0, leaving the analyzer setting of the CARS signal in principle undetermined.

Using this concept, transient CARS measurements were performed on Magnesiumtetraphenylporphyrin

oscillatory structure comprised in the signal of Fig. 2, the power spectrum density (PSD) of the time domain signal was calculated with the fast Fourier transform method (FFT), which is shown in Fig. 4. The exponentially

Figure 3: Cuts of the CARS signal (Fig. 2), plotted at three different wavenumber positions.

Table 1: FFT wavenumber (νFFT) and CARS

wavenumber (˜νCARS) locations of the peaks in the

PSD plot (Fig. 4). The peaks correspond to quantum

beatings between the vibrational modes νk and νj of

MgOEP. νk and νj are assigned from ns-CARS data

(a) and resonance Raman data (b) of Ref. 23.

Figure 4: Power spectrum density (PSD) of the oscillating

contributions to the transient CARS signal of Fig. 2.

7

(MgTPP), where the detected coherent vibrational motion is selected by the polarization direction of the analyzer used for the CARS signal beam. The electronic resonance conditions in these measurements have been the same as for the MgOEP measurements. The excitation

-1wavenumber was 1400 cm . Panel (a) of Fig. 5 shows the PSD of the transient CARS signal for the same magic angle polarization arrangement (analyzer polarization -

0 60 with respect to the pump/Stokes polarizations) like in the measurements on MgOEP. Four peaks are observable in the resulting signal. The assignment of the contributing modes and their respective symmetries are given in Table 2. Almost no cw data are available for the MgTPP system because of the strong fluorescence background in resonance Raman spectra (Q-band excitation). Therefore, the assignment is mainly based on results on

24 NiTPP.

Assuming that the CARS depolarization ratios for conventional CARS are valid for our measurements, the contribution to the signal should vanish for an analyzer

0 0 setting of 0 for modes of a symmetry, and at -78 for 2g

modes of b and b symmetry. Panel (b) of Fig. 5 shows 1g 2g

the PSD of the transient signal for an analyzer setting of 00 . The peak structure is basically the same as in the magic

angle measurement. However, the intensity of the peak originating from two b modes has strongly increased, 1g

whereas the two peaks containing an a mode have 2g

considerably decreased in relative intensity. The PSD shown in panel (c) of Fig. 5, depicting the measurement at

0an analyzer setting of -78 , is similar to the one given in panel (a). The peaks containing modes with a symmetry 2g

are now clearly dominating. All features containing contributions from b and b modes are now even further 1g 2g

suppressed.

The presented results for the three different polarization geometries are in accordance with the expectation that the detected dynamics depend strongly on the employed polarization geometry. The tentative mode assignment in Table 2, from NiTPP data, is confirmed.

4.2 Selective Excitation of Electronic Ground State Vibrational Motion in Polydiacetylenes

The experiments discussed in the following give another example of the application of CARS spectroscopy for the investigation and manipulation of ultrafast dynamics in complex molecular systems. The experiments were performed on diacetylene single crystals containing less than 1% polymer molecules. Fig. 6 shows the chemical structure of polydiacetylenes (PDAs). They have a large

Figure 5: PSD of the transient CARS signal of

MgTPP for three different polarization geometries: 0 0 0a) CARS signal analyzer setting -60 , b) 0 , c) -78

Table 2: Mode assignment to the PSD peaks of the

polarization sensitive fs-CARS measurements on

MgTPP (Fig. 5).

8

also Ref. 28). The main reason for the change of transition probability for the different vibrational motions is the strong coupling of the considered chain modes to the π electron system of the polymer backbone. The PDAs perform a geometric relaxation after they are pumped to their free excitonic state with a femtosecond laser pulse. The formation of self-trapped excitons (STE) follows in short time. Pham et al. suggested a transition from a butatrienic mesomeric chain structure to an acetylenic one, where the STE is self-trapped in the C=C double bonds. The transition to the electronic ground state (with acetylenic backbone structure) therefore should favor the mode, which is Fermi coupled to the mode.

In the presented CARS experiments, the chirp has been estimated by measuring the time-and wavenumber-resolved non-resonant CARS signal of a thin glass plate

27 (300 µm). In this experiment, T was kept fixed while τ 1

has been varied. The resulting three-dimensional plot [anti-Stokes intensity = f(signal wavenumber, τ )] was 1

used to minimize the chirp for this experiment.

third order susceptibility and are well suited for an investigation by means of nonlinear spectroscopical methods. Several types of PDAs embedded in single crystals of their monomers have been also investigated with wavenumber-resolved nanosecond CARS

25 spectroscopy. With femtosecond time - and wave 26 number - resolved CARS one can not only generate the

vibrational coherent motion of PDAs in their electronic ground state, but also distinguish in the anti-Stokes signal contributions from different modes. Figure 7 shows the resonance Raman spectrum of the PDAs. The spectrum is determined by a few chain modes, which are excited due to their strong coupling to the delocalized π electron system of the polymer backbone resonantly excited by the lasers. The wavelengths of the pump and probe lasers in the experiments were chosen to be 605 nm, which is

1close to resonance with the B transition to the excitonic u

state of the polymer. The Stokes laser was tuned to 652 nm in order to coherently excite the vibrational modes in the ground states. Here, the four vibrational modes and were excited coherently with the

22 femtosecond pump and Stokes laser pulses.

The general idea of the experiments discussed in the following was to influence the excitation of different vibrational modes in the electronic ground state of the PDA molecules. It is shown that this goal can be reached by varying the time dependent phase of the pump and/or Stokes lasers as well as making use of the dynamics on the electronic excited PES. As already mentioned in the theory section, there are two factors, which decide on the outcome of the pump-dump process: (i) the dynamics evolving on the excited PES during a delay time τ 1

between pump and Stokes lasers and (ii) the chirp of the exciting laser pulses as discussed above. Recently, the dynamics of the electronically excited state (exciton) of

27 the PDAs has been investigated, which is a basis for the presented work. In the following, two experiments are discussed.

In the first experiment, the chirp of the laser pulses was minimized (completely chirp-free conditions can never be reached in a real experiment). By varying the delay time τ between the pump and Stokes pulses, there is still 1

an influence of the chirp, but, the effect should now be dominated by the dynamics in the excited state. Due to this dynamics, as a function of τ Franck-Condon 1

windows are opened at different positions of the PESs. For example, this should give rise to an enhancement of the vibrational modes (C=C stretching + Fermi resonance with side group CH scissors vibration) as 2

27 discussed in the contribution by Chen et al. (compare

.

Figure 6: The molecular structure of the diacetylene monomer

and polymer unit. The polymer chain (backbone) can be

described by two mesomeric structures: the acetylenic (left-

hand side) and the butatrienic (right-hand side).

Figure 7 : Resonance Raman spectrum displaying the

vibrational modes excited by the femtosecond pump and Stokes

pulses.

9

Conclusions

Applications of CARS (coherent anti-Stokes Raman scattering) in the time domain have been presented on two examples. By employing fs-CARS, coherent vibrational motion in the electronic ground state of two different types of large polyatomic systems has been excited and probed. It was shown that for the porphyrin molecules the method is capable of providing a detailed mapping of the dynamics of a multitude of excited modes. It was also demonstrated that fs-CARS yields the dephasing behavior and spectral information at the same time. The detected vibrational dynamics can be efficiently selected by varying the polarization direction of the CARS signal analyzer. Also for diacetylene polymers, the vibrational motions in their electronic ground state have been excited by stimulated Raman scattering with two femtosecond laser pulses. Different

Figure 8 shows an anti-Stokes spectrum taken from TS6 PDA at τ = -80 fs and T = 0 fs. The shaded areas show the 1

Figure 8: CARS spectra taken from TS6 PDA at τ = -80 fs and T 1

= 0 fs. The shaded areas show the result of a fit with three Gaussian functions.

Figure 9: (a) The wavenumber-resolved CARS spectra at three different τ. T was chosen to be 0 fs. (b) The relative population of 1

~ν/ ν and ν modes as a function of τ calculated by means of Eqn. 11.2 2' 3 1

~ ~

Figure 10: Controlled mode excitation in the electronic

ground state: (a) minimized chirp, (b) chirped pulse.

10

Kiefer W, J. Raman Spectrosc. 30 (1999) 807.

27. Chen T., Vierheilig A., Waltner P., Kiefer W., Materny A., Chem. Phys. Lett, 325 (2000) 176.

28. Pham T.A., Daunois A., Merle J.-C., Moigne J. Le, Bigot J.Y., Phys. Rev. Lett. 74 (1995) 904.

29. Kobayashi T., Yoshizama M., Stamm U., Taiji M., Hasegawa M., J. Opt. Soc. Am. 7 (1990) 1558.

30. Judson R.S., Rabitz H., Phys. Rev. Lett 68(1992) 1500.

31. Assion A., Baumert T., Bergt M., Brixner T., Kiefer B., Seyfried V., Strehle M., Gerber G., Science 282 (1998) 919.

vibrational modes were enhanced or suppressed by optimizing the time delay between the pump and Stokes

11

of frequency components in time results in the lengthening of an otherwise bandwidth limited ultrafast laser pulse. For positively chirped pulse leading edge of the pulse is red shifted and the trailing edge is blue shifted with respect to the central frequency of the pulse. Negative chirp corresponds to the opposite effect. Several experimental and theoretical developments have made linear chirped pulse control a very attractive field of

8-14research . In typical single-photon absorption and emission processes, cross-sections are proportional to the magnitude of the transition dipole moment, and as such the role of laser chirp is rarely considered. In “coherent control” experiments, however, high intensity chirped pulse single-photon experiments show systematic trends that are mostly dictated by the laser pulse property. In single photon processes, such laser induced control that occurs irrespective of exact molecular properties is an

7important aspect of active coherent control . However, the situation is quite different in multi-photon processes where the relative phase of the interacting photons with

15,16the chromophore is important . The intensities involved in the multi-photon processes are typically high enough to induce driven-oscillator interactions as compared to the simple harmonic perturbative interactions for the single-photon case. Thus, it is expected that chirp can induce systematic effects on two-photon processes and we also expect such laser induced effects to be mostly dictated by laser pulse characteristics rather than the choice of molecules or solvent. We systematically show here, effect of chirp on TPA and TPF depends on a specific system environment and so such studies in liquid phase are more demanding and more complicated in the sense, one now has to take care of the additional solute-solvent interactions also.

One of the first and foremost difficulties in the process of control of condensed phase two-photon processes that we register lies in the fact that the quantification of the two-photon process by specifying a measure of TPCS is no longer possible for chirped pulses. Thus, we have concentrated in our study only on the raw-experimental data that are available from the two-photon processes. We take advantage of the fact that two-photon absorption as well as the two-photon fluorescence measurements can,

Abstract

We show that experimental modulation of two-photon absorption and fluorescence is possible by simple phase ordering a femtosecond laser pulse into a linearly frequency chirped pulse. However, the modulation is non-monotonic due to the strong impact of solvent properties. Systematic effects occur mostly only over a limited range of chirp since it is an interplay of two opposing effects on two-photon processes-linear chirp enhancing it while the associated pulse broadening reducing it.

Keywords: Chirped Femtosecond laser pulses, Enhanced Two-Photon Cross-section

Introduction

Among coherent spectroscopic techniques, two-photon processes are difficult to control-more so in the condensed phase. Model calculations and feedback-loop

1-3 coherent control experiments have often failed to come up with simple frequency chirped pulses that have predictable results. Two-photon processes are characterized by a measure of their two-photon cross-sections (TPCS). Design and synthesis of molecules with large TPCS measured through absorption or fluorescence has received a lot of recent attention due to potential applications in two-photon imaging, optical limiting, two-photon photodynamic therapy, etc. Typically, modulation in TPCS is achieved with increased electron delocalization in molecules by synthetic design

4-6strategies . However, in addition to the high challenges in synthetic skill, research in the design and synthesis of molecules with high TPCS faces the bottleneck of laser interaction characteristics. Thus, wavelength tunable TPCS of molecules is intricate and further laser control parameters are certainly desirable.

We present here the effect of linearly chirped laser pulses on two-photon fluorescence and absorption processes in

7condensed phase. Frequency chirping is one of the easiest pulse modulation schemes. Chirping essentially refers to the process of arranging the frequency components in a laser pulse with certain phase ordering. Linear ordering can be easily achieved by dispersing the ultrafast pulses through a pair of gratings. This ordering

Attempt to control femtosecond two-photon processes in solution

Amit Nag, Sumit Ashtekar and Debabrata GoswamiDepartment of Chemistry, Indian Institute of Technology Kanpur, Uttar Pradesh - 208016, India

12

20our double pass 4f geometry pulse shaper , which comprise of only one diffraction grating and a curved mirror each.

We generate the chirps by changing the focal distance ‘f’ between the flat mirror (in the Fourier plane) and the curved mirror, keeping the overall 4f distance of the shaper constant. The signatures of the chirps produced from this setup remain the same: as the distance between curved mirror and flat mirror increases or decreases, we get negative or positive linear chirps respectively. The complete characterization of the chirped pulses is done by our homebuilt intensity autocorrelation and STRUT (Spectrally and Temporally Resolved Upconversion

21Technique) . Unlike autocorrelation, the STRUT can recover both the amplitude and phase of the pulse and gives the sign of the chirp. Figure 1 shows the various autocorrelation traces of the pulses from oscillator Mira at 800 nm for different chirps. It is evident from the figure, as the pulse gets chirped from its transform limited position, it gets lengthened in time. Representative STRUT traces for a transform limited and a chirped pulse form kHz Odin is also shown in the figures 2 and 3 respectively.

Results and Discussions

The calculation of the chirp parameter is as following.

The phase (ϕ(ω)) of the laser pulse which is centered at ω , can be expanded around ω to second order in ω:0 0

(1)

Here, the second order term is responsible for group

velocity dispersion. In fact, is linear chirp

coefficient (chirp parameter in the frequency domain) introduced by the compressor and is defined as second derivative of the spectral phase at the center frequency. The linear chirp coefficient (β) can be calculated using

22-24the equation : where τ is the

pulse duration of the chirped laser pulse and τ is the 0

chirp-free pulse duration of the transform limited pulse in FWHM. The experimental error in the chirp value calculated from the equation mentioned above is about ±5%.

We studied the TPA and TPF properties of the chromophores using linearly chirped laser pulses and compared it to its transform-limited position. We find the sign and the extent of linear chirp affects these nonlinear properties of the chromophores. However, since the

in fact, give rise to identifying the same two-photon 17property . We find that, in general, the phase dependence

exists only over a limited range of chirp since the associated pulse broadening effect of chirp eventually reduces the pulse intensity, which overwhelms the chirp effect. As, the effect is highly solvent dependent, it is not possible to easily identify a systematic effect that is dictated only by the characteristics of the laser chirp.

Experimental

We target our study on the chemically diverse range of important dyes: cis-Ru(II)(bpy) Cl ion complex 2 2

(Rubpy), Rhodamine-6G (Rh6G), Rhodamine-101 (Rh101) and Octaethylporphyrin (OEP) free base. Two-photon studies were performed either by an open aperture (OA) z-scan technique or through two-photon

17fluorescence (TPF) measurement in a single setup . We used both the kHz amplified laser system (ODIN, Quantronix Inc.) and the high repetition rate MHz laser Mira for the experiments. We ensured that the laser intensity at the focus for all experiments remains around 1

2GW/cm . Experimental details of the setup and chemical structures of the dyes have been given earlier. All samples were bought from Sigma-Aldrich (spectra-grade) and

-3used without further purification at a concentration of 10 M for OA z-scan and TPF experiments.

The production of chirped ultrafast laser pulses is straightforward. For, our kHz amplifier system Odin, we generate linear chirps from the built-in suitably modified

18,19compressor system inside the amplifier . As we increase the spacing between the compressor gratings relative to the optimum position for minimum pulse duration of 50 fs, we generate a negatively chirped pulse. Conversely, we obtain the positively chirped pulse by decreasing the inter-grating distance. For the production of linearly chirped pulse from MHz laser Mira, we use

Figure 1. Different autocorrelation traces at different chirp values from Mira laser. Pulse width lengthens as the chirp increases.

13

Figure 2. STRUT trace of a transform-limited pulse of kHz amplifier Odin at 800nm.

Figure 3. STRUT trace of a negatively chirped (blue to red) pulse of kHz amplifier Odin at 800nm.

OA z-scan experiments with kHz amplifier Odin at 800 nm are performed to get a measure of the TPA for these chromophores. For both Rubpy and Rh101 [figures 4(a) & (b)] TPA is enhanced mainly in negative direction of the chirp in DCM solvent up to a certain degree of chirping beyond which the pulse width effect overwhelms and TPA diminishes. The enhancement is roughly about 15% for Rubpy and about 10% for Rh101, both in DCM solvent. But in CHCl solvent medium, we 3

do not see any modulation in the TPA for either of the chromophores. So, our conjecture is: The phenomenon is

principle of calculating a TPCS value is no longer valid for the chromophore when we chirp a pulse, we will limit our discussion with respect to the raw values of the TPA and TPF only. We calculated the raw TPA values from the area under the curve of the TPA signals as well as the area under raw TPF signal. We have shown that two-photon

25processes are highly solvent dependent also . On that note, we also varied the solvents in our study of chirp effect. Figure 4 shows the dependence of TPA of the chromophores Rubpy, Rh101 and OEP, upon linear chirping.

14

bandwidth limited longer pulses from Mira laser produce lesser TPF for Rh6G dye (fig. 6b) compared to the transform limit condition. Thus, a distinct control with chirp is evident for the two-photon processes although it is highly dependent on the particular system and solvent medium, revealed from our present study.

28A recent study , of optimal feedback control of two-photon fluorescence in Coumarin 515 based on genetic algorithm shows that TPF depends on laser chirps and TPF is enhanced by 20% from the near transform limited condition upon chirping. In their case, a positively

highly solvent dependent. However, similar study with octaethyl porphyrin in DCM does not show any similar feature in either direction of the chirp (fig. 4c). The data shows almost invariant nature upon chirping. We also measured the TPF signal of Rhodamine 6G dye at different values of chirping from 800 nm Mira pulse in MeOH and DMF solvents (fig. 5). We find enhancement of TPF signal in Rh6G in methanol in both directions of the chirp, but more so in the negative direction again. Maximum enhancement obtained was 10% by chirping in the negative direction in MeOH. Again, modulation of TPF intensity in DMF is very negligible. So, one thing is

25clear here. As we have shown in our earlier work , solvent viscosity can play important role in modulating the nonlinear properties of the chromophores. Here, CHCl and DMF both are much viscous solvents than the 3

other two, which are less effective in this enhancement process.

The observation of preferential enhancement for one chirp sign (negative) for the chrmophores implies that the observed enhancements are not due to the pulse width effects, they rather depend on the magnitude and sign of

26,27the chirp . Hence coherence of the laser field plays an important role in this process. To prove this statement, we have also systematically studied the variation of SHG intensities at different values of chirp (fig.6a). This study was performed by simply replacing the chromophore solution by a thin BBO crystal (0.1 mm thick) and SHG signal was collected by a photodiode. We find, integrated SHG intensity at different chirp symmetrically decay

2around 0 fs (fig. 6a), which has the most conversion efficiency under the transform-limited condition as compared to any of the chirped cases. This confirms that there is nothing systematic in the laser pulse causing the enhancements in the TPA process. Similarly, the

Figure 5. Effect of linear chirping on TPF of Rh6G at 800nm.

Experiments were performed using Mira Oscillator.Figure 4. Effect of linear chirping on TPA properties of (a)

Rubpy, (b) Rh101, and (c) OEP at 800 nm. Experiments were

performed using kHz amplifier Odin.

Figure 6. (a) Measure of second harmonic generation (SHG) signal as a function of chirp parameter, β. (b) Bandwidth limited TPF of Rh6G in MeOH solvent at 800 nm. Both the experiments were performed using oscillator Mira.

15

5. Das, S.; Nag, A.; Goswami, D.; Bharadwaj, P.K. J. Am. Chem. Soc. 2006, 128, 402.

6. Misra, R.; Kumar, R.; Chandrashekar, T.K.; Suresh, C.H. ; Nag, A.; Goswami, D. J. Am. Chem. Soc. 2006, 128,16083.

7. Goswami, D. Phy. Reports 2003, 374, 385.

8. Melinger, J.S.; Gandhi, S.R.; Hariharan, A. ; Goswami, D.; Warren, W.S. J. Chem. Phys. 1994,101,6439.

9. Melinger, J.S.; Gandhi, S.R.; Hariharan, A.; Tull, J.X.; Warren, W.S. Phys. Rev. Lett. 1992, 68, 2000.

10. Krause, J. L.; Whitnell, R. M.; Wilson, K. R.; Yan, Y.; Mukamel, S. J. Chem. Phys. 1993, 99, 6562.

11. Cerullo, G.; Bardeen, C. J.; Wang, Q.; Shank, C. V. Chem. Phys. Lett. 1996, 262, 362.

12. Pastirk, I.; Brown, E. J.; Zhang, Q.; Dantus, M. J. Chem. Phys. 1998, 108, 4375.

13. Yakovlev, V. V.; Bardeen, C. J.; Che, J.; Cao, J.; Wilson, K. R. J. Chem. Phys. 1998, 108, 2309.

14. Cao, J.; Che, J.; Wilson, K. R. J. Phys. Chem. 1998, 102, 4284.

15. Meshulach, D.; Silberberg Y. Nature 1998, 396, 239.

16. Hosseini, S.A.; Goswami, D. Phys. Rev. A 2001, 64, 033410.

17. Nag, A; De, A.K; Goswami, D. J. Phys. B 2009, 42, 065103.

18. Mathur, D.; Rajgara, F. A. J. Chem. Phys. 2004, 120, 5616.

19. Strickalnd D.; Mourou, G. Opt. Commun. 1985, 56, 219.

20. Nag, A.; Chaphekar, P.A.; Goswami, D. (under review in Rev. Of Sci. Instr.).

21. Nag, A. Ph.D Thesis: Accurate quantification and control of two-photon processes, Indian Institute of Technology Kanpur, India, 2009.

22. Treacy, E. B. IEEE J. of Quantum Electronics 1969, 5, 454.

23. Hong, K.-H.; Sung, J. H.; Lee, Y. S.; Nam, C. H. Opt. Commun. 2002, 213, 193.

24. Siegman, A. E. Lasers, University Science Books, Mill Valley, California, USA, 1986.

25. Nag, A; Goswami, D. J. Photochem. Photobiol. A 2009, 206, 188.

26. Assion, A.; Baumart, T.; Bergt, M.; Brixner, T.; Kiefer, B.; Sayfried, V.; Strehle, M.; Gerber, G. Science 1998, 282, 919.

27. Daniel, C.; Full, J.; González, L.; Kaposta, C.; Krenz, M.; Lupulescu, C.; Manz, J.; Minemoto, S.; Oppel, M.; Francisco, P. R.; Vajda, S.; Wöste, L. Chem. Phys. 2001, 267, 247.

28. Zhang, S.; Sun, Z.; Zhang, X.; Xu, Y. ; Wang, Z.; Xu, Z.; Li, R. Chem. Phys. Lett. 2005, 415, 346.

29. Goswami, T.; Karthick Kumar, S. K.; Dutta, A.; Goswami, D. Chem. Phys. 2009, 360, 47.

chirped pulse favored the effective population transfer in the two-photon transition and enhanced the TPF intensity of the dye. But as we show here from our study with different chromophores, the enhancement need not be always in the positive direction of the chirp only, rather it heavily depends on system to system. Different Solvents also play an important role in modulating the population transfer process in two-photon transitions. We also have

29recently reported a comprehensive study of laser induced molecular fragmentation of n-propyl benzene using chirped pulses. There also, we find the enhancement of relative yields of the smaller fragment ions are mainly favored by the negative direction of the linear chirp.

A closer look at the results indicates that there is a competition between the chirp-induced enhancement of TPA and TPF for the chromophores and the effect of pulse width which decreases these processes. At very low chirps (close to transform limit), we see a quadratic effect of the chirping reminiscent of the adiabatic processes. However, the pulsewidth effect on the intensity is soon drastic enough to counter the enhancement with chirp and the TPA falls off after reaching the maxima with chirping. This, in fact, explains the reduced two-photon processes and difficulties of typical fiber microscopy experiments with femtosecond lasers. However, our experiments establish that the effect of chirp on two-photon process is distinct but is only observable until the pulsewidth parameter is not overwhelming.

Summary and conclusions

We have demonstrated that the phase characteristics of the femtosecond laser pulse play a very important role in modulating the TPA and TPF processes of the chromophores in liquid phase. The phenomena depend on the sign of chirp. Though the role of chirp in these processes is distinct, high solvent dependence is a constraint for clear understanding the issue. After the initial enhancement, pulsewidth effects overwhelm the chirp effect in all the cases.

References

1. R.J Gordon,.; Rice, S.A. Annu. Rev. Phys. Chem. 1997, 48, 601.

2. Shapiro, M.; Brumer,P. Advances in Atomic, Molecular and Optical Physics,42, ed. B Bederson and H. Walther (San Diego: Academic Press,1999, p 287.

3. Goswami, D. J. Opt. B: Quantum Semiclass. Opt. 2005, 7, S265.

4. Rath, H.; Sankar, J.; PrabhuRaja, V. ; Chandrashekar, T.K. ; Nag, A.; Goswami, D. J. Am. Chem. Soc. 2005, 127, 11608.

16

cross-polarising filter. Slight changes in the plane of

polarisation will thus cause variations in the detected

light intensity after the second filter. Variation in the laser

light intensity as a function of applied field yields the

magnetic hysteresis curve. However, the background

noises such as sample vibration and noise due to the stray

light cause distortion to the magnetic hysteresis curve and

should be minimized. A.C modulation techniques

together with the lock-in detection help in improving the

single to noise ratio. Several devices have been proposed

to modulate the state of the polarization of the incident

light such as, mechanical modulator (spinning analyser

prism and vibrating half shaded polariser) and electronic

modulator (Pockels cell and photo-elastic modulator). In

the set-up shown in figure 1, a photo-elastic modulator

(PEM) is being used as a modulator.

Figure 1: Block diagram of the MOKE set up

Light that passing through the PEM receives a

periodically changing retardation, given by

(1)

where δ is the retardation amplitude and f is the ο

modulation frequency. PEM modulated light is then

reflected by the sample. For p polarized light, if the

sample is nonmagnetic then the reflected light is purely p-

polarized. However, if the sample is magnetic, then the

reflected light consists of a s-component (E ) in addition s

to the p-component (E ). Therefore, measuring the E is p s

Introduction

The current general thrust in nanotechnology and

nanostructures has generated a great deal of interest in the

development of techniques capable of producing and

characterizing such structures. The case of magnetic thin

films and layered structures is an especially challenging

one, which is economically and technologically

important in spintronics and the recording media

industry. One of the powerful techniques used for the

analysis of thin ferromagnetic samples is based on the

Kerr effect, described by the Reverend John Kerr in 1877

[1]. This consists of the rotation of the polarization plane

of a light beam when reflected from a magnetized

surface. The magnitude of this change in polarization is

proportional to the magnetization of the sample.

Macroscopically this effect arises from the anti-

symmetric, off-diagonal elements in the dielectric tensor

[2]. Magneto-optics can also be described in the context

of microscopic quantum theory [3], where spin-

dependent dielectric constant is a consequence of the

spin-orbit interaction, which couples the electron spin to

its motion. Using this magneto-optic effect (Kerr effect)

one probes a quantity proportional to the magnetization

in a surface layer, the thickness of which is determined by

the optical absorption coefficient of the material at the

wavelength used for the experiments. For the metals in

the visible region this surface layer is typically around 20

nm. The application of the magneto-optic Kerr effect to

surface magnetism better known by its acronym MOKE,

began in 1985. The technique is popular because it

combines several advantages with respect to other

techniques. Present write-up does not deal with the

theoretical aspects of MOKE, but rather presents

experimental details, with some in-situ results on pure

cobalt film and alloy film of FINEMET.

Experimental set-up

The experimental setup normally involves passing laser

light through a polarising filter and then reflecting the

light off the sample. The light then passes through another

Magneto Optical Kerr Effect (MOKE) for study of thin film magnetism

Dileep Kumar, Ajay GuptaUGC-DAE Consortium for Scientific Research,

Khandwa Road, Indore 452017, India

17

gold protecting overlayer is given in the figure 3. The

clear hysteresis loop of Fe film shows that it is possible to

detect the magnetic properties of thin film down to very

small thickness. It may be noted that the active Fe film in

this may be less then that of the 2.5 nm as a magnetically

dead layer formation is expected at the interface between

Fe film and the non-magnetic protecting layer. It may be

noted that the y-axis is proportional to the magnetization

of the film. However, it is difficult to estimate the

absolute magnetization from Kerr intensity, as it depends

upon many factors including the geometry of

measurement, sample surface condition etc. On the other

hand, x-axis is absolute, and one can easily determine

properties like coercivity, exchange bias etc.

Since MOKE is a highly sensitive and fast technique, it

has contributed substantially to the progress made in the

area of thin film magnetism, for examples, in the

understanding of ultra thin film magnetic anisotropies

[4], oscillatory exchange coupling [5], critical

phenomena [6], and spin reorientation transition [7].

In-situ MOKE studies

Since MOKE is a non-contact technique, it is possible to

do in-situ measurements on ultrathin films inside an

ultra-high vacuum deposition chamber. In the case of

ultra thin films in-situ measurements are highly desirable,

as exposure to the ambient would result in formation of an

oxide layer at the surface, which can significantly affect

the magnetic properties of the film. Even a nonmagnetic

over-layers which is sometimes used to protect the film

against oxidation, can modify the magnetic properties

substantially. Furthermore, by continuously monitoring

the magnetic hysteresis of a growing magnetic film in-

situ, a detailed thickness dependence of magnetic

the goal of this experimental set-up. This can be realized

by keeping an analyser in crossed position to eliminate

the E component. However, in general, analyzer is kept at p

an angle δ from the crossed position. It can be easily

shown that the intensity measured by the detector is,

(2)

(3)

φ′ and φ′′ are Kerr rotation and ellipticity respectively

and are lineally proportional to the magnetization. The

signal from the detector is sent to measuring devices that

is lock-in amplifier. The PEM supplies a frequency

reference signal to the lock-in amplifier at the same

frequency at which light was modulated. The measured

intensity though lock-in amplifier as a function of applied

field (H) yields the magnetic hysteresis curve.

Depending on the direction of the magnetization vector

with respect to the reflection surface and the plane of

incidence, MOKE can be done in three configurations

viz., longitudinal, polar and transverse (figure 2). (a)

Longitudinal MOKE, where the magnetic field is applied

parallel to the plane of the film and the plane of incidence

of the light. This geometry provides a signal proportional

to the component of magnetization that is parallel to the

film plane and the plane of incidence of the light. (b) In

Polar MOKE magnetic field is applied perpendicular to

the plane of the film. This geometry provides a signal

proportional to the component of magnetization that is

perpendicular to the film plane. (c) In transverse MOKE

magnetic field is applied perpendicular to the plane of

incidence of the light. A change in the intensity of p-

polarized incident light is detected. This geometry

provides a signal proportional to the component of

magnetization that is parallel to the film plane but

perpendicular to the plane of incidence of the light. In

first two ceases, there will be a change in the state of

polarization due to the magneto optical effects and in the

transverse mode it is the intensity, which changes in

proportional to the magneto-optical interaction.

A typical hysteresis loop of 2.5 nm Fe film with 2.0 nm

Figure 3: Hysteresis loops of 2.5 nm Fe layer on float

glass substrate with 2.0 nm protective Au layer.

Figure 2: Three possible configurations in the magneto optical

Kerr effect experiments: a) longitudinal, b) polar, c) transverse

18

ferromagnetism in Co film at this thickness. The

observed loop at ~0.5 nm film thickness shows that the

sensitivity of the in-situ MOKE set-up is quite good.

The inset of Figure 5b shows variation of Kerr signal with

film thickness. Initially Kerr signal increases linearly

with Co layer thickness upto ~12.0 nm. However, for

higher thicknesses it exhibits a saturation effect and

become constant after a thickness of ~ 25 nm.

This suggests the surface sensitivity of the technique

which depends on the penetration of the laser. In the case

of Co film one can infer that MOKE is sensitive up to a

depth of ~25 nm.

properties can be obtained in a single experiment. The in-

situ method thus makes it practical to study the influence

on the magnetic properties of the growth parameters such

as, substrate temperature, chamber pressure, and film

growth rate. In view of the above facts, a facility has been

developed at UGC-DAE Consortium for Scientific

Research, Indore for in-situ MOKE measurement in a

UHV deposition chamber. The hysteresis loop is

measured using the Kerr effect by shining a laser beam on

the sample surface without changing the sample position

during the growth of the film.

Figure 4 shows in-situ MOKE set-up, in which a

polarized laser light gets reflected at 45° from a Pt mirror

to enter the UHV chamber through a quartz window

coated with an anti-reflection coating. A pair of

Helmholtz coils inside the chamber creat magnetic field

at the sample. The chamber also has facility for in-situ

magnetoresistance measurement. This MOKE set-up

system is used extensively for in-situ study of magnetism

of ultra thin films. Various in-situ measurements on pure

cobalt, FINEMET alloy and compound FeN films have

been performed at thicknesses ranging from a fraction of

a nm to a few nm. A few representative examples of such

measurements are discussed in brief in the following.

Development of magnetic properties of Co film as a

function of thickness

Figure 5a shows development of MOKE hysteresis loops

of a Co film, deposited using electron beam evaporation

on a float glass substrate, as a function of film thickness.

A significant MOKE signal starts appearing at a film

thickness of ~0.5 nm. A finite coercivity is first observed

at a thickness of 0.7nm, indicating the onset of the

Figure 4: Schematic diagram of the UHV chamber equipped

with MOKE and electron beam deposition techniques.

Figure 5: (a) Few representative hysteresis loops of Co film at

different thicknesses, (b) Kerr signal as a function of the Co

film thickness. Data is fitted by a straight line. Inset of the figure

shows variation of Kerr signal up to 80nm of thickness

(a)

(b)

19

coercivity exhibits a slow decrease as the effect of

domain wall pinning at the suface irragularities decreases

with thickness.

Evolution of magnetic anisotropy in polycrystalline Co

film

In order to study possible magnetic anisotropy at different

stages of Co film growth on float glass, azimuthal angle

dependence of hysteresis loop was measured in-situ after

For ultra-thin films, the Kerr rotation in the longitudinal

geometry is given by the relation: [8]:

(4)

where Q is the magneto-optical constant, d is the

thickness of the magnetic layer and θ is the angle of

incidence of the laser beam from the surface normal.

Therefore, for a fixed measurement geometry, the Kerr

signal is linearly proportional to the thickness of the film.

Figure 5b shows the dependence of Kerr signal on film

thickness in the range 0 to 3nm. In this thickness range,

Kerr signal does exhibit a linear dependence. The

intercept of the straight line on the x-axis gives the

thickness of magnetic dead layer if any. In the present

case of Co film on glass substrate, there is no magnetic

dead layer at the interface.

Variation of coercivity and sheet resistance (R ) with Co

film thickness

In figure 6, the variation of the coercivity and sheet

resistance (R ) up to a film thickness of 3 nm is plotted. Co

Information about the film morphology obtained from

resistivity measurements, can be used to understand the

film thickness dependence of coercivity. High sheet

resistance up to a film thickness of 0.5nm suggests an

island structure with almost no connectivity between the

islands. Sudden drop in resistivity around 0.7nm suggests

development of a percolating cluster. With further

increase in the film thickness, connectivity between

grains keeps on increasing, resulting in a further decrease

in resisitivity. After a film thickness of 1.5nm, sheet

resistance exhibits only a slow linear variation. This may

be due to the fact that, once a continuous film is formed,

thickness dependence of resistivity comes because of

finite surface scattering of the conduction electrons; for

film thickness less that the electron mean free path in the

bulk Co, surface/interface scattering contributes

significantly to the resistivity. However, with increasing

film thickness this contribution decreases, giving rise to a

slow decrease in resistivity. Development of a finite

coercivity at a thickness of 0.7nm at which the

percolating cluster first forms, suggests that below this

thickness clusters are superparamagnetic, resulting in

zero coercivity. As the cluster size increases with

increasing film thickness, superparamagnetism is

suppressed and the coercivity increases. Coercivity

reaches its maximum value at a thickness at which a

continuous film is formed. Beyond this thickness

Figure 6: Variation of sheet resistance (R ) and Co

coercivity (Hc) as a function of Co film thickness.

Figure 7: Polar plots of azimuthal angle dependence of

coercivity (H ) at three thicknesses of Co film. Corresponding c

0hysteresis loops along easy direction (φ = 0 ) and hard direction 0 (φ = 90 ) are also plotted.

20

ripple pattern with pronounced amplitude was created on

Si (100) substrate by low energy (500 eV) Ar ion beam

erosion (figure 8a). Ripples are formed with their wave

vector along the direction of the bombarding ions. Co

thin films were deposited on this rippled substrate by

electron beam evaporation and the magnetic properties

were investigated by in-situ MOKE.

The polar graph shown in figure 8b represents the

variation of coercivity with respect to the azimuthal

rotation of a 18nm thick film. It is clear from the polar

graph that the Co ultrathin film shows a strong uniaxial

magnetic anisotropy with the hard axis along the wave

vector orientation of the ripples. This anisotropy may be

attributed to the magneto-elastic effects due to the

anisotropic tensile or compressive stresses developed at

the edges of the ripples during the deposition of the film.

The magnetic anisotropy decreases with increasing film

thickness. However, it persists even after a thickness of

120nm, though the amplitude of the ripples on the Si(100)

substrate is only a few nanometers.

Temperature dependence of magnetization in thin

film of FeCuNbSiB alloy

Temperature dependent magnetic behaviour of thin films

of well known soft magnetic alloy Fe Cu Nb Si B 73.5 1 3 16.5 6

(FINEMET) has also been studied in-situ. Thin film of 41

nm thickness on float glass substrate was produced by in-

situ sputtering a target made of the ribbons of FINEMET

alloy of above composition. Measurements as a function

of temperature (upto 723K) have been done in order to

investigate temperature dependent magnetic properties

of the film. Sample temperature was gradually increased

with 1ºC in 5 sec and continuous MOKE measurements

with magnetic field applied along easy axis were done

during heating of the film.

some specific film thicknesses. Figure 7 gives azimuthal

angle dependence of H as obtained from the hysteresis c

loop measurements at three representative points C , C 1 2

and C which corresponds to 1.0 nm, 1.6 nm and 2.8 nm 3

thickness of Co film respectively. One can see that up to a

thickness of 1.6 nm, the Co film is magnetically isotropic

in nature. However, at a thickness of 2.8 nm, a significant

uniaxial magnetic anisotropy developes. Structural

studies done using x-ray diffraction (XRD) and atomic

force microscopy (AFM) suggest that the origin of

magnetic anisotropy in polycrystalline Co films could be

understood in term of stresses, which are generated when

the islands coalesce to form continuous film.

Magnetic anisotropy in Co films on nano-patterned Si

(100) substrate

Towards achieving higher densities of magnetic

recording, a major challenge is to tune the magnetic

anisotropy in thin films in magnitude as well as in

direction. A nano-patterned substrate can modulate the

morphological and magnetic properties of the magnetic

film deposited on it. The correlation between the

nanoscale ripple patterns on the Si (100) substrate and the

magnetic properties of Co thin film deposited on it has

been investigated using in-situ MOKE. A well ordered

Figure 8: (a) AFM image of the ion bombarded Si surface.

(b) Azimuthal angle dependence of coercivity of 18.0 nm

thick Co film deposited on rippled Si. Zero of the azimuthal

angle is perpendicular to the ripple wave vector.

250 nm

(a)

(b)

Figure 9: Variation of coercivity along easy axix

of a FINEMET film as a function of temperature.

250 nm

21

material under study. The fact that it is a non-contact

technique, makes it possible to study magnetic properties

of thin films in-situ inside a UHV deposition chamber.

In-situ measurements allow one to study the properties of

ultra-thin films without any protective overlayer which

can modify their magnetic properties. Evolution of the

magnetic properties of a polycrystalline cobalt film with

thickness has been studied using such a set-up.

Simultaneous measurement of the resistivity of the film

allows one to correlate the evolution of morphology with

that of the magnetic properties. As another example,

temperature dependent magnetic properties of an

amorphous FeCuNbSiB film have been studied. An

interplay between structural and magnetic phase

transitions lead to a complex temperature dependence of

the coercivity of the film.

References:

1. Kerr J., Philos. Mag. J. Sci. 5, 321(1877).

2. Azzam R. M. A. and Bashra N. M.,”Ellipsometry

and Polarized Light”, NorthHolland, Amsterdam

(1977).

3. Hulme H.R., Proc. Roy. Soc. A 135, 237(1932);

Kittel C., Phys. Rev. 83, 208(1951); Argyres P. N.,

Phys. Rev. 97, 334(1955).

4. Heinrich B. and Cochran J. F., Adv. Phys. 42,

523(1993); Moog E. R. and Bader S. D.,

Superlattices Microstruct. 1, 543(1985).

5. Purcell S. T., Folkerts W., Johnson M. T., McGee N.

W. E., Jager K., Stegge J. Aan de, Zeper W. P., and

Gruenberg P., Phys. Rev. Lett. 67, 907(1991).

6. Qiu Z. Q., Pearson J., and Bader S. D., Phys. Rev. B

49, 8797(1994); Schumann F. O., Buckley M. E.,

and Bland J. A. C., ibid. 50, 16424(1994).

7. Qiu Z. Q., Pearson J., and Bader S. D., Phys. Rev.

Lett. 70, 1006 (1993); A.Berger and H. Hopster,

ibid. 76, 1996(1996).

8. You C.Y. and Shin S.C., J.Magn. Magn. Matter.

573, 198(1999).

9. Reininger T., Hafmann B., kronmuller H., J Magn.

Magn. Mater. 111 (1992) 220.

10. Franco V., Conde C. F., Conde A., and Kiss L. F.,

Phys. Rev. B 72, (2005) 174424.

Figure 9 gives coercivity of the film along easy axis as a

function of temperature. It is found that in as prepared

state coercivity of the film is about 20 Oe, which is about

2 orders higher than that of FINEMET ribbons [9]. It may

be noted that initially coercivity shows a decrease with

temperature upto ~665K. Around 670 K, loop disappears

and again appears at around 725 K. With further increase

in the temperature, it exhibits a sharp increase in the

coercivity. In order to elucidate the structural changes

occurring with temperature and to correlate the same with

observed changes in the magnetic properties, ex-situ x-

ray diffraction measurements were also done on a

separate set of as- deposited films of the same thickness,

after annealing at 598 K, 673K and 723K temperatures

(results not shown). It is found that up to 598 K the film

remains amorphous. After annealing at 673K, film

consists of a mixture of nanograins of 8nm diameter

embedded in the amorphous matrix. . At 723K the volume

fraction as well as the size of nanograins increases

substantially. Thus, one can conclude that, the initial

decrease in coercivity can be attributed to structural

relaxation as well as relaxation of internal stresses in the

amorphous phase itself. In the temperature 663 to 723K

the hysteresis loop is not observed because the

temperature is above the curie temperature of amorphous

phase. Although Curie temperature of nanograins is

higher, they exhibit super-paramagnetic relaxation

because of small size. With increasing volume fraction

and size of nanograins, magnetic interaction among them

increases and above 723K it results in ferromagnetic

ordering and reappearance of the hysteresis loop. Above

723 K the amorphous phase is in paramagnetic state and

the coupling between nanograins takes place only via

weak dipolar interaction. This is the reason for sharp

increase in the coercivity with temperature [10].

Conclusions

In conclusion, it is shown that MOKE is a powerful

technique for the study of thin film magnetism, and has

got several advantages as compare to other techniques

such as SQUID and VSM. It is not only a fast method but

also a very sensitive one that can compete with the best

SQUID, especially to study the magnetism of ultra-thin

films. It can detect the magnetization even in a fraction of

a monolayer thick film of ferromagnetic material. The

penetration depth of the laser beam in the sample lies in

the range 20-30 nm, which allows a good sampling of the

22

linewidth of the ‘dark’ resonance can be made ultra-

narrow, in the range of sub-KHz, by controlling the laser-

atom interaction parameters. Such ultra-narrow

linewidth spectral features provide convenient basis for

time and frequency standard and they are at the origin of

the exciting applications mentioned above [1-10]. The

phenomenon of EIT is accompanied by a sharp change in

the dispersion of the medium and these changes are

fundamental for the studies of sub-luminal and super-

luminal light propagation. The article is organized as

follows: In Sec.2 we discuss the coherent laser-atom

interaction, which forms the basis for the understanding

of EIT. In Sec.3 a representative, but inexpensive,

experimental configuration for laboratory realization of

EIT is presented. Typical experimental results pertaining

to EIT are presented in Sec.4. As an application of EIT,

we discuss the development of a compact precision

atomic clock in Sec.5.

Coherent Laser-Atom Interaction

EIT is a quantum mechanical phenomenon, which can be

explained by quantum-mechanical description of atom-

field interaction [1]. The core physics issues here are the

atomic coherence and quantum interference, and their use

for achieving control of the interaction between atomic

sample and electromagnetic fields. In order to understand

these issues succinctly let us consider a two level system

(see Fig.1) consisting of ground level |a⟩ and excited level

|b⟩ and excited by a near-resonant coherent field of

frequency ω.

Abstract

Electromagnetically induced transparency (EIT) is a

phenomenon that originates from the quantum

interference between excitation pathways in a multi-level

atomic system that is driven by two or more coherent

fields. The phase coherent atomic ensemble generated in

such interaction process provides a basis of new

generation of precision spectroscopic measurement

techniques, which afford sub-natural frequency

resolution. We discuss here the basic physics underlying

EIT, its realization with simple experimental set-up and

finally its use as a time and frequency standard.

Introduction

Precision spectroscopy in a coherently prepared atomic

medium is one of the frontline areas in contemporary

physics encompassing a wide spectrum of problems, both

very basic and very applied [1-10]. It is now well

established that coherent preparation leads to remarkable

changes in the optical properties of the medium and these

manifest in the observation of several new phenomena

such as electromagnetically induced transparency (EIT),

coherent population trapping (CPT) and lasing without

population inversion (LWPI) [1]. These effects originate

from the phenomenon of quantum interference between

excitation pathways in a multilevel atomic system, which

is coherently driven by two or more electromagnetic

fields.

In this article we focus on EIT, which is an important

coherent phenomenon utilized for spectroscopy with sub-

natural resolution and for cutting edge technologies like

precision atomic clock and ultra-sensitive magnetometer

[2-4]. EIT is a quantum optical phenomenon based on the

destructive interference between the competing

excitation pathways in a three level atomic system [1].

The destructive interference makes the medium

transparent to one of the radiation field interacting with

the atomic medium, which is otherwise absorbing for the

same field. The result is generation of a ‘dark’ or non-

absorptive resonance in contrast to the ‘bright’ or

absorptive resonance of standard spectroscopy. The

Spectroscopy in Coherently Prepared Atomic Medium and its Applications

Y.B. Kale, Niharika Singh, Ayan Ray and B.N. JagatapLaser & Plasma Technology Division,

Bhabha Atomic Research Centre, Mumbai 400 085, India

Fig.1: Two-level atom coherently driven by a laser field of

frequency ω and Rabi frequency Ω. The frequency detuning

from the atomic resonance ∆. The dressed states are

represented by |S ⟩.1,2

23

For simplicity we consider here the Λ system (see Fig.2a

and Fig.3a) to discuss the EIT phenomenon. In this

system the transition |a⟩ ↔ |c⟩ is electric dipole forbidden

but |a⟩ ↔ |b⟩ and |c⟩ ↔ |b⟩ are dipole allowed. In the

absence of pump beam the probe laser scans through the

absorption profile and that results in the usual Doppler

broadened profile. However the presence of pump beam

in the atomic medium manipulates the probe absorption

spectrum. The pump beam dresses the transition |a⟩ ↔ |b⟩ to give rise to the dressed states |S ⟩. When probed from 1,2

the level |c⟩, these two dressed states give rise to a pair of

peaks on the absorption spectrum, which is the usual

Autler-Townes doublet. The generalized coherence

induced by pump field can be described in terms of the

induced polarization for the probe absorption, ρ, which cb

has a functional form [5]:

(2)

We may assume here γ = γ for the sake of simplicity and ba bc

further define natural linewidth Γ= γ + γ . Note here is ba bc

the non-radiative incoherent decay for the transition |a⟩ → |c⟩, which needs to be considered for the complete

description of the system. Generally the incoherent decay

γ is governed by transit time broadening in coherent ac

pump-probe spectroscopy and for alkali atoms in a vapor

cell at room temperature, typical values of γ ~ .001Γ.ac

The above description is valid for a stationary atom and

we need to generalize Eq.(2) for atoms with a thermal

velocity distribution in a vapor cell. An atom with

velocity sees the pump and probe Doppler shifted, i.e.,

ω (1±v/c) and ω (1±v/c) where ± signs apply for the pu pr

counter- and co-propagating and co-propagating lasers.

Now using the Doppler shifted detuning ∆ and ∆ in Eq. pu pr

The semi-classical Hamiltonian of this system may be

written as H = H + H where is the Hamiltonian of the 0 I

.bare atom (in the absence of the field) and H =-d E is the 0

interaction Hamiltonian with d as the atomic dipole

moment and E as the electric field associated with the

radiation field. The states |a⟩ and |b⟩ are the eigenstates of

H and they are called the bare atomic states. The 0

eigenstates |S⟩ of H are called as the dressed states, which

are the superposition of the bare atomic states. For a two-

level system, the energies of the dressed states |S ⟩ are 1,2

given by

where ∆ = ω - ω is the frequency detuning, ω is the ab ab

frequency of transition |a⟩ → |b⟩ and is the

Rabi frequency. These dressed states are represented in

Fig.1. In coherent spectroscopy, we observe the

signatures of the dressed states rather than those of the

bare atomic states. In order to look for the existence of

these dressed states we require a third level and a probe

laser field. This calls for a three-level atomic system

excited by a bi-chromatic radiation field. Such three level

systems provide basic configurations for the observation

of EIT.

There exist a variety of three-level atomic systems

depending on the way the excitation is organized. They

are referred as lambda (Λ), Vee (V) and ladder (Ξ)

configurations as illustrated in Fig.2. Such level

configurations can be conveniently constructed using the

hyperfine level manifold of D or D transitions of alkali 1 2

atoms. In Fig.3 we show examples of Λ and V excitation 133schemes in Cs atom. Note here that the wavelength of

the transition 6s → 6p is ~ 852 nm and consequently it 1/2 3/2

can be accessed by the single mode tunable external

cavity diode laser (ECDL).

H0

Fig.2: Level scheme presentation of (a) Λ (b) V and (c) Ξ systems. Here ∆ ( ) and ( ) are respectively the pu

detuning and Rabi frequency of the pump (probe) laser field

from the corresponding atomic resonance and γ are the ij

radiative decays corresponding to the transitions | i⟩ → | j⟩.

∆ ΩΩpr pu pr

Fig. 3: Energy level scheme for (a) Λ and (b) V systems in 133Cs D transition. The bracketed entries represent the 2

separation between the adjacent hyperfine levels in MHz.

24

limited by damping mechanisms present in the system

e.g. spontaneous decay (Γ) and incoherent decay (γ) ac

between ground states. Intuitively one may conclude that

the optical pumping must be strong enough to

compensate for those loss channels. The optical pumping 2is defined as Ω / Γ [7] and it should be strong enough to pu

overcome incoherent decay, i.e.,

(4)

A detailed analysis by Javan el al [7] has shown that the

linewidth of EIT (Γ) is given byEIT

(5)

where and and D is the Doppler

width. Eq. (5) has an important consequence to follow: at

low pump intensity Γ is proportional to Ω. However EIT

2At higher pump power it varies as Ω indicating the pu

power broadening of the EIT signal.

It is to be noted here that the EIT linewidth observed

under V and Ξ systems is comparatively broader than that

of Λ system. This is due to the higher coherence

dephasing rate present in V and Ξ systems [8]. The

coherence dephasing rate is a function of spontaneous

decay terms and is zero for an ideal Λ system whereas for

V system the dephasing is highest. The coherence

dephasing actually indicates leakage of atoms from the

optically pumped three level scheme and introduces

decoherence in the closed system EIT picture.

3. Experimental Configuration

The functional block diagram of a typical experimental

arrangement for EIT demonstration is shown in Fig.5.

Two external cavity diode lasers, ECDL1 and ECDL2,

are used as the probe and pump lasers and sent co-

propagating through a room temperature Cs vapor cell of

~ 25 mm length. The probe and pump laser beams are

linearly polarized. Both lasers have ~1 MHz short-term

linewidth observed over a period of ~ 100 ms. The probe

beam power is kept at 200µW and the pump beam power

is held at a nominal value of 1-8 mW for the EIT

experiment. About 4% of the main probe beam power is

used in a SAS set up with a room temperature vapor cell

of 50 mm length. Doppler-free resonances thus obtained

serve as frequency markers. In a similar manner SAS

spectrum for the pump laser is also obtained. The pump

pu

(2) and integrating over the Maxwell-Boltzmann velocity

distribution, we obtain the average polarization, . The

probe absorption is related to the imaginary part of .

The probe absorption spectrum under pump-induced

coherence consists of AT doublet structure for a

stationary atom, while for an inhomogenously broadened

medium a transparency window appears at ∆ = ∆. pu pu

Under this condition the probe laser passes through the

medium without any absorption. The sudden increase in

transparency of the medium gives rise to a sharp signal,

which in essence is EIT signal. In Fig.4, we show this

situation simulated for a typical set of laser-atom

interaction parameters.

Physically the EIT phenomenon can be described in

terms of the absorption of the probe laser over the dressed

states. The principal effect of the pump beam is to

introduce an ac-Stark splitting [6] of the excited atomic

state. The probe beam thus couples |c⟩ to these two

dressed states |S ⟩ instead of the single excited state |b⟩. 1,2

If the splitting of the dressed states is comparable to the

excited state width, then one can expect interference in

the amplitudes of the transitions |c⟩ → |S ⟩. A destructive 1,2

interference of the transition amplitudes gives rise to

cancellation of absorption, which in essence is EIT.

The most important property of the EIT signal, which

makes it a good choice as frequency reference, is its ultra-

narrow linewidth. The linewidth of the EIT signal is

Fig. 4: Simulated probe absorption spectrum, i.e. intensity vs probe laser detuning (∆ ), for a three-level Λ system of Fig.3a. pr

The data used is Ω = 40 MHz, ∆ =0 MHz and Ω =0.1 MHz. The pu pu pr

dotted curve shows the Autler-Townes doublet for a stationary atom. The intensity scale for this curve is provided by the y-axis on the left. The solid curve is a result of Doppler averaging of Cs vapor corresponding to the Doppler width of 378 MHz. The Intensity scale for this curve is given by the y-axis on the right. The sharp transparency at ∆ = ∆ is a result of EIT.pr pu

25

(cf. Eq. 5) and make sure that we are operating in x<1

region. Here Γ ∝ Ω is still valid as our observed EIT pu

linewidth is quite close to the theoretical value of ~1.6

MHz (cf. Eq.5).

Investigation of pump laser parameter dependence on

such ultra-narrow EIT signal under Λ level scheme is

difficult to investigate. This is because the small change

in detuning and intensity introduce obscurity in EIT

signal as neighboring AT components start encroaching

in the narrow frequency window (Γ). In order to bring EIT

the salient features of EIT under variation of pump

laser is locked on the appropriate hyperfine transition

with 6S F = 4 → 6P F' = 3,4,5 manifold by using 1/2 3/2

frequency modulation spectroscopy (FMS). The fixed

frequency of the pump laser, as locked above is further

shifted appropriately using an acousto-optic modulator

(AOM) so that the effect of ∆ on dressed levels can be pu

studied in details.

In the discussion that follows we consider two level

configurations, namely, Λ and V, in Cs D transition. We 2

use Λ configuration to demonstrate sub-natural linewidth

EIT. We then use the V configuration to draw attention to

the general features of EIT and its dependence on laser-

atom interaction parameters. We may mention here that

the V configuration results in a broad EIT peak due to

high coherent dephasing rate. Unlike EIT under Λ

configuration, which easily gets smeared out with small

fluctuations in pump laser, the EIT in V system is more

robust. Finally we add here that the EIT in V system is not

pure and it contains the effect of increased probe

absorption due to pump field saturation.

4. Results and Discussion

Fig. 6 shows the EIT signal observed in Λ system of Cs D 2

transition. The pump laser frequency is resonant with

6S F = 4 → 6P F' = 4 hyperfine transition while the 1/2 3/2

probe laser scans over 6S F = 3 → 6P F' = 2,3,4. The 1/2 3/2

sub-natural linewidth EIT signal appears at the position

of 6S F = 3 → 6P F' = 4. Inset of Fig. 6 shows 1/2 3/2

Lorentzian fit of the EIT signal and fitted FWHM is 2.9

MHz, which is well below the natural linewidth Γ(=5.3

MHz).

For the experiment of Fig.6, we calculate the value of x

Fig.5: Experimental block diagram of coherent pump-probe spectroscopy. ECDL1 and ECDL2 are probe and pump lasers respectively. SAS: saturation absorption set up, AOM: acousto optic modulator, PD :Photodiode:

Fig. 6: Simultaneous recording of (a) EIT under Λ system and (b) SAS of the probe laser. In SAS various resonances are indicated as (i): F=3→F'=2, (ii): crossover, (iii): , (iv): crossover and (v):F=3→F'=4. The pump laser is resonant with F=4→F'=4 hyperfine component. Lorentzian fitting of EIT signal (inset) shows FWHM ~ 2.9 MHz. Here Ω and Ω pu pr

are 25 and 5 MHz.

F=3→F'=3

Fig.7: EIT spectrum (a) under V system with SAS background. In SAS various resonances are indicated as (i):F=4→F'=5, (ii): crossover, (iii): F=4→F'=4, (iv): crossover and (v): F=4→F'=3. The pump laser is resonant with F=4→F'=4 transition For (a) pump and probe powers are 8 mW and 200 µW respectively. The EIT linewidth is 18 MHz under V system.

26

EIT in Atomic Clock

The ultra-narrow ‘dark’ resonance generated in

coherently prepared Cs/Rb atomic vapour provides an

excellent basis for an atomic clock. Note here that one

second is defined as the duration of 9,192,631,770 cycles

of microwave absorbed or emitted by the ground 133hyperfine transition of Cs (cf. Fig.3). Now consider two

laser beams whose frequency difference (∆ω) matches L

with the frequency separation of the ground hyperfine

levels (∆ω). Such a bi-chromatic field is used to drive a Λ a

configuration in Cs atoms (cf. Fig.3a). The EIT signal will

be maximized when ∆ω=∆ω. This principle is used to L a

construct a compact atomic clock based on the ‘dark’

resonance. The Λ configuration is further preferred since

it provides an ultra-narrow EIT resonance as discussed

earlier. In what follows we discuss the ideas underlying

the ‘dark’ resonance based atomic clock.

In the presence of longitudinal magnetic field, the

degeneracy of the hyperfine levels is lifted. These

magnetic states (m ) together with a circularly polarized F

bi-chromatic field can split ultra-narrow EIT resonance

into (2m +1) components depending on the satisfaction F

of Λ condition for the magnetic transitions. These signals

with line widths of few tens of Hz can be used as an ultra-

precision frequency reference. However it is to be noted 'that the Zeeman component m =0→m =0 does not suffer F F

shift due to the magnetic field and that provides an ideal

choice as frequency discriminator for stabilizing a laser.

Fig. 10 shows the functional block diagram of an EIT

based atomic clock. Here the output of a vertical cavity

parameters, we take recourse to EIT in V system in the

discussion that follows. The broad EIT spectrum in V

system is preserved over a certain range of detuning and

intensity variations of pump laser. Fig. 7 shows the result

of V system EIT where pump laser is resonant with

6S F=4→6P F'=4 transition. The EIT signal appears at 1/2 3/2

the position of hyperfine transition.

The effects of pump power variation and pump frequency

detuning are shown in Figs. 8-9. Fig.8 demonstrates that

with increase in pump power EIT signal becomes broad

due to power broadening.

Fig. 9 exhibits that the narrowest (~ 18 MHz) EIT signal

[9] is obtained when the pump laser is at exact resonance

with 6S F=4→6P F'=4 hyperfine transition. This signa1/2 3/2

much broader compared to that observed with Λ system

for the reasons discussed earlier.

l is

Fig. 8: Pump power dependence of EIT signal under V system. Probe power is 200 µW and pump power is varied as (a) 40 µW, (b) 1.6 mW, (c) 2.5 mW.

Fig. 9: Pump frequency dependence of probe transmission signal for V system. Spectra (a)-(b) show pump frequency detuning from 6S F=4→6P F'=5 transition by: (a) 0 MHz 1/2 3/2

and (b) 125 MHz. The pump power is 8 mW and probe power is 200 µW.

Fig 10: Schematic block diagram of the all optical atomic clock based on ultra-narrow resonance generated from EIT/CPT. 10 MHz clock signal is obtained from OCXO, which is in phase lock with radio frequency generator.

27

theoretical and experimental activity directed towards the

understanding of EIT in four-level atomic systems [10].

The ‘dark’ resonance associated with such a three-photon

absorption process is characterized by ultra-narrow

linewidth, high contrast, leading-order light shift

cancellation and less sensitivity to the environmental

effects. These features are of particular importance for

atomic frequency standards. Also interesting is the

phenomenon of electromagnetically induced absorption

(EIA), which arises from the constructive quantum

interference in contrast to the destructive interference in

the EIT phenomena. In summary the physics and

technology associated with EIT and related phenomena

will continue to keep the scientists and technologists

occupied in the years to come.

References:

1. Marangos J.P., J. Mod. Opt., 45, 471 (1998).

2. Kilin S. Ya., Kapale K.T., and Scully M. O., Phys.

Rev. Lett., 100, 173601 (2008).

3. Knappe S., Shah V., Schwindt P.D., Hollberg L.,

Kitching J., Liew L., and Moreland J., Appl. Phys.

Lett., 85, 1460, 2004.

4. Stähler M., Knappe S., Affolderbach C., Kemp W.

and Wynands R., Euro. Phys. Lett., 54, 323, 2001.

5. Vemuri G., Agarwal G. S., and Rao B. D.

Nageswara, Phys. Rev. A, 53, 2842, (1996).

6. Foot C.J., Atomic Physics, Oxford Master Series,

Oxford Univ. Press (2005).

7. Javan A., Korcharovskaya O., Lee H. and Scully

M.O., Phys. Rev. A, 66, 013805 (2002)

8. Fullton D.J., Shepherd S., Moseley R.R., Sinclair

B.D. and Dunn M.H., Phys. Rev. A, 52, 2302

(1995).

9. Ray A., Pradhan S., Manohar K.G. and Jagatap

B.N., Laser Phys., 17, 1353 (2007).

10. Champenois C., Morigi G. and Eschner J., Phys.

Rev. A 74, 053404 (2006).

surface emitting laser (VCSEL) is modulated at a

frequency equal to the difference between ground level

hyperfine transitions of alkali atom like Rb or Cs to

generate the required sidebands and sent through an

alkali vapor cell.

A solenoid, which generates small magnetic field (~ few

mG) in the longitudinal direction, is wrapped around the

vapor cell. When the bi-chromatic field from the VCSEL

passes through the vapor cell the light fields interact with

Zeeman sublevels and give rise to ultra-narrow

components of EIT signal. A low frequency modulation

(~ kHz) is applied to the drive current of the VCSEL to

facilitate frequency modulation of laser and subsequent

harmonic detection of Zeeman components of EIT signal.

This helps in locking the laser wavelength to the line 'centre of a Zeeman component (m =0→m =0). An F F

additional low frequency modulation (~ few hundred Hz)

is used to modulate the VCO carrier frequency and

facilitate similar phase sensitive detection of absorption

profile. A control loop is used to stabilize the VCO to

ground state hyperfine energy gap of alkali atoms. The

trigger pulse signal coming out of a crystal oscillator,

which is in phase lock with the VCO, is used as a time

standard.

Atomic clocks working on ‘dark’ resonance have been

demonstrated in very recent years. These clocks are

small in size and portable, and therefore expected to be

extremely useful in synchronization of communication

networks and in improved military applications of global

positioning systems.

Conclusion

We have discussed here the theoretical and experimental

ideas underlying EIT in an inhomogenously broadened

atomic medium. The physical mechanism of EIT is

addressed from the point of view of coherence in a three-

level system. We have presented here experimental

results on the observation of EIT with sub-natural

linewidth and the dependence of EIT on the laser-atom

interaction parameters. As an application we have

discussed qualitatively ‘dark’ resonance based atomic

clock. In recent years there have been increasing

28

2. Ultrafast Pump-Probe Transient Absorption Spectrometer

Most of the fundamental photophysical and photochemical processes, such as photodissociation, photoisomerization or configurational relaxation, transfer of electrons and protons are the dynamical processes involving the mechanical motion of electrons and/ or atomic nuclei. Considering the speed of atomic motion is ~1 km/s, to record atomic-scale dynamics over a distance of an Angström, the average time required is ~ 100 fs. Hence, using kinetic spectrometers having a time-

-15resolution of a few tens of a femtosecond (10 s), we can freeze the molecular structure far from equilibrium and follow the atomic motions in a molecule in real time.

Figure 2.1 : Pump - probe spectroscopy

The technique for studying the time-resolved spectroscopic behavior of the photoinduced chemical processes was first developed by Nobel laurates, Norrish and Porter known as ‘flash photolysis’. Subsequently, there was a quick development of this technique with rapid developments of ultrafast lasers and associated optoelctronics. The principle of this ‘pump-probe technique’. is schematically presented in Fig. 2.1 and the present experimental set up for femtosecond transient absorption spectrometer working in RPCD, BARC in Fig. 2.2. In this technique, the excitation (pump) and the probe pulses are generated from the same laser to ensure perfect synchronization between the two pulses without

1. Introduction

In the interactions of high-energy incident particles i.e., photons, electrons, heavy charged particles, neutrons and other particles with matter, the succession of events that follow the absorption of their energies has been classified into three characteristic temporal stages: physical, physico-chemical and chemical. In principle there is no distinction between physical and chemical effects in nature, however, it is profitable to view the radiation effect depending on the investigator’s choice of discipline. Radiation physics or radiation chemistry are designated as viewing the radiation effect either from the point of view of the incident particle or from that of the interacting medium respectively. Thus the study of the charge, rate of energy loss, range and penetration etc. of the incident particle, any of them may change with interaction, constitutes the radiation physics. Investigation of the matter interacting with the radiation to produce chemical changes, charge separation, luminescence etc. are the subject of radiation chemistry. It is evident that radiation chemistry is the link between radiation physics and radiation biology. The above three stages in ascending orders of time ( pt = – log t (s) = 18 – 0 ) can have additionally biochemical and biological stages at longer time scale. The time scale in radiobiology may exceed several years or generations if genetic effects are considered. Thus various disciplines connected with radiation action all begin with radiation physics and radiation chemistry. Based on the knowledge of these disciplines from basic research, on one hand it is applied to engineering and industrial applications while on the other it caters to the radiation biology and medical applications.

We at Radiation & Photochemistry Division [1.1] of Bhabha Atomic Research Centre are pursuing investigations in different thrust areas of these disciplines. However, the past twenty years has seen an explosion of interest because of their pivotal role in both chemistry and biology. In the present article, I would like to describe some of the fast and ultrafast laser based facilities developed in our lab over the years and shares some of the excitements from such studies.

Advances in Laser Photochemical Research at Radiation & Photochemistry Division (RPCD), BARC

Sisir K. SarkarRadiation & Photochemistry Division,

Bhabha Atomic Research Centre, Mumbai-400 085, INDIA

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Dynamics of chemical bond-breaking process:

Photochemists have dreamt for a long time to break a particular bond of a molecule selectively by using laser.

Here, we present our investigation of the laser induced bond-breaking process and the photodissociation dynamics of mercuric iodide (HgI ) in ethanol solution 2

[2.4]. Steady state spectroscopic studies reported earlier revealed that when HgI is excited by a photon of 330 nm 2

wavelength to one of its dissociative electronic excited state (HgI *, the electronically excited molecule), the 2

molecule is dissociated into two fragments, mercurus #, iodide (HgI vibrationally hot molecule in the ground

electronic state) and iodine atom (I) (equation 3). In this #process, the HgI fragment is produced in the ground

2 +electronic state (X Σ), but with a large amount of excess vibrational energy. The iodine atom is also produced in the ground electronic state. Using the ultrafast transient absorption technique, the dynamics of the bond breaking and energy flow process could be followed-up in the photodissociation reaction of HgI in ethanol solution.2

(1)

HgI , being a tri-atomic linear molecule, it has four 2

fundamental modes - one symmetric stretch, one asymmetric stretch and two degenerate bending modes

-1corresponding to the frequencies 155, 237 and 33 cm , respectively. Symmetric stretch and bending modes are bound motions but asymmetric stretch can provide the dissociative motion. This mode has the fundamental

-1frequency, ν = 237 cm and its time period of oscillation 0

is 141 fs.

On excitation of HgI molecule onto one of the excited 2

electronic dissociative states, the HgI * system evolves 2

along the dissociative potential energy surface (Fig. 2.3) This motion can be described by the separation of the

any time-jitter. When a part of the pump pulse is focussed onto water or glass plates, the intense peak power can generate a ‘continuum’ light of same pulse duration having a broad spectrum covering the entire visible and near IR region (400 – 1000 nm). This continuum pulse can be used for probing the absorption of the transient species created by the other part of the laser pulse. In a transient absorption experiment, the change in absorbance due to photoexcitation of the sample is measured in double-beam spectrophotometric method, by splitting the probe beam into two, one of which sees the excited sample and the other the unexcited one.

Fig.2.2 Laser system for tunable pump and tunable probe transient absorption spectroscopy.

Information regarding the excited state absorption (ESA), which occurs as positive absorption, or stimulated emission (SE) or bleaching, both of which are occurring as negative absorption, can be obtained by comparing the integrated intensities of the two probe pulses detected by integrating photodiodes (slow detectors) and boxcar averagers. The time-resolved measurements are performed using an optical delay rail. We at RPCD embarked in this area in 1990’s and built the first femtosecond spectrometer using a colliding pulse mode locked (CPM) laser [2.1]. Subsequently, this was replaced by Ti:S oscillator-amplifier laser system improving the time resolution and sensitivity of the technique [2.2-2.3]. We have two such spectrometers working in our lab addressing various ultrafat phenomena.

Currently, development of optical parametric oscillator (OPO) or amplifier (OPA) coupled with the difference frequency generator (DFG), the pump-probe transient absorption spectroscopy has also been extended to the infrared, which is a fingerprint region for the vibration of most of the molecular bonds. Hence the time-resolved infrared spectroscopy can also provide temporal as well as structural dynamics. Fig. 2.3: Schematic representation of the photodissociation

reaction of HgI in a dissociative excited electronic state and the 2

concept of the transition state in the photodissociation reaction.

30

separated from the residual fundamental laser light using a dichroic mirror and is used for the sample excitation, while the residual fundamental light is used as the gate pulse.

The fluorescence (ω) originating from the sample is fl

focused onto an up-conversion crystal (0.5 mm thick BBO crystal), using two elliptical mirrors. The residual

fundamental beam (gate pulse, ω) is first directed to an g

optical delay line and subsequently focused onto the up-conversion crystal to generate the sum-frequency light

(ω), which is detected by using a photon counting system s

1,2(cf. figure 3.1). The working principle of the upconversion instrument is shown in Figure 3.2, where the integrated sum frequency light at each delay of the gate pulse (shaded areas in figure 3.2B) is a measure of the fluorescence intensity at the respective delays (cf. figure 3.2C). Table 1 lists the key features of the present up-conversion instrument.

Ultrafast Electron Transfer (ET) Reaction Dynamics:

The dynamics of electron transfer (ET) reactions between coumarin acceptors and amine donors have been investigated in detail using the present fluorescence up-

3conversion instrument. In these experiments, the donor amines have been used directly as the solvents such that the acceptor coumarin molecules are always in contact with the donor amine molecules. Because of this contact nature of the donors and acceptors, the ET occurs very fast under non-diffusive condition

Figure 3.3 shows the fluorescence decay profile of coumarin-151 (C151) dye in dimethylaniline (DMAN) solvent. The fluorescence lifetime of C151 in DMAN is thus measured to be ~180 fs. Unlike in DMAN solvent, the fluorescence lifetime of C151 in nonpolar solvent like ethyl acetate, having the similar polarity as that of DMAN, is found to be quite long, ~2 ns. Thus, there is a

#HgI and I fragments due to stretching of one of the two identical Hg-I bonds due to asymmetric stretch motion. When the separation between the two fragments reaches to about 5Å, they become free from each other’s influence and the molecule is considered to be dissociated into two fragments. Different molecular

configurations of IHg I, which have different internuclear separations between the I and HgI fragments existing at different times between photoexcitation and dissociation, can be designated as the transition states. After creation of HgI * state on photoexcitation, it is 2

possible to probe the transition states as well as the #product state, HgI , using probe light of appropriate

colour.

3. Femtosecond Fluorescence Up-Conversion Facility

The block diagram and the photograph of the femtosecond fluorescence up-conversion instrument developed in RPCD, BARC, are shown in Figure 4. In this instrument the laser output from a Ti:sapphire oscillator (~800 nm) is first passed through a harmonic

nd rdgenerator unit, where either the 2 (~400 nm) or the 3 harmonic (~266 nm) of the laser light is generated using suitable BBO crystals. Harmonic light thus generated is

Figure 3.1. The Schematic diagram (A) and the actual photograph (B) of the femtosecond fluorescence up-conversion instrument developed in RPCD, BARC.

Table-1: Key feature of the upconversion instrument developed in Radiation & Photochemistry Division.

Temporal resolution Upto 100 fs

Excitation wavelength 400±20 nm and 266±10 nm

Emission Spectral Range 300-750 nm

Time range few femtosecond to about 1.7 ns

Type of sample solution, gas phase, thinfilms and solid samples

Polarization Study Ultrafast fluorescenceanisotropy

31

as the donors the orientational restriction for the ultrafast ET reaction is unusually high compared to that with aromatic amine donors. Present results are rationalized on the basis of the nature of the highest occupied molecular orbitals (HOMO) of the amine donors involved. Due to the n-character of the HOMO of the aliphatic amines, even in the donor-acceptor close contact, a good fraction of the donor-acceptor pairs cannot have suitable electronic coupling and accordingly the coumarin-aliphatic amine systems show quite large

orientational restriction for ultrafast ET. With the π-character of the HOMO of the aromatic amines, almost all the donor-acceptor close contact pairs can give very high electronic coupling and hence display negligible orientational restriction for the ultrafast ET. Presently the instrument is in extensive use to investigate various fast and ultrafast chemical processes in condensed phase to understand the dynamics and mechanism involved in these processes.

4. Molecular Beam Resonance Enhanced Multiphoton Ionization Time-of-flight (MB-REMPI-TOF ) Spectrometer

Molecular Beam-Resonance Enhanced Multiphoton Ionization -Time-of-Flight (MB-REMPI-TOF) is a highly sophisticated analytical detection system [4.1], having advantages of three techniques embedded in its name. The main interest in the development of MB-REMPI-TOFMS system is to develop a highly sensitive and selective technique, complementary to Laser Induced Fluorescence technique, for investigating pho tod i s soc i a t i on dynamics . Rad i a t i on & Photochemistry Division has previously developed, and extensively employed, the Laser Photolysis-Laser induced Fluorescence (LP-LIF) technique for investigation of gas-phase reaction dynamics [4.2]. REMPI, being ionization based detection technique, is much more sensitive, versatile and can be applied to systems not amenable to LIF. The MB-REMPI-TOFMS system consists of differentially-pumped two vacuum chambers, expansion and ionization chambers. Fig. 4.1 is the picture of the assembled set up. In this setup, the molecular beam is generated by using a pulsed valve and a skimmer. The nozzle-skimmer assembly is mounted on the end-flange of the expansion chamber. Two optical ports are provided on the opposite sides of ionization chamber for accessing the molecular beam formed by the nozzle-skimmer assembly. The TOF mass spectrometer assembly is mounted vertically on the side arm. The two important parameters of the set-up namely, the resolution and the detection limit, have been determined by generating a molecular beam of aniline by seeding 1%

large decrease in the fluorescence lifetime of the dye in DMAN solvent, which is a manifestation of ultrafast ET in C151-DMAN system. In the present study, fluorescence decays have also been measured for C151-DMAN and other coumarin-amine systems, either in neat amine solutions or with very high amine concentrations, to understand how the orientation of the amines with respect to the coumarin dyes affect the ultrafast ET reactions. Our results suggest that with aliphatic amines

Figure 3.2. Working principle of fluorescence up-conversion instrument

Figure 3.3. Fluorescence transient decay of coumarin-151 dye in N,N-dimethylaniline solvent.

32

atmospheric chemistry, bromoform is one of them. The following are some of the important dissociation pathways of bromoform, based on energetics,

CHBr →CBr +HBr 245 kJ/mol, (1)3 2

→CHBr +Br 258 kJ/mol, (2)2

→CHBr+Br 349 kJ/mol, (3)2

→CBr+HBr+Br 530 kJ/mol. (4)

It is well known that in the photolysis of bromoform [4.4], Br atom elimination is a major channel and in reactions 2 and 4, Br atom may be formed in the ground state

5 2 5Br =(4p P ) or the spin-orbit excited state Br =(4p 3/2 3/2 1/2

2P ), hereafter, referred to as Br and Br*, respectively. 1/2

Hence , we measured Br /Br* ra t io in the photodissociation of bromoform at ~ 234 nm. The (2+1) REMPI transitions of Br and Br* atoms, in the wavelength region of 230-235 nm, are used to probe Br and Br* atoms. The laser pulses, in 230-235 nm range, were generated from a Quantel dye laser, TDL 90, using frequency-doubling and mixing module. In all the experiments, the same laser beam was employed as a

aniline in helium, and recording its REMPI-TOF spectrum at 293.77 nm. For the present configuration, using one metre long flight tube, the resolution has been found to be about 400, and detection limit is better than

6 310 species per cm . The system developed has provision for simultaneous LIF detection as well.

In a photodissociation experiment, the nozzle-skimmer assembly is utilized to reduce the population of the higher internal states of the dissociating molecules, we have prepared the supersonic molecular beam of chemical species of interest by hydrodynamic expansion of the molecules seeded in a buffer gas, such as helium or argon, through a nozzle, and skimming the free-jet thus formed. The generated molecular beam is intersected in the extraction region of a Wiley-McLaren type TOF mass spectrometer [4.3] by the photolysis-cum -probe laser beam. The ion signal was gate integrated by a boxcar, averaged for 30 laser shots, and fed into an interface (SRS 245) for A/D conversion.

Photodissociation Dynamics of halomethane:

We have carried out the photodissociation dynamics of various chloro and bromo hydrocarbons of relevance to

Fig. 4.1: A picture of the MB-REMPI-TOF setup.

Fig. 4.2 : Schematic of the setup

Fig.4.3: Typical TOF spectra of Br produced from photolysis of CHBr .3

Fig.4.4: The REMPI spectra of Br and Br* produced from CHBr .3

33

the visible region are Rhodamine 590, DCM and Coumarin 500. The dye solutions prepared in methanol are circulated using a dye circulator. The output energy from the dye laser is in the range of 10-50 mJ/pulse.

The sample is placed on the path of the laser beam in a

standard 1 × 1 cm cuvette. Raman scattering from the 0irradiated sample is collected at 90 to the laser beam by a

camera lens (f/1.4 from Nikon) which is then focused on to the spectrograph (Triax 550, Jobin Yvon, France). An edge filter is placed between the sample cell and the spectrograph to reject the intense laser light. An intensified charge coupled device (ICCD, Andor) is mounted at the exit port of the spectrograph to detect the scattered light. The detector is operated in gated mode and in synchronization with the laser beam in order to avoid spurious background signal. The spectrograph and ICCD are interfaced to the computer and the Raman scattered light is plotted using a plotter.

SERS Studies of CGA on Ag Surface:

For Surface-enhanced Raman scattering (SERS), colloidal silver was prepared by the reduction of silver nitrate with sodium citrate. SERS spectra of chlorogenic acid (CGA) were recorded at room temperature using the

3+532 nm line, from a Nd :YAG laser (Brilliant B). The laser power used to excite the Raman spectrum was 30

pump and a probe, i.e., for both photodissociation of the parent molecule and ionization of the photoproducts Br and Br* atoms. Fig. 4.3 shows a typical TOF spectrum of the Br product from CHBr . Two peaks in the TOF are due 3

to two isotopes of Br, at 79 and 81 mass units. The REMPI spectra was obtained by recording the integrated TOF signal (with a gate covering both m/z=79 and m/z=81 signal), as a function of the probe laser wavelength. The typical spectrum obtained is shown in Fig. 4.4. The spectrum matches very well with the literature reported values.

The relative quantum yields of Br and Br* were extracted from the relative integrated signal intensities in the TOF spectrum in Fig. 4.4. From the integrated areas for the

2two-photon transitions of Br* and Br, the Br* ( P )/Br 1/2

2( P ) ratio was estimated to be 1.4. In a purely statistical 3/2

process, the ratio should be the same as the ratio of the 2J+1 values, which is 0.5. The wide difference between the observed and the statistically estimated values can not be accounted for by the Boltzmann factor. Presently, the underlying interactions between the states in the exit channel are being investigated.

5. Time Resolved Resonance Raman (TR3) Equipment

Recently we have fabricated a Raman instrument for various Raman applications, viz. surface-enhanced Raman, resonance Raman and time-resolved resonance Raman scattering. In Fig. 5.1 is shown the Raman instrument that has been tested for surface-enhanced Raman scattering application. The Raman instrument involves an irradiation source to interact with the sample, collection optics to gather the scattered light, a spectrograph to disperse the scattered light and a detector to identify the scattered light. The irradiation source as

3+shown in Fig. 5.1 is a Nd :YAG (Brilliant B) pumped tunable dye laser (TDL-90 system from Quantel, France).

3+The Nd :YAG laser has a repetition rate of 13 Hz with pulse energy 750 mJ/pulse at the fundamental wavelength i.e. 1064 nm, 350 mJ/pulse at the second harmonic, 532 nm and 145 mJ/pulse at the third harmonic output, i.e. 355 nm. The tunable dye laser is pumped by the second and third harmonics, i.e. 532 and 355 nm of the Brilliant B. The dye laser consists of an Oscillator, which generates the lasing, the pre-amplifier and the amplifier, through which the amplified output laser energy is generated. The dye laser is tunable in the range of 420-750 nm and the laser lines can be separated using a grating with 2400 l/mm. The dyes used for the lasing in

Fig. 5.1: Instrumentation for Raman applications

34

mW. The Raman scattered light as mentioned earlier was

collected at the 90° geometry and detected using a Triax 500 spectrograph and an ICCD together with an edge

-1filter, covering a spectral range of 150-1700 cm .

SERS is a phenomenon resulting in strongly enhanced Raman signals from molecules that have been adsorbed over nanometer-sized metallic colloids/films.[2.1-2.3] An important aspect of SERS is its potential for probing the interaction between various adsorbates and metallic surfaces. SERS has become an increasingly popular technique not only for studying the molecules at trace concentrations but also in estimating their possible

4-8orientations on the metal surfaces. SERS of CGA has been investigated using our Raman instrument. CGA is an important plant metabolite with anti-viral and anti-bacterial properties and thus, it is useful to study its surface adsorption characteristics. The structure of CGA is shown in Fig. 5.2. The SERS spectra of CGA were recorded at different concentrations is shown in Fig. 5.3. Strong peaks are observed at 1212, 1363, 1489 and 1583

-1cm and are assigned to phenyl ring CH bend, CO 2

symmetric stretch, ring CO stretch and the phenyl ring stretch, respectively. From the enhanced bands observed in the SERS spectra it has been inferred that CGA is chemisorbed to the silver surface through the oxygen atoms of the carboxylate group.

6. Coherent Control of Chemical Reaction

Controlling the outcome of chemical reactions by

Fig. 5.2: Structure of Chlorogenic acid (CGA)

Fig. 6.1: Experimental Setup: The Stokes laser pulse of coherent anti-Stokes Raman scattering (CARS) process (bottom, left) is shaped by a phase modulator setup (bottom, right). The spectrum resulting from this experiment serves as feedback for the evolutionary algorithm (top, right), which controls the pulse shaper during the optimization.

-5 -4Fig. 5.3: SERS spectra at (a) 5×10 M, (b) 1×10 -4M and (c) 2×10 M concentrations of CGA

specifically tailored femtosecond laser pulses is a fascinating perspective, which has recently become experimentally feasible and recently, successful control experiments have been demonstrated the great versatility of this technique to selectively influence light matter interaction in complex systems. The concept of coherent control is to adjust the spectral and temporal characteristics of the excitation light to the molecular resonances and dynamics, such that these can be selectively addressed and manipulated. This has implications on enriching our understanding on mode selective chemistry and control of molecular dynamics. This technique has the potential to control chemical reactions and develop various exciting future applications.

The experimental realization of feedback controlled self learning loop requires a high power femtosecond laser system, frequency conversion units to have suitable wavelengths for the molecular system under investigation, a computer controlled pulse shaper, and an optimization algorithm based on evolutionary strategies. The wide spectra inherent to femtosecond laser pulses will be manipulated in the pulse shaper to generate an electric field profile, which adapts to the desired result. The search of the optimal field will be guided by an automated learning loop, which employs a direct feedback from the experimental output. The optimized pulse will provide physical insight into the control process. The feedback-controlled pulse shaping has been successfully applied to control the molecular dynamics in complex systems. The closed-loop process is shown in

35

the evolutionary algorithm to control the spectral phase of the Stokes pulse.

The upcoming facility will provide in-depth understanding of laser control of a number of physical and chemical processes. These include processes such as isomerization, fragmentation, photo-association, charge transfer, ionization, etc., which involve manipulation of nuclear as well as electron dynamics [6.2].

Summarizing the transition from observation science to control science requires a three-fold attack: development of instruments that are more precise and more flexible than those used for observation science, creation of theories and concepts beyond those we currently possess and finally new approach to training and funding.

Acknowledgements

I would like to thank my colleagues in RPCD upon whose work this article is written Support and funding from BARC, DAE for sustained activity in these areas are acknowledged.

References1.1 RPCD webpage http://www.barc.gov.in/cg/rpcd/

Homepage1.html2.1 Singh A. K., Bhasikuttan A. C., Palit D. K. and Mittal J. P. J.

Phys. Chem. A 104 (2000) 7002 - 7009 2.2 Singh A. K., Ramakrishna G., Ghosh H. N. and Palit D. K. J.

Phys. Chem. A. 2004, 108, 2583 – 2597*2.3 Palit D. K. , Proceedings of the National Academy of

Sciences, 2009, 101A, 1-162.4 Palit D. K. et. al. J. Chem. Phys. 1993, 99, 7273 Ultrafast

Phenomena IX, 1994, 501 3.1 Bebelaar D., Rev. Sci. Instrum. 57, 1986, 11163.2 Mahr H., Hirch M. D., Opt. Commun. 13, 1975, 963.3 Singh P. K., Nath S., Bhasikuttan A. C., Kumbhakar M.,

*Mohanty J., Sarkar S. K., Mukherjee T., Pal H ., J. Chem. Phys. 129, 2008, 114504

4.1 McGiven W. S., Li R., Zou P. and North S.W., J. Chem. Phys. 111 (1999) 5771

*4.2 Naik P. D., Kumar Awadhesh, Upadhyaya H. P., Bajaj P. N ., Sarkar S. K., Lasers in Chemistry, Vol.1, Ed. M. Lackner, Wiley-VCH Verlag GmbH (2008), p.463.

4.3 Wiley W. C., McLaren I. H., Rev. Sci. Instrum. 26 (1955) 1150

4.4 Xu D., Francisco J. S., Huang J., Jackson W. M., J. Chem. Phys. 117 (2002) 2578

5.1 Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Field, M. S. Chem. Rev. 1999, 99, 2957

5.2 Biswas, N.; Thomas, S.; Kapoor, S.; Mishra, A.;Watergaonkar, S; Mukherjee, T. J. Chem. Phys. 2008, 129, 184702

5.3 Biswas, N.; Thomas, S.; Sarkar, A.; Mukherjee, T.; Kapoor, *S . J. Phys. Chem. C 2009, 113, 7091

6.1 Konradi J., Singh A. K., and Materny A., Phys. Chem. Chem. Phys., 20, 3574 (2005)

*6.2 Singh A. K. and Sarkar S. K ., KIRAN, 17, 18 (2006)*Authors to be contacted for further information on the facilities

figure 6.1.

Over the years, we at RPCD, BARC have been actively involved in the development of ‘state-of-the-art’ experimental facilities for using sophisticated fs laser systems for the study of molecular dynamics as illustrated in section 2-5.. The major focus in previous projects was to understand the dynamics of elementary reactions at microscopic level. Our group has made a very useful contribution in the field of ultrafast chemical reaction dynamics in condensed phase. In recent years, the challenge has shifted from the study of molecular dynamics to control over molecular processes. In the ongoing XIth plan project on Coherent Control of Chemical Reactions” we desire to utilize the acquired knowledge for more realistic systems of technological importance.

We have already demonstrated [6.1] coherent control techniques to influence and control the multimode dynamics of polyatomic molecules in solution phase (Fig. 6.2) in collaboration with the group of Prof. Arnulf Materny of Jacobs University, Bremen, Germany.

In the CARS spectrum of toluene, three bands can be

detected, which for simplicity are labeled by 1 (≈1000 -1 -1 -1cm ), 2 (≈800 cm ), and 3 (≈1200 cm ) (Fig. 6.2). Due to

the nonlinear character of the coherent anti-Stokes Raman excitation, the CARS spectrum of toluene is dominated by band 1 in the considered spectral range. This makes a selective enhancement of the weak spectral lines assigned to the bands 2 and 3 very challenging. Performing an optimization of phase shape of the Stokes resulted in dramatic changes of the anti-Stokes spectra. Here, the Raman band ratio served as fitness function for

Fig. 6.2: Mode Selective Excitation: fs CARS spectra taken with transform limited pulses (A) and phase shaped (B,C) Stokes pulses from Toluene (left side), with FROG traces of the corresponding Stokes pulses

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