Optics Communications 282 (2009) 1183–1185

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Optics Communications

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A novel demonstration of photoacoustic Raman spectroscopy with combinedstimulated Raman pumping in H2 molecule

Ramesh C. Sharma *

Department of Physics and Astronomy, The University of Manchester, Oxford Road, M13 9PL Manchester, United Kingdom

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 March 2008Received in revised form 26 November 2008Accepted 26 November 2008

Key words:Photoacoustic Raman spectroscopyStimulated Raman pumpingNonlinear spectroscopyH2 moleculeIR spectroscopyLaser spectroscopy

0030-4018/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.optcom.2008.11.088

* Corresponding author. Tel./fax: +44 161 306 3963E-mail address: Ramesh.Sharma@manchester.ac.uk

We present the experimental demonstration of a novel, efficient, and vibrational selective technique toprepare population in vibrational level v00 = 1 using the stimulated Raman pumping. Photoacoustic Ramansignal has been studied in non-radiative transitions in the molecule H2 (v00 = 0) and (v00 = 1). The popula-tion fraction in the v00 = 1 level can be estimated by using combined photoacoustic Raman spectroscopywith stimulated Raman pumping for the first time.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, measurements of trace greenhouse gases suchas CO2 and CH4 have become important to avoid environmentalpollution problems. Continuous monitoring of the leak of inflam-mable gases, such as H2, CH4 and H2S, is also important in chemicalplants and along gas pipe-lines from the view point of safety. Laserspectroscopy provides sensitive detection techniques for variousmolecules in the atmosphere. Absorption spectroscopy andphotoacoustic spectroscopy (PAS) in the infra-red (IR) region areuseful methods for this purpose. However, these techniques de-mand a tunable IR laser source. The photoacoustic Raman spectros-copy (PARS) was proposed by Barrett and Berry [1]. In this case,two visible lasers with frequencies (xp) and (xs) were used insteadof an IR laser, and the frequency difference (xp �xs) was tuned tobe equal to the Raman shift frequency (xR). The thermal relaxationof molecules from the upper vibrational state was detected as theacoustic wave. In various kinds of factories, in chemical plants, oralong gas pipe-lines, continuous monitoring for any leakage of inflammable gases such as H2, CH4, and H2S is very necessary. Forsuch applications, nonlinear Raman spectroscopy can be applied.Nonlinear Raman spectroscopy has the merit that it can excitethe vibrational level of gas molecules in the IR region withoutthe use of a tunable IR laser [2–4]. However, traditional nonlinearRaman spectroscopy requires two lasers whose frequency differ-

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ence should coincide with the Raman frequency of the samplegas. Usually a fixed-frequency laser and a tunable laser are usedfor this purpose.

Development of high power lasers has given rise to several non-linear spectroscopic technique based on Raman Scattering such ascoherent antistokes Raman scattering (CARS), stimulated Ramanscattering (SRS) [2,3]. PARS is also based on the stimulated Ramanspectroscopy. Previously, a new scheme for nonlinear Raman spec-troscopy without the requirement of a tunable laser and with onlya fixed-frequency laser and a Raman shifter was proposed [2–4].The various kinds of nonlinear Raman spectroscopy techniquessuch as PARS [4–11].

We have demonstrated for the first time here a combined tech-nique the PARS with stimulated Raman pumping (SRP) of H2 mol-ecules in vapour phase. Vibrational level v00 = 1 is populated by thetechnique stimulated Raman pumping. The population fraction ofthe vibrational level v00 = 0 and v00 = 1 level can be estimated byusing PARS without SRP and with SRP technique. PARS signal ismonitored based on non-radiative transitions in the molecularsample using a sensitive microphone coupled with a pre-amplifier.

2. Experimental

A second harmonic radiation at 532 nm from the Nd-YAG laser,pulsed width 5–7 ns at a repetition rate of 10 Hz is divided by intwo parts 20% and 80%. The 80% of 532 nm radiation is pumpedthe tunable dye laser using excitation of mixer DCM and LDS 698dye solution in methanol. The band width of the dye laser is

ωp

Non-radiativerelaxation

a

b

c

ωs

ωp – ωs = ωR

Fig. 2. Energy level diagram for the PARS in the region IR wavelength, using thepump and stokes beams.

1184 R.C. Sharma / Optics Communications 282 (2009) 1183–1185

0.05 nm. The setup of PARS was composed of two pulsed beams;one from an Nd:YAG laser at 532 nm with 16 mJ and the otherfrom an Nd:YAG laser pumped-dye laser emitting in the range680–685 nm with 4–7 mJ. With the use of a dichroic mirror, bothbeams propagating along the same direction are temporarily andspatially overlapped in the center of the photoacoustic cell. The532 nm radiation fixed has been used as a pump beam (xp) andtunable lasing beam from the dye laser as a stokes beam (xs) forthe photoacoustic Raman signal. The experimental setup is illus-trated in Fig. 1a–c.

The photoacoustic (PA) cell with quartz windows at Brewsterangle at the ends was made from 2.5 cm diameter and 15 cm longPyrex glass tube. The design of the PA cell is given in Fig. 1a. Twoneedle valves were connected to the PA cell. One valve was usedto seal the sample reservoir, and the other was used to connectthe cell to the vacuum pump. A sensitive microphone (SennheiserKE-4-211-2) was fitted at the center of the sample cell and propershielding was made to minimize the external electrical pickup. Themicrophone was safely housed in a Teflon tube with 2 cm lengthand 1 cm diameter. PA cell was found to be suitable for experi-ments in a free running mode in which the sample flow rate care-

Mic

Pre-Amp

BoxcarComputer

PA Cell DM2

DM1

P1

Oscilloscope

Nd-

YAG

Las

er

DyeLaser

Triggered

L1

PA Cell

Microphone

7.0 cms

3.5 cms3.5 cms

2.5 cms 2.5 cms1.0 cms

Mic

Pre-Amp

BoxcarComputer

PA CellDM2

DM1P1

OscilloscopeNd-

YAG

Las

er

DyeLaser

Raman Cell

Dispersive Prism

DM3DM4

Nd-YAG Laser

L1L2

Triggered

Delay generator

a

b

c

Fig. 1. A design of the photoacoustic cell (a), PARS setup without the SRP (b), Acombined setup PARS with SRP (c).

fully controlled by a needle valve. H2 gas is introduced through aside-arms after evacuation of the PA cell. Microphone was safelymaintained throughout this experiment. The photoacoustic spec-trum was recorded about at 2 Torr pressure of H2 gas. PARS, it asample is illuminated by pulsed two laser beams temporally andspatially coincident, interactions can occur that produce selectivepopulation of a two energy levels of the sample (Fig. 2). The condi-tions for which this selective population occurs are: The transitioninvolving energy levels must be Raman active, i.e. the transitionmust involve a change in the molecular polarization. The frequencydifference of the two incident laser beams must be adjusted toequal the frequency of this Raman active transitionðx0p �x0s ¼ x0RÞ. As the excited molecules in this energy level relaxnon-radiatively heat collision in the H2 molecule, a pressure fluctu-ations is generated that is detected by a sensitive microphone andthen fed a boxcar averager (EG&G Model 162) to improve signal tonoise ratio. The output signal is then recorded by the personalcomputer. SRP technique was employed in the experimental setup(Fig. 1c) to prepare the H2 population in the v00 = 1 level. The secondharmonic of Nd:YAG laser at 532 nm was focused into a Ramancell, 100 cm in length and 5 cm in diameter filled with 9 atm H2

gas, to give out a strong first stokes and anti-stokes Raman shiftat about 683 and 436 nm, respectively. The output beams at 532and 683 nm were measured to be 14 and 17 mJ, respectively, priorto the photoacoustic (PA) cell, while the beam at 436 nm was fil-tered out. Then these two beams were softly focused, with a focallength of 70 cm, and reflected by a dichroic mirror into the centerof the PA cell, overlapping with the probe beam which was trans-mitted through the same dichroic mirror. Delay time were adjustedto be 5 ns each between of both Nd-YAG lasers. The non-radiativetransitions in the combined technique SRP-PARS is monitored by asensitive microphone and amplified by a pre-amplifier. The signalis processed to the boxcar averager to improve the signal to noiseratio.

3. Results and discussion

PARS spectra are recorded in the frequency range 3880–3936 cm�1 in Fig. 3a & b. PARS spectra are with SRP in Fig. 3aand without SRP technique (Fig. 3b). The frequency of the pumpbeam was fixed ðx0pÞ and the frequency of stokes beam ðx0sÞ wastunable. The pump ðx0pÞ was excited from the vibrational levelv00 = 0 of the H2 molecule. The spectrum is recorded asðx0p �x0s ¼ x0RÞ. ðx0RÞ is a Raman active in a non-radiative transi-tions. PARS spectrum is recorded in the frequency range 3880–3936 cm�1 in Fig. 3b with SRP technique. In the case, the vibra-tional level v00 = 1 is populated by the method of SRP. During suchshort delay time the prepared H2 vibrational level (v00 = 1) popula-tion could not be relaxed via collisions [12]. Fig. 2 shows the PARS

35093088 3930

3880 3905 3930

3.0

6.0

9.0

3.0

6.0

9.0

12.0

b

aIn

tens

ity (a

.u.)

Inte

nsity

(a.u

.)

cm-1

cm-1

Q (j=0)

Q ( j=1)

Q (j=2)

Q ( j=3)

v =0, j = 0 -3

Fig. 3. Photoacoustic Raman spectrum without SRP (a), photoacoustic Ramanspectrum with combined technique SRP-PARS (b).

R.C. Sharma / Optics Communications 282 (2009) 1183–1185 1185

spectra of four Q-branch transitions of H2 (v00 = 0 ? 1) with andwithout the SRP application. The intensity alternation is causedby the nuclear spin I1/2 and thus the statistical weights for the evenand odd rotational lines are 1 and 3, respectively [13]. The basictheory of PARS has been described by Refs. [7,10,14]. PARS signalintensity is directly proportional as in Eq. (1),

IPARS / r � L � N � Ip � Is ð1Þ

where N is the molecular number density, IP and IS are the intensi-ties of pump and Stokes laser beams, respectively, r is the Ramancross section for laser beams and L is the effective interaction lengthbetween sample and two laser beams. We have estimated popula-tions of the vibrational levels of H2 (v00 = 0) and (v00 = 1) from the re-corded photoacoustic Raman spectra (Fig. 3a and b) using Eq. (1).Shown in Fig. 3b, a decrease of 25% in populations of all rotational

lines of the v00 = 0 level suggests that the (v00 = 1, j = 0–3) levelsshould be excited simultaneously by SRP, different from only a sin-gle ro-vibrational level (H2 v00 = 1, j = 1) prepared in a supersonic jetexperiment [15]. The vibrational population is evaluated as a sum ofthe corresponding rotational peak areas. Accordingly, the popula-tion fraction in v00 = 1 pumped by SRP can be estimated to be 10%of the total vibrational population in our work. A value more than30% was previously reported by detection of resonance-enhancedmultiphoton ionization �REMPI, under the beam energies of25 mJ/pulse each for the wavelength at 532 and 683 nm througha 20 cm focal lens into the chamber [15]. The deviation in the beamenergy densities and the pressure in the Raman shifter may causethe difference of the SRP efficiency.

4. Conclusion

We have studied PARS of H2 (v00 = 0) and (v00 = 1) molecule basedon the non-radiative de-excitation in the IR frequency range 3880–3896 cm�1, using the two visible pulsed laser beams pump andstokes. The pump beam was fixed and stokes beam was tunablein this work. The vibrational level (v00 = 1) was populated by thetechnique SRP using the generated two beams from the Raman cell532 and 683 nm. A population fraction of (23 ± 4)% pumped to thev00 = 1 level may be estimated with the combined technique SRP-PARS measurement for the first time. This combined technique isapplicable in physical sciences, for the studies population effi-ciency, vibronic selective, non-radiatively de-excitation in the in-fra-red wavelength region without tunable infra-red laser.

Acknowledgement

Author is thankful to Professor Mitsuo Koshi, The University ofTokyo, Japan, Professor Jagdish P Singh, ICET, Mississippi State Uni-versity, USA, Professor S.N. Thakur, BHU, Varanasi, India for thediscussion.

References

[1] J.J. Barrett, M.J. Berry, Appl. Phys. Lett. 34 (1979) 144.[2] Y. Oki, N. Kawada, T. Ogawa, Y. Abe, M. Maede, Jpn. J. Appl. Phys. 36 (1997)

1172.[3] Y. Oki, N. Kawada, Y. Abe, M. Maeda, Opt. Commun. 61 (1999) 57.[4] Y. Oki, S. Nakazono, Y. Nonaka, M. Maeda, Opt. Lett. 25 (2000) 1040.[5] D.R. Siebert, G.A. West, J.J. Barrett, Appl. Opt. 19 (1980) 53.[6] A. Melchior, I. Bar, S. Rosenwaks, J. Phys. Chem. A 102 (1998) 7273.[7] G.A. West, J.J. Barrett, D.R. Siebert, K.V. Reddy, Rev. Sci. Instrum. 54 (1983)

7977.[8] J.J. Barret, D.F. Heller, J. Opt. Soc. Am. 71 (1981) 1299.[9] K.K. Das, Y.V. Rostovtsev, K.K. Lehmann, M.O. Scully, Opt. Commun. 246 (2005)

551.[10] Y. Yu, K. Lin, X. Zhou, H. Wang, S. Liu, X. Ma, J. Raman Spectrosc. 38 (2007)

1206.[11] A. Owyoung, C.W. Patterson, R.S. McDowell, Chem. Phys. Lett. 59 (1978) 156.[12] R.L. Farrow, D.W. Chandler, J. Chem. Phys. 89 (1988) 1994.[13] G. Herzberg, Molecular Spectra and Molecular Structure: I. Spectra of Diatomic

Molecules, Van Nostrand Reinhold, Toronto, 1950. p. 139.[14] A.C. Eckbreth, P.W. Schreiber, in: A.B. Harvey (Ed.), Chemical Application of

Nonlinear Raman Spectroscopy, Academic, New York, 1981, p. 47.[15] D.A.V. Kliner, D.E. Adelman, R.N. Zare, J. Chem. Phys. 95 (1991) 1648.