An unusual cytokine:Ig-domain interaction revealed in the crystal ...
Strong-field physics revealed through time-domain spectroscopy
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Strong-field physics Strong-field physics revealed through time- revealed through time- domain spectroscopy domain spectroscopy Grad Grad student: student: Li Fang Li Fang Funding Funding : : NSF-AMO NSF-AMO May 30, 2009 May 30, 2009 XI Cross Border Workshop on Laser Science XI Cross Border Workshop on Laser Science University of Ottawa, Ottawa, Canada University of Ottawa, Ottawa, Canada George N. George N. Gibson Gibson University of University of Connecticut Connecticut Department of Department of Physics Physics
description
Strong-field physics revealed through time-domain spectroscopy. George N. Gibson University of Connecticut Department of Physics. Grad student: Li Fang Funding : NSF-AMO. May 30, 2009 XI Cross Border Workshop on Laser Science University of Ottawa, Ottawa, Canada. Motivation. - PowerPoint PPT Presentation
Transcript of Strong-field physics revealed through time-domain spectroscopy
Strong-field physics Strong-field physics revealed through time-revealed through time-domain spectroscopydomain spectroscopy
Grad Grad student:student:
Li FangLi Fang
FundingFunding::NSF-AMONSF-AMOMay 30, 2009May 30, 2009
XI Cross Border Workshop on Laser ScienceXI Cross Border Workshop on Laser ScienceUniversity of Ottawa, Ottawa, CanadaUniversity of Ottawa, Ottawa, Canada
George N. George N. GibsonGibsonUniversity of University of ConnecticutConnecticut
Department of Department of PhysicsPhysics
MotivationMotivation Vibrational motion in pump-probe Vibrational motion in pump-probe
experiments reveals the role of experiments reveals the role of electronically excited intermediate electronically excited intermediate statesstates..
This raises questions about This raises questions about how the how the intermediate states are populatedintermediate states are populated. Also, . Also, we can study how they couple to the final we can study how they couple to the final states that we detect.states that we detect.
We observe inner-orbital ionization, which We observe inner-orbital ionization, which has important consequences for has important consequences for HHG and HHG and quantum tomographyquantum tomography of molecular orbitals. of molecular orbitals.
Pump-probe experiment Pump-probe experiment with fixed wavelengths.with fixed wavelengths.
3 6 9 12 150
2
4
6
8
10
12
14
Ene
rgy
[eV
]
Internuclear separation, R [a.u.]
I22+
I2+ + I
I1+ + I1+Pump
Probe
In these In these experimentsexperiments
we used a we used a standardstandard
Ti:Sapphire Ti:Sapphire laser:laser:
800 nm800 nm23 fs pulse 23 fs pulse
durationduration1 kHz rep. rate1 kHz rep. rate
Pump-probe Pump-probe spectroscopy on Ispectroscopy on I22
2+2+
Internuclear separation of dissociating molecule
EnhancedIonization at Rc
EnhancedExcitation
Lots of vibrational Lots of vibrational structure in pump-probe structure in pump-probe
experiments experiments
Vibrational structureVibrational structureDepends on:Depends on: wavelength (400 to 800 nm).wavelength (400 to 800 nm). relative intensity of pump and probe.relative intensity of pump and probe. polarization of pump and probe.polarization of pump and probe. dissociation channel.dissociation channel. We learn something different from We learn something different from
each signal.each signal.
Will try to cover several examples of Will try to cover several examples of vibrational excitation.vibrational excitation.
II2+2+ pump-probe data pump-probe data
(2,0) vibrational signal(2,0) vibrational signal Amplitude of vibrations so large that Amplitude of vibrations so large that
we can measure changes in KER, we can measure changes in KER, besides the signal strength.besides the signal strength.
Know final state – want to identify Know final state – want to identify intermediate state.intermediate state.
II22 potential potential energy energy curvescurves
Simulation of A stateSimulation of A state
Simulation resultsSimulation results
From simulations:
- Vibrational period- Wavepacket structure- (2,0) state
What about the What about the dynamics?dynamics? How is the A-state populated?How is the A-state populated?
II22 I I22++ (I (I22
++)* - resonant excitation?)* - resonant excitation?
II22 (I (I22++)* directly – innershell ionization?)* directly – innershell ionization?
No resonant transition from X to A state No resonant transition from X to A state in Iin I22
++..
From polarization From polarization studiesstudies
The A state is only produced with the The A state is only produced with the field field perpendicular to the molecular perpendicular to the molecular axisaxis. This is opposite to most other . This is opposite to most other examples of strong field ionization in examples of strong field ionization in molecules.molecules.
The A state only The A state only ionizes to the (2,0)ionizes to the (2,0) state!?state!?Usually, there is a branching ratio Usually, there is a branching ratio between the (1,1) and (2,0) states, but between the (1,1) and (2,0) states, but what is the orbital structure of (2,0)?what is the orbital structure of (2,0)?
Ionization of A to (2,0) stronger with Ionization of A to (2,0) stronger with parallel polarization.parallel polarization.
Implications for HHG Implications for HHG and QTand QT
We can readily see ionization from We can readily see ionization from orbitals besides the HOMO.orbitals besides the HOMO.
Admixture of HOMO-1 depends on Admixture of HOMO-1 depends on angle.angle.
Could be a major problem for Could be a major problem for quantum tomography, although quantum tomography, although this could explain some this could explain some anomalous results.anomalous results.
(2,0) potential curve (2,0) potential curve retrievalretrieval
It appears that I22+ has a truly bound potential
well, as opposed to the quasi-bound ground state curves. This is an excimer-like system – bound in the excited state, dissociating in the ground state. Perhaps, we can form a UV laser out of this.
Wavelength-dependent pump probe scheme
Change inner and outer turning points of the wave packet by tuning the coupling wavelength.
Femtosecond laser pulses:Pump pulse: variable wavelength. (517 nm, 560 nm and 600 nm.) Probe pulse: 800 nm.
I2+ spectrum: vibrations in signal strength and kinetic energy release (KER) for different pump pulse wavelength [517nm, 560 nm and 600 nm]
Vib
rati
onal
per
iod
(fs)
X-B coupling wavelength (nm)
Simulation: trapped population in the (2,0) potential well
The (2,0) potential curve measured from the A state of I2
+ in our previous work:
pump-probe delay=180 fs
PRA 73, 023418 (2006)
..31.6,..48.1,60
))(exp(1)(1
02
uaRuameVD
VRRDRV
ee
ee
I2+ + In+ dissociation channels
Neutral ground state Neutral ground state vibrations in Ivibrations in I22
Oscillations in the data appear to Oscillations in the data appear to come from the X state of neutral Icome from the X state of neutral I22..
Measured the vibrational frequency Measured the vibrational frequency and the revival time.and the revival time.
Revival Revival structurestructure
0 5 10 15 20 25 30 352.64
2.65
2.66
2.67
2.68
2.69 0 5 10 15 20 25 30 351.00
1.02
1.04
1.06
1.08
1.10
6.20 6.25 6.30 6.35 6.40 6.450
1
2
3
(b) SimulationR
(Å
)
Pump-probe delay (ps)
(a) DataDis
soci
atio
n en
ergy
(eV
)
Pow
er s
pect
rum
[ar
b. u
nit]
Freqency [1/ps]
FFT of simulation FFT of data
Vibrational frequencyVibrational frequencyMeasuredMeasured 211.0211.00.7 cm0.7 cm-1-1
KnownKnown 215.1 cm215.1 cm-1-1 Finite tempFinite temp 210.3 cm210.3 cm-1-1
Raman scattering/Bond Raman scattering/Bond softeningsoftening
Raman Raman transitions are transitions are made possible made possible through coupling through coupling to an excited to an excited electronic state. electronic state. This coupling This coupling also gives rise to also gives rise to bond softening, bond softening, which is well which is well known to occur known to occur in Hin H22
++..
h
Raman transition
Distortion of potentialcurve through bond-softening
R-dependentionization
LochfrassLochfrass New mechanism for vibrational excitation: New mechanism for vibrational excitation:
“Lochfrass”“Lochfrass”R-dependent ionization distorts the ground R-dependent ionization distorts the ground state wavefunction creating vibrational motion.state wavefunction creating vibrational motion.
Seen by Ergler Seen by Ergler et et alal. PRL . PRL 9797, , 103004 (2006) in 103004 (2006) in DD22
++..
Lochfrass vs. Bond Lochfrass vs. Bond softeningsoftening
Can distinguish these two effects Can distinguish these two effects through the phase of the signal.through the phase of the signal.
0 200 400 600
2.00
2.01
2.02
2.03
Bond-softening Lochfrass
<R
> [
a.u.
]
Pump-probe delay [fs]
LFLF = = BSBS = = /2./2.
Iodine vs. DeuteriumIodine vs. Deuterium
S/SS/Saveave = 0.60 = 0.60
Iodine better resolved:Iodine better resolved:23 fs pulse/155 fs period = 0.15 (iodine)23 fs pulse/155 fs period = 0.15 (iodine)7 fs pulse/11 fs period = 0.64 7 fs pulse/11 fs period = 0.64
(deuterium)(deuterium) Iodine signal huge:Iodine signal huge:
S/SS/Saveave = 0.10 = 0.10
Variations in kinetic Variations in kinetic energyenergy Amplitude of the Amplitude of the
motions is so large motions is so large we can see we can see variations in KER or variations in KER or <R>.<R>.
2.5 3.0 3.5 4.0
0
1
10
12
14
16
18
18
19
20
21
22
30
35
R-dependentionization
Initialwavefunction
Final vibrational wavepacket
Internuclear separation, R
Pot
enti
al e
nerg
y
Req,ion
R(Å)
I+
2 X
g,3/2
=0
I2+ 2 p
oten
tial
ene
rgy
(eV
)I2+
2 (2,0)
I2 X
gI 2, I+ 2 p
oten
tial
ene
rgy
(eV
)
Req,GES
Probe pulse
Temperature effectsTemperature effects Deuterium vibrationally cold at room Deuterium vibrationally cold at room
temperaturetemperatureIodine vibrationally hot at room temperatureIodine vibrationally hot at room temperature
Coherent control is supposed to get worse at Coherent control is supposed to get worse at high temperatures!!! But, we see a huge high temperatures!!! But, we see a huge effect.effect.
Intensity dependence also unusualIntensity dependence also unusual We fit <R> = We fit <R> = Rcos(Rcos(t+t+) +R) +Raveave
As intensity increases, As intensity increases, R increases, RR increases, Raveave decreases.decreases.
Intensity dependenceIntensity dependence
Also, for Lochfrass signal strength should Also, for Lochfrass signal strength should decrease with increasing intensity, as is decrease with increasing intensity, as is seen.seen.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Internuclear separation, R [atomic units]
Pot
entia
l ene
rgy
[eV
]
v = 1
v = 2
v = 3
v = 4
v = 5
But, RBut, Raveave temperature: temperature:
T T decreasesdecreases while while R R increasesincreases!!!!!!
We have an incoherent sea of We have an incoherent sea of thermally populated thermally populated
vibrational states in which we vibrational states in which we ionize a coherent hole:ionize a coherent hole:
So, we need a density matrix approach.So, we need a density matrix approach.
Density matrix for a 2-Density matrix for a 2-level modellevel model
For a thermal systemFor a thermal system
where where pp11(T)(T) and and pp22(T)(T) are the Boltzmann are the Boltzmann factors. This cannot factors. This cannot be written as a be written as a superposition of superposition of state vectors.state vectors.
e
go
)(0
0)()(
2
1
Tp
TpTi
Time evolution of Time evolution of We can write:We can write:
These we can evolve in time.These we can evolve in time.
10
00,
00
01
,)()()(
)2()1(
)2(2
)1(1
TpTpti
Coherent interaction – use Coherent interaction – use pulse for maximum coherencepulse for maximum coherence
Off diagonal terms have Off diagonal terms have opposite phases. This opposite phases. This means that as the means that as the temperature increases, ptemperature increases, p11 and pand p22 will tend to cancel will tend to cancel out and the coherence will out and the coherence will decrease.decrease.
21
212
21221
21
2
221
)2(
21
2
221
)1(
))()((
))()(()(
,
tii
tii
f
tii
tii
ftii
tii
f
o
o
o
o
o
o
eTpTp
eTpTpT
e
e
e
e
R-dependent ionization – R-dependent ionization – assume only the right well assume only the right well
ionizes.ionizes. ff = ( = (gg + + ee)/2)/2
Trace(Trace() = ½ due to ) = ½ due to ionizationionization
41
41
41
41
)1(ti
ti
o
o
e
e
What about excited state?
)(41
41
41
41
)2( Te
efti
ti
o
o
NOTEMPERATUREDEPENDENCE!
Expectation value of R, Expectation value of R, <R><R>
)()( 2112 oRRTraceR
))()()(sin( 21 TpTptRR oo
Coherent
)cos(2
tR
R ooLochfrass
The expectation values are /2 out of phase for the two interactions as expected.
Comparison of two Comparison of two interactionsinteractions
Coherent Coherent interactionsinteractions::
Off diagonal terms Off diagonal terms are imaginary.are imaginary.
Off diagonal terms Off diagonal terms of upper and lower of upper and lower states have states have opposite signs and opposite signs and tend to cancel out.tend to cancel out.
R-dependent R-dependent ionizationionization
Off-diagonal terms Off-diagonal terms are real.are real.
No sign change, so No sign change, so population in the population in the upper state not a upper state not a problem.problem.
Motion produced by coherent interactions and Lochfrass are /2 out of phase.
““Real” (many level) Real” (many level) molecular systemmolecular system
Include electronic Include electronic coupling to excited coupling to excited state.state.
Use I(R) based on Use I(R) based on ADK rates. Probably ADK rates. Probably not a good not a good approximation but it approximation but it gives R dependence.gives R dependence.
Include Include = 0 - 14 = 0 - 14
h
Raman transition
Distortion of potentialcurve through bond-softening
Generalize equationsGeneralize equations
10
0
000
),()(ottU
)()()( TpTf
/
1,1,2 RR
Same conclusionsSame conclusionsFor bond-softeningFor bond-softening Off-diagonal terms are imaginary Off-diagonal terms are imaginary
and opposite in sign to next higher and opposite in sign to next higher state. state. 1212
(1)(1) - -1212(2)(2)
R decreases and <R decreases and <> increases > increases with temperature.with temperature.
For LochfrassFor Lochfrass Off diagonal terms are real and have Off diagonal terms are real and have
the same sign. the same sign. 1212(1)(1) 1212
(2)(2)
R increases and <R increases and <> decreases > decreases with temperature.with temperature.
Excitation from Lochfrass will always Excitation from Lochfrass will always yield real off diagonal elements with yield real off diagonal elements with the same sign for excitation and the same sign for excitation and deexcitation [f(R) is the survival deexcitation [f(R) is the survival probablility]:probablility]:
dRRfRRc
dRRfRRc
)()()(
)()()(
2*112
1*221
R and R and <<>>
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.00 0.03 0.06 0.09 0.12 0.150.00
0.05
0.10
0.15
0.20
0.25
<v>
<v> - initial <v>
f - bondsoftening
<v>f - Lochfrass
kBT [eV]
R [
a.u.
]
Bondsoftening actual max
Lochfrass actual max
Density matrix elementsDensity matrix elements
1 2 3 4 5
0.00
0.03
0.06
0.09
0.12
0.15
12
345
1 2 3 4 5
0.0
0.2
0.4
0.6
0.8
1.0
12
345
1 2 3 4 5
0.00
0.01
0.02
0.03
0.04
0.05
12
345
1 2 3 4 5
0.0
0.2
0.4
0.6
0.8
1.0
12
345
nm
mn
Lochfrass
nm/m
ax
nm
mn
Bond-softening
nm
mn
nm/m
ax
nm
mn
ConclusionsConclusionsCoherent reversible interactionsCoherent reversible interactions Off-diagonal elements are imaginaryOff-diagonal elements are imaginary Excitation from one state to another is out-Excitation from one state to another is out-
of-phase with the reverse process leading of-phase with the reverse process leading to a loss of coherence at high temperatureto a loss of coherence at high temperature
Cooling not possibleCooling not possibleIrreversible dissipative interactionsIrreversible dissipative interactions Off-diagonal elements are realOff-diagonal elements are real Excitation and de-excitation are in phase Excitation and de-excitation are in phase
leading to enhanced coherence at high leading to enhanced coherence at high temperaturetemperature
Cooling is possibleCooling is possible
ConclusionsConclusions Excitation of the A-state of Excitation of the A-state of
II22++ through inner-orbital through inner-orbital
ionizationionization
Excitation of the B-state of IExcitation of the B-state of I22 to populate the bound region to populate the bound region of (2,0) state of Iof (2,0) state of I22
2+2+
Vibrational excitation Vibrational excitation through tunneling ionization.through tunneling ionization.
Laser SystemLaser System
• Ti:Sapphire 800 nm OscillatorTi:Sapphire 800 nm Oscillator• Multipass AmplifierMultipass Amplifier• 750 750 J pulses @ 1 KHzJ pulses @ 1 KHz• Transform Limited, 25 fs Transform Limited, 25 fs
pulsespulses• Can double to 400 nmCan double to 400 nm• Have a pump-probe setupHave a pump-probe setup
Ion Time-of-Flight Ion Time-of-Flight SpectrometerSpectrometer
Laser
Drift Tube MCPConical Anode
Parabolic Mirror
AMP
DiscriminatorTDCPC
Phase lagPhase lag
Ionization geometryIonization geometry
Ionization geometryIonization geometry
II2+2+ pump-probe data pump-probe data
