laser-induced vibrational motion through impulsive ionization
DESCRIPTION
Laser-induced vibrational motion through impulsive ionization. George N. Gibson University of Connecticut Department of Physics. Grad students: Li Fang, Brad Moser Funding : NSF-AMO. October 19, 2007 University of New Mexico Albuquerque, NM. Motivation. - PowerPoint PPT PresentationTRANSCRIPT
Laser-induced vibrational Laser-induced vibrational motion through impulsive motion through impulsive
ionizationionizationGrad Grad
students:students:Li Fang, Brad Li Fang, Brad
MoserMoser
FundingFunding::NSF-AMONSF-AMOOctober 19, 2007October 19, 2007
University of New MexicoUniversity of New MexicoAlbuquerque, NMAlbuquerque, NM
George N. George N. GibsonGibsonUniversity of University of ConnecticutConnecticut
Department of Department of PhysicsPhysics
MotivationMotivation Excitation of molecules by strong laser Excitation of molecules by strong laser
fields is not well-studied.fields is not well-studied. Excitation can have positive benefits, Excitation can have positive benefits,
such as producing inversions in the VUV such as producing inversions in the VUV and providing spectroscopy of highly and providing spectroscopy of highly excited states of molecules. excited states of molecules. Excited Excited states of Hstates of H22
++ have have nevernever been studied been studied before!before!
Can be detrimental to certain Can be detrimental to certain applications, such as quantum applications, such as quantum tomography of molecular orbitals.tomography of molecular orbitals.
How to detect excitationHow to detect excitation
TOF experiments are very common, but TOF experiments are very common, but are not sensitive to excitation, except are not sensitive to excitation, except in one case: Charge Asymmetric in one case: Charge Asymmetric Dissociation.Dissociation.
II222+2+ I I2+2+ + I + I0+0+ has ~8 eV more energy has ~8 eV more energy
thanthan I I22
2+2+ I I1+1+ + I + I1+1+
Also see NAlso see N226+6+ N N4+4+ + N + N2+2+, which has , which has
more than 30 eV energy than the more than 30 eV energy than the symmetric channel.symmetric channel.
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
Used 80 Used 80 J pumpJ pumpand 20 and 20 J probe.J probe.
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 structure Depends on wavelength (800 vs 400 Depends on wavelength (800 vs 400
nm).nm). Depends on relative intensity of pump Depends on relative intensity of pump
and probe.and probe. Depends on polarization of pump and Depends on polarization of pump and
probe.probe. Depends on dissociation channel.Depends on dissociation channel.
Will focus on one example: the (2,0) Will focus on one example: the (2,0) channel with 400 nm pump and probe.channel with 400 nm pump and probe.
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
II2+2+ pump-probe data pump-probe data
(2,0) vibrational signal(2,0) vibrational signal Final state is electronically excited.Final state is electronically excited. See very large amplitude motion, See very large amplitude motion,
can measure amplitude and phase can measure amplitude and phase modulation.modulation.
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
(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.
What about the What about the dynamics?dynamics? How are the states populated?How are the states 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
++..
Ionization geometryIonization geometry
Ionization geometryIonization geometry
From polarization From polarization studiesstudies
The A state is only produced with the The A state is only produced with the field perpendicular to the molecular field perpendicular to the molecular axis. This is opposite to all other axis. This is opposite to all other examples of strong field ionization in examples of strong field ionization in molecules.molecules.
The A state only ionizes to the (2,0) The A state only 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.
Conclusions from IConclusions from I22
Can identify excited molecular states Can identify excited molecular states from vibrational signature.from vibrational signature.
Can perform novel molecular Can perform novel molecular spectroscopy.spectroscopy.
Can learn about the strong-field Can learn about the strong-field tunneling ionization process, tunneling ionization process, especially details about the angular especially details about the angular dependence.dependence.
Could be a major problem for Could be a major problem for quantum tomography.quantum tomography.
Ground state vibrationsGround state vibrations
““Lochfrass” J. Ullrich & Lochfrass” J. Ullrich & A. SaenzA. Saenz
TOF DataTOF Data
Phase lagPhase lag
Phase lagPhase lag
SimulationsSimulations
Thermal effectsThermal effects
ConclusionsConclusions
We see large amplitude ground We see large amplitude ground oscillations in neutral iodine molecules.oscillations in neutral iodine molecules.
We believe them to result from We believe them to result from Lochfrass or R-dependent ionization of Lochfrass or R-dependent ionization of the vibrational wavefunction.the vibrational wavefunction.
From simulations, we conclude that the From simulations, we conclude that the amplitude of the coherent vibrations is amplitude of the coherent vibrations is larger for larger temperature.larger for larger temperature.
This is very different from all other This is very different from all other coherent control schemes that we coherent control schemes that we are aware of.are aware of.