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.