Transcript Slide 1
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
Excitation of molecules by strong laser fields is not well-studied.
Excitation can have positive benefits, such as producing inversions in the VUV and providing spectroscopy of highly excited states of molecules. Excited states of H 2 + have never been studied before!
Can be detrimental to certain applications, such as quantum tomography of molecular orbitals.
How to detect excitation
TOF experiments are very common, but are not sensitive to excitation, except in one case: Charge Asymmetric Dissociation.
I 2 2+ I 2 2+
I I 2+ 1+ + I + I 0+ 1+ has ~8 eV more energy than Also see N channel.
2 6+
N 4+ + N 2+ , which has more than 30 eV energy than the symmetric
Pump-probe experiment with fixed wavelengths.
14 I 2 2+ 12 10 I 2+ + I 4 2 8
Probe
6 0 3
Pump
I 1+ + I 1+ 6 9 12 Internuclear separation, R [a.u.] 15
In these experiments we used a standard Ti:Sapphire laser: 800 nm 23 fs pulse duration 1 kHz rep. rate Used 80
J pump and 20
J probe.
Pump-probe spectroscopy on I 2 2+
Enhanced Excitation
Enhanced Ionization at R c Internuclear separation of dissociating molecule
Counts/shot
Lots of vibrational structure in pump-probe experiments
Vibrational structure
Depends on wavelength (800 vs 400 nm).
Depends on relative intensity of pump and probe.
Depends on polarization of pump and probe.
Depends on dissociation channel.
Will focus on one example: the (2,0) channel with 400 nm pump and probe.
Laser System
• • • • • • Ti:Sapphire 800 nm Oscillator Multipass Amplifier 750 J pulses @ 1 KHz Transform Limited, 25 fs pulses Can double to 400 nm Have a pump-probe setup
Ion Time-of-Flight Spectrometer
Parabolic Mirror Drift Tube MCP Conical Anode AMP Laser TDC Discriminator PC
I 2+ pump-probe data
(2,0) vibrational signal
Final state is electronically excited.
See very large amplitude motion, can measure amplitude and phase modulation.
Know final state – want to identify intermediate state.
I 2 potential energy curves
Simulation of A state
Simulation results From simulations: - Vibrational period - Wavepacket structure - (2,0) state
(2,0) potential curve retrieval It appears that I 2 2+ 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 dynamics?
How are the states populated?
I 2 I 2 + (I 2 + )* - resonant excitation?
I 2 (I 2 + )* directly – innershell ionization?
No resonant transition from X to A state in I 2 + .
Ionization geometry
Ionization geometry
From polarization studies
The A state is only produced with the field perpendicular to the molecular axis. This is opposite to all other examples of strong field ionization in molecules.
The A state only ionizes to the (2,0) state!?
Usually, there is a branching ratio between the (1,1) and (2,0) states, but what is the orbital structure of (2,0)?
Ionization of A to (2,0) stronger with parallel polarization.
Conclusions from I 2
Can identify excited molecular states from vibrational signature.
Can perform novel molecular spectroscopy.
Can learn about the strong-field tunneling ionization process, especially details about the angular dependence.
Could be a major problem for quantum tomography.
Ground state vibrations
“Lochfrass” J. Ullrich & A. Saenz
TOF Data
Phase lag
Phase lag
Simulations
Thermal effects
Conclusions
We see large amplitude ground oscillations in neutral iodine molecules.
We believe them to result from Lochfrass or R dependent ionization of the vibrational wavefunction.
From simulations, we conclude that the amplitude of the coherent vibrations is larger for larger temperature.
This is very different from all other coherent control schemes that we are aware of.