Laser-induced vibrational motion through impulsive ionization

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

2. 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 H2+ have never been studied before! • Can be detrimental to certain applications, such as quantum tomography of molecular orbitals.

3. How to detect excitation • TOF experiments are very common, but are not sensitive to excitation, except in one case: Charge Asymmetric Dissociation. • I22+  I2+ + I0+ has ~8 eV more energy than I22+  I1+ + I1+ • Also see N26+  N4+ + N2+, which has more than 30 eV energy than the symmetric channel.

4. Pump-probe experiment with fixed wavelengths. 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. Probe Pump

5. Pump-probe spectroscopy on I22+ Enhanced Excitation Enhanced Ionization at Rc Internuclear separation of dissociating molecule

6. Lots of vibrational structure in pump-probe experiments

7. 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.

8. 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

9. Ion Time-of-Flight Spectrometer

10. I2+ pump-probe data

11. (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.

12. I2 potential energy curves

13. Simulation of A state

14. Simulation results From simulations: - Vibrational period- Wavepacket structure- (2,0) state

15. (2,0) potential curve retrieval 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.

16. What about the dynamics? • How are the states populated? • I2 I2+  (I2+)* - resonant excitation? • I2  (I2+)* directly – innershell ionization? • No resonant transition from X to A state in I2+.

17. Ionization geometry

18. Ionization geometry

19. 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.

20. Conclusions from I2 • 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.

21. Ground state vibrations

22. “Lochfrass” J. Ullrich & A. Saenz

23. TOF Data

24. Phase lag

25. Phase lag

26. Simulations

27. Thermal effects

28. 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.