1. Laser-induced vibrational motion through impulsive ionization Grad students: Li Fang, Brad Moser Funding : NSF-AMO October 19, 2007 University of New Mexico Albuquerque, NM George N. Gibson University of Connecticut Department of Physics

2. Motivation Excitation of molecules by strong laser fields is not well-studied. 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! 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. 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. TOF experiments are very common, but are not sensitive to excitation, except in one case: Charge Asymmetric Dissociation. I 2 2+  I 2+ + I 0+ has ~8 eV more energy than I 2 2+  I 1+ + I 1+ I 2 2+  I 2+ + I 0+ has ~8 eV more energy than I 2 2+  I 1+ + I 1+ Also see N 2 6+  N 4+ + N 2+, which has more than 30 eV energy than the symmetric channel. Also see N 2 6+  N 4+ + N 2+, which has more than 30 eV energy than the symmetric channel.

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

5. Pump-probe spectroscopy on I 2 2+ Internuclear separation of dissociating molecule Enhanced Ionization at R c Enhanced Excitation

6. Lots of vibrational structure in pump-probe experiments

7. Vibrational structure Depends on wavelength (800 vs 400 nm). Depends on wavelength (800 vs 400 nm). Depends on relative intensity of pump and probe. Depends on relative intensity of pump and probe. Depends on polarization of pump and probe. Depends on polarization of pump and probe. Depends on dissociation channel. Depends on dissociation channel. Will focus on one example: the (2,0) channel with 400 nm pump and probe. Will focus on one example: the (2,0) channel with 400 nm pump and probe.

8. Laser System Ti:Sapphire 800 nm Oscillator Ti:Sapphire 800 nm Oscillator Multipass Amplifier Multipass Amplifier 750  J pulses @ 1 KHz 750  J pulses @ 1 KHz Transform Limited, 25 fs pulses Transform Limited, 25 fs pulses Can double to 400 nm Can double to 400 nm Have a pump-probe setup Have a pump-probe setup

9. Ion Time-of-Flight Spectrometer

10. I 2+ pump-probe data

11. (2,0) vibrational signal Final state is electronically excited. Final state is electronically excited. See very large amplitude motion, can measure amplitude and phase modulation. See very large amplitude motion, can measure amplitude and phase modulation. Know final state – want to identify intermediate state. Know final state – want to identify intermediate state.

12. I 2 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 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.

16. What about the dynamics? How are the states populated? How are the states populated? I 2  I 2 +  (I 2 + )* - resonant excitation? I 2  I 2 +  (I 2 + )* - resonant excitation? I 2  (I 2 + )* directly – innershell ionization? I 2  (I 2 + )* directly – innershell ionization? No resonant transition from X to A state in I 2 +. No resonant transition from X to A state in I 2 +.

17. 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 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)? 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. Ionization of A to (2,0) stronger with parallel polarization.

20. Conclusions from I 2 Can identify excited molecular states from vibrational signature. Can identify excited molecular states from vibrational signature. Can perform novel molecular spectroscopy. Can perform novel molecular spectroscopy. Can learn about the strong-field tunneling ionization process, especially details about the angular dependence. Can learn about the strong-field tunneling ionization process, especially details about the angular dependence. Could be a major problem for quantum tomography. Could be a major problem for quantum tomography.

21. Ground state vibrations

22. “Lochfrass” J. Ullrich & A. Saenz

23. TOF Data

24. Phase lag

26. Simulations

27. Thermal effects

28. Conclusions We see large amplitude ground oscillations in neutral iodine molecules. 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. 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. 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. This is very different from all other coherent control schemes that we are aware of.