Strong Field Molecular Ionization in the Impulsive Limit: Freezing Vibrations with Short Pulses

Abstract

We study strong-field molecular ionization as a function of pulse duration. Experimental measurements of the photoelectron yield for a number of molecules reveal competition between different ionization continua (cationic states) which depends strongly on pulse duration. Surprisingly, in the limit of short pulse duration, we find that a single ionic continuum dominates the yield, whereas multiple continua are produced for longer pulses. Using calculations which take vibrational dynamics into account, we interpret our results in terms of nuclear motion and non-adiabatic dynamics during the ionization process.

Time-resolved measurement of internal conversion dynamics in strong-field molecular ionization

“…The ponderomotive potential is estimated to be approximately 0.5-0.7 eV based on previous work [24,29]. …”

“…In our previous work studying CH 2 IBr, we found that vibrational dynamics and nonadiabatic transitions following resonant excitation of the neutral molecule could be suppressed by using laser pulses with a duration shorter than the vibrational period [24]. By making the pulses sufficiently short (<10 fs), we were able to "freeze out" the nuclear motion, preventing population transfer via nonadiabatic coupling, resulting in ionization to only a single state of the cation.…”

“…1. This picture is based on our earlier calculations and measurements [24,29], which indicate that an excited neutral (Rydberg) state (R 1 ), correlated with the first excited state of the cation (D 1 ), comes into resonance during the laser pulse. The reaction coordinate in this picture is the CH 2 wagging mode, which was found to be the only one for which there was significant displacement of excited state minima at the Franck-Condon (FC) point, and crossing between states facilitating nonadiabatic transitions.…”

“…We measure the momentum-resolved photoelectron yield as a function of pump-probe delay, allowing us to see how the population is transferred between electronic states of the neutral molecule following excitation. The experimental results are interpreted with calculations of strong-field ionization which include vibrational dynamics on the neutral excited states as well as nonadiabatic couplings [24,30]. Dynamics in the molecular cation have been studied in earlier publications [31,32], but are not relevant to the current study since the photoelectron spectrum is not affected by dynamics in the cation following ionization [33].…”

Ionic dynamics underlying strong-field dissociative molecular ionization

“…III that along the nuclear coordinate which largely describes the motion on D 0 and D 1 after ionization, there are 1-photon resonances in CH 2 IBr near the FC while there is only one 2-photon resonance in CF 3 I that is far away from the FC. We note that the relative change in the D 0 and D 1 yields with pulse duration is due to non-adiabatic dynamics in intermediate neutral states, which we considered in a separate study [25].…”