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

Abstract

We time-resolve coupled electronic and nuclear dynamics during strong-field molecular ionization by measuring the momentum-resolved photoelectron yield as a function of pump-probe delay for a pair of strong-field laser pulses. The sub-10 fs pulses are generated using a specially designed ultrafast optical pulse shaper and the electrons are measured using velocity map imaging. Our measurements, in conjunction with calculations that solve the time-dependent Schrödinger equation, allow us to time-resolve resonance enhanced strong-field ionization and break it down into three basic steps: (1) Stark shifted resonant excitation of a high-lying neutral state of the molecule, (2) nonadiabatic dynamics (internal conversion) in which multiple electronic states are coupled, and (3) coupling to the continuum (ionization).

Sequential and direct ionic excitation in the strong-field ionization of 1-butene molecules

“…We remark that nuclear dynamics was very recently confirmed to play a role in post-ionization excitation. 50,51 Future studies of the correlated photoelectron spectra as a function of intensity, pulse duration, and wavelength within a pump-probe scheme would be very helpful for developing a deeper understanding, in particular for photon energies above the gap energy. Finally, it would be interesting to explore how general our conclusions are by performing similar studies in other molecular systems.…”

Strong-field- versus weak-field-ionization pump-probe spectroscopy

“…the difference in the measurements comes from the different sensitivities that WFI and SFI have to the excited state dynamics. We believe that this longer decay in the SFI CH 2 I 2 pumpprobe signal is due to a multiphoton resonance which enhances the ion yield as the molecule dissociates [19][20][21][22][23][24][25]28]. As the wavepacket dissociates on the neutral excited state, the molecular structure can be such that n-photons (n < IP/hν) from the strong field probe can come into resonance with an intermediate state, between the excited state and the continuum, increasing the ionization rate.…”

“…Time resolved ionization spectroscopy [15][16][17][18][19][20] offers the advantage over optical spectroscopies that it is always possible to ionize, regardless of the character of the excited state. The near threshold ionization of valence electrons from excited states (for which the cross section is large) can be accomplished either in the weak field regime, with the absorption of a single ultraviolet (UV) or vacuum ultraviolet (VUV) photon (∼ 6-10 eV), or in the strong field regime, with the absorption of multiple low energy near infrared photons (∼ 1-3 eV) [19][20][21][22][23][24][25].…”