Simulating resonantly enhanced strong-field ionization with shaped femtosecond pulses
Our understanding of strong-field ionization is still rudimentary due to the complex interplay of multiphoton transitions between initial, final, and intermediate resonant states, their dynamic Stark shift, the ponderomotive potential and many more fundamental processes. Strong laser pulses are necessary to investigate these processes and the results can be measured e.g. as photoelectron spectra. The question tackled in this STSM was, how the signals and dynamics of strong-field ionization change if different parameters of the laser pulse are varied. To answer the question, we used our previously developed method to simulate molecular strong-field ionization (SFI) in the presence of vibrational motion. Within this method, the ionic continuum is described by Legendre polynomials and the treatment of intermediate non-resonant states of the yet neutral molecule is simplified by the so-called adiabatic elimination of off-resonant neutral states.
We investigated pulses with a so-called triangular phase, whereby a pulse can be separated into two subpulses with a tunable time delay between them. As a result of such pulses, we obtained interestingly shaped photoelectron spectra in our simulations. The system under study is a one-dimensional model for Iodobromomethane (CH2BrI) including four electronic states of the neutral and three ionic states. Looking at the photoelecton spectrum, we found additional peaks when using shaped laser pulses compared to the outcome with unshaped pulses. Further investigation revealed that the new peaks stem from the same electronic states as the previously observed peaks, i.e., a peak originating from D1 splits into two. Further investigations and comparison to experiment are necessary to further understand these results.