Probing charge migration in bromoacetylene using frequency-matched ionization

We present how strong field ionization (SFI) can be used as a coherent probe to measure the charge migration (CM) in bromoacetylene. This simple “two-site” molecule has shown CM between Br and carbon/carbon triple bond sites on a femtosecond time scale in [A. S. Folorunso et al. Phys. Rev. Lett. 126, 133002 (2021)]. The dynamics were simulated using real-time time dependent density functional theory (RT-TDDFT) approach with auxiliary basis sets, complex absorbing potentials, and  tuned range-separated functionals. To initiate the CM, we create a localized hole on Br site using constrained density functional theory, and to probe, a strong AC field was applied perpendicular to the CM axis (molecular axis), i.e. field-free CM. We observe that time evolution of the hole via CM affects the ionization yields in multiple ways. First, the yield is increased when the field frequency and the CM frequency are commensurate. Second, in the commensurate case, the ionization yields are enhanced when the field time delay is such that the hole is on the triple bond when the field is a maximal. Additional calculations with different electron donating and withdrawing groups were also studied to determine the effect of hole contrast on yield. These results demonstrate how SFI is a promising technique for observing hole dynamics during CM.