Strong-field Ionization as a Probe of Charge Migration

Charge migration (CM) is an attosecond coherent process where electron holes move across molecules in a particle-like way. There has been intense effort to study this process both theoretically and experimentally, but many questions remain about how CM is affected by the chemical properties of a molecule, and what techniques can be used to measure CM. To address these, we present a simulation study showing how strong-field ionization (SFI) can be used to measure CM in brominated conjugated molecules. In this modality, a strong infrared laser pulse induces CM via ionization SFI, and then another (or a two-color configuration) probes the dynamics via modulations in the second ionization yield. In this work, the electron dynamics were simulated using real-time time dependent density functional theory (RT-TDDFT) with Gaussian basis sets, complex absorbing potentials, and tuned range-separated functionals. To emulate the first ionization, a localized hole is created via constrained DFT, and for the second ionization an explicit field is used, with the polarization perpendicular to the CM direction. We observe that the second ionization is modulated such that the ionization is enhanced when the field maximum is coincident in time with the hole being on a pi-bond, i.e., when there is extra electron density around the bromine. We interpret these results using a hopping model of CM, where the hole travels between lone-pair electrons and pi bonds.