Frequency dependence of electron dynamics during femtosecond laser resonant photoionization of Li₄ cluster

This study investigates the resonant effects on nonlinear photoionization of Li₄ cluster under femtosecond laser irradiation. The real-time and real-space time-dependent density functional theory is applied to describe the optical linear response and nonlinear electron dynamics during the laser-material interactions. The calculations describe well the behaviors of the ionization process for both the off resonance and on resonance cases. For given laser intensities, the frequency dependence of electron ionization and energy absorption on multiphoton and/or tunnel ionization mechanisms is investigated. Theoretical results show that (1) the off resonance behaves as a classical oscillator whereas resonance leads to an internal excitation with subsequent emission of electrons after the laser termination; (2) when multiphoton ionization dominates, the frequency dependence of emitted electrons and absorbed energy coincides with trend of the spectral cross section for photon absorption; (3) while when tunnel ionization becomes significant, the aforementioned correlation gradually disappears, and the enhancement of resonant effect on photoionization is not manifest.