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Abstract

A joint experimental and theoretical study of four-photon ionization of Li is performed using a pump-probe setup with the laser frequency tuned in between the excitation energies of the 2s–4p and 2s–4f three-photon transitions. The three-dimensional photoelectron momentum pattern is recorded as a function of the delay time between two otherwise identical laser pulses. The process is modeled by numerically solving the time-dependent Schrödinger equation. While complicated interference patterns arise in the electron spectrum when the two pulses partially overlap in time, the expected quantum-beat phenomenon between Li atoms in either the 4p or the 4f state are seen when the fast oscillations involving the 2s initial state are averaged over. The qualitative agreement between the predicted and observed beating patterns is very good.