Two-path interference in the resonance-enhanced few-photon ionization of lithium atoms

We investigate the resonance-enhanced few-photon ionization of an atomic Li target at a photon energy near the resonance between the 2s ground and 2p excited states. For this system, the ground-state ionization resembles an atomic “double slit”, because it can proceed through the 2p resonances with the magnetic quantum number m being either −1 or +1. In our experiment, the target can be prepared in one of the polarized excited 2p states before subjecting it to the ionizing radiation, thereby effectively closing one of the two slits. This procedure makes it possible to extract the interference term between the two pathways to obtain complex phase information on the final state. The analysis of our experimental results is supported by an ab initio model based on the numerical solution of the time-dependent Schroedinger equation.