Orbital Angular Momentum Transfer in the Photoionization of Excited Lithium Using Optical Schemes with Vortex Beams

Optical vortex beams carry orbital angular momentum (OAM) due to the phase variation of their wavefront. OAM can be transferred to atomic and molecular systems, and lead to a modification of the standard selection rules for electronic transitions. In particular, the transfer of OAM manifests itself in the structure of the photoelectron momentum distribution (PMD). In gas-phase atoms, one difficulty is that OAM transfer is only efficient near the center of the beam, i.e., exactly where the field vanishes. In this work, we explore the efficiency of OAM transfer for excited lithium atoms near the vortex singularity by solving the time-dependent Schrödinger equation in a spatially inhomogeneous field. The computed efficiency of OAM transfer and the characteristics of the PMDs are well-reproduced by a perturbative approach. We also investigate two-color schemes where a UV pulse triggers the first ionization step while an optical vortex mediates continuum-continuum transitions.