Photoelectron momentum distribution of ground- and excited-state lithium atoms induced by extreme-ultraviolet photon absorption

We perform a theoretical investigation of the photoelectron momentum distribution (PMD) after ionization of a lithium (Li) atom, initially prepared in the ground (1s22s) or excited (1s22p) state, by extreme ultraviolet (XUV) radiation of frequency 85 eV that was previously measured in an experiment with the free-electron laser in Hamburg (2009 Phys. Rev. Lett. 103 103008). The experiment revealed three distinct peaks in PMD with absolute momentum values of 2.4, 1.2 and 0.8 atomic units (a.u.), respectively. We carry out an all-electron calculation based on time-dependent density functional theory (TDDFT) and find that the two outer PMD peaks (2.4 and 1.2 a.u.) are well described by the single ionization from the valence (2s) and core (1s) electron shells, while the innermost peak (0.8 a.u.) is not reproduced. However, the peak at this position is found in the TDDFT calculation of a Li+ ion (1s2) subject to the same XUV radiation, thus the TDDFT calculations are in good agreement with the experimental measurements if one assumes that the innermost peak originates from a Li+ ion rather than a neutral atom. We estimate possible production of Li+ ions by the pumping infrared laser used in the experiment to prepare the target atoms in the 1s22p excited state prior to interaction with the XUV field.