Direct excitation of Li I (nl) levels in keV Li-He collisions

Abstract
The results of a combined experimental and theoretical investigation of the excitation processes in Li-He collisions in the 1-100 keV energy range are presented. Experimentally the emission cross sections have been measured for the two lowest members of each of the 22S-n2P, 22P-n2S and 22P-n2D Li I Rydberg series, which are the strongest optical transitions observed. Due to the long lifetimes of the higher excited levels of the projectile the problem of radiation equilibrium in the collision cell requires attention. Theoretically, the collision has been treated as a quasi-one-electron system and close-coupling solutions to the impact-parameter equations have been obtained, using the (2s, 2p0, 2p+or-1, 3s, 3p0, 3p+or-1, 3d0, 3d+or-1, 3d+or-2) basis. A Hartree-Fock frozen-core potential has been used to describe the direct electrostatic interaction between the Li valence electron and the helium atom.