Two-electron model of Li(2s-2p) and Na(3s-3p) excitation in Li-Na high-energy collisions

Abstract

A two-electron model of direct excitation in Li-Na collisions is developed, based upon one-electron eigenstates of the separated atoms. A valence-bond-type two-electron atomic basis, antisymmetrised and modified by the translational phase factors, is used in the expansion of the electronic scattering state. The time-dependent Schrodinger equation is solved by the impact parameter method assuming a rectilinear trajectory and constant relative velocity of the alkali cores. Solutions of the resulting close-coupled equations have been obtained in the high-energy limit which defines a model with complete neglect of exchange contributions. Various sets of basis functions, Li(2s/2p)Na(3s/3p), involving single and/or double (simultaneous) excitation of the alkali atoms have been used. The aim was to obtain first estimates of Li(2p) and Na(3p) excitation cross sections and polarisations as functions of collision energy and to probe the details of the direct-excitation mechanism. The single-excitation predictions match recent experimental cross sections at 50-60 keV centre-of-mass energy. The double-excitation contributions are significant but the present calculations tend to overemphasize their importance. Excitation out of the collision plane, an allowed two-electron excitation process, is important at centre-of-mass energies above 100 keV.