An R-matrix with pseudo states calculation of electron-impact excitation in Ar

Abstract

Electron-impact excitation of neutral argon is of both fundamental and applied interest. It has been studied theoretically and experimentally for many years and excitation data for argon are of significant importance to the modelling of a variety of laboratory plasmas. To date, the theoretical calculations of excitation for this atom have not included the effects of coupling to the high Rydberg states and the target continuum. Here we present the results of the first R-matrix with pseudo states (RMPS) calculation of excitation in neutral argon. Although the spin–orbit interaction has a significant effect on the target structure for this atom, the pseudo-state expansion that is required to accurately represent the target continuum is sufficiently large that we have performed the present calculation in pure LS coupling. Both differential and total excitation cross sections resulting from this RMPS calculation are compared to those from a standard R-matrix calculation without pseudo states. These comparisons indicate that the effects of continuum coupling are large even for transitions from the ground term to the lowest excited terms, and although they are most pronounced above the ionization limit, they are still important at lower energies. This demonstrates the need for a full intermediate-coupling RMPS calculation between individual levels in argon, and this will present a formidable computational challenge.