Near-threshold K-shell photoionisation in argon

Abstract

The K-shell photoionisation cross section of argon has been calculated in the length and velocity gauge using the Dirac-Fock method. The relaxation is taken into account completely by constructing separate solutions for the ground state and the final 1s^-1 hole state and by including all the resulting overlap matrix elements between the initial- and final-state one-electron four spinors in the transition amplitude. The post-collision interaction has been taken into account quantum mechanically by incorporating the lowest-order scattering theory amplitude into the cross section. The authors' results reveal dramatic relaxation effects in the threshold region bringing the theoretical cross section into close agreement with experiment. The cross section is nearly gauge independent with only the E1 multipole contributing significantly. The post-collision interaction is found to lower the cross section by 15% at threshold. This result is attributed to inadequacies in the lowest-order treatment of the influence of the residual ionic field on the photoelectron.