Three-photon above-threshold ionization of magnesium

Three-photon ionization cross sections from the ground state, and two-photon ionization from the 3¹P(3s3p) state have been calculated for Mg in the region between the 3p and 4s ionization thresholds. These processes include an above-threshold ionization process for the last absorbed photon. We have used the Green’s-function method in the Feshbach formalism and an L² close coupling approach, with a basis of L² integrable B-spline functions. We report the positions, widths, and assignments of a number of relevant ¹P^o and¹F^o doubly excited Feshbach states. We also observe unusually high cross sections due to a bound-bound transition from the 3¹S(3s²) to the 3¹P(3s3p) state, a core excitation process, and the population of an intermediate doubly excited state. Both total cross sections and angular distributions are reported.