Energy dependence of photoelectron angular distributions from two- and four-photon ionization of Mg in the vicinity of the 3p2 1S0 doubly excited state

Abstract

We report on measurements concerning the variation of photoelectron angular distributions (PADs) from two- and four-photon ionization of ground state Mg, as the laser wavelength is scanned across the 3p2 1S0 resonance. The energy dependence of the asymmetry parameters determined from the PADs and corresponding to two-photon ionization is found to be in good agreement with prior theoretical predictions. For the four-photon PADs there are at present no theoretical results. Nevertheless, the resonant character is found considerably red-shifted in accordance with earlier experimental and theoretical studies of the single and double ionization yields. In either case, the PADs show a dominant s-wave on-resonance contribution, while the d-wave contribution apparently dominates under non-resonant excitation within the scanned wavelength range. The PADs offer detailed wavelength- and channel-dependent information on the dynamics of the autoionization resonances under pulsed laser excitation, corroborating and elucidating the conclusions reached from ionization yield studies.