R-matrix calculation of photoionisation from Rydberg states in strontium

This paper presents the first detailed theoretical calculation of numerous observables investigated in Sr using stepwise laser excitation techniques. Measurements concern the 5p 1 2/ns and 5p 3 2/ns J=1 autoionising levels excited through the 5sns 1 S 0 Rydberg states; isolated resonances as well as interacting autoionising 5pns levels have been investigated. Results on the excitation spectra, the positions and widths of the levels as well as on the branching ratios and the angular distributions of the photoelectrons have been reported. All the above-mentioned observables are calculated through a combination of the eigenchannel R matrix and the multichannel quantum defect (MQDT) methods. Short-range parameters used by MQDT to describe final-state channel interactions are determined by performing R-matrix calculations in LS coupling and including spin-orbit effects through a geometric (jj/LS) frame transformation. The calculation of the required dipole matrix elements is based on the isolated core excitation approximation. All the measured quantities are accurately reproduced except the asymmetry parameters characterising the angular distributions of electrons ejected to the Sr + 5s level, these latter quantities being found to be the most sensitive to the interseries interaction. In addition, the calculated MQDT parameters are compared with those previously obtained empirically from experiment.