Eigenchannel R-matrix calculation of the J=1 odd-parity spectrum of barium

Abstract

The J=1 odd-parity spectrum of barium is studied through a combination of the eigenchannel R-matrix and multichannel quantum defect (MQDT) methods. Short-range parameters used by MQDT to describe electronic channel interactions are determined by performing R-matrix calculations in LS coupling and the (LS/jj) frame transformation is used to include fine-structure effects. This approach has proven to be an efficient tool for describing lighter alkaline earths but its ability to handle Ba, where spin-orbit interaction is stronger, is analysed. The calculations reproduce well the perturbations of the principal Rydberg series 6snp 1.3P1 by eight low-lying doubly excited levels as well as most of the features observed in the photoabsorption spectrum 6s2-J=1' up to the 6p3/2 threshold. Although some discrepancies were found, the considerable overall success obtained by describing spin-orbit effects in Ba with the (LS/jj) frame transformation instead of including them explicitly in the short-range calculation is a great surprise.