R-matrix calculation of eigenchannel multichannel quantum defect parameters for strontium

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Abstract
Within the framework of the eigenchannel R-matrix method, theoretical calculations of the multichannel quantum defect (MQDT) parameters have been performed for 1.3Po, 1.3De and 1.3Se spectra of Sr. For the singlet spectra, both discrete and autoionisation regions below the Sr+ 4d threshold are investigated, while the analysis of the triplets is restricted to the bound spectra below the Sr+ 5s threshold. Near the Sr+5s threshold, the calculated parameters are in good agreement with those previously fitted to spectroscopic data. The main improvements with respect to the empirical MQDT studies concern evidence of the large energy dependence of the parameters and identification of isolated perturbers of the 1D2 and 1S0 spectra. In addition, the energy levels and the spectral intensities are accurately reproduced and channel mixing and correlation effects in doubly excited states are analysed in detail. A close similarity is found between the 5snp-4d np 1Po mixing in Sr and the msnp-mpns 1Po mixing in the light alkaline earths Be and Mg. Such near invariance does not hold for the 1De and 1Se spectra.