Photoionization of atomic chlorine near the K-edge

The photoionization cross section for atomic Cl in the vicinity of the 1s threshold has been investigated using R-matrix methodology. Specifically, the resonances leading up to the first two 1s ionization thresholds, the 1s2s$^{\mathrm{2}}$2p$^{\mathrm{6}}$3s$^{\mathrm{2}}$3p$^{\mathrm{5}}$ $^{\mathrm{3,1}}$P states of Cl$^{\mathrm{+}}$,$^{\mathrm{\thinspace }}$have been examined in detail. In addition to the 1s2s$^{\mathrm{2}}$2p$^{\mathrm{6}}$3s$^{\mathrm{2}}$3p$^{\mathrm{6}}$ $^{\mathrm{2}}$S resonance, which arises from a 1s$\to $3p transition that is possible owing to the open shell nature of the Cl atom, there are six resonances series leading up to the two thresholds: \textbraceleft 1s2s$^{\mathrm{2}}$2p$^{\mathrm{6}}$3s$^{\mathrm{2}}$3p$^{\mathrm{5}}$ $^{\mathrm{3,1}}$P\textbraceright np $^{\mathrm{2}}$S, $^{\mathrm{2}}$P, $^{\mathrm{2}}$D. The results show that the 1s$\to $3p resonances is by far the strongest, as might be expected, and the energy and shape are in rather good agreement with experiment [1]. Furthermore, this lowest $^{\mathrm{2}}$S resonance ``robs'' oscillator strength from the resonances of the \textbraceleft 1s2s$^{\mathrm{2}}$2p$^{\mathrm{6}}$3s$^{\mathrm{2}}$3p$^{\mathrm{5}}$ $^{\mathrm{3}}$P\textbraceright np $^{\mathrm{2}}$S series, which are very much weaker than their $^{\mathrm{2}}$P and $^{\mathrm{2}}$D counterparts; there is no 1s$\to $3p resonance in the $^{\mathrm{2}}$P and $^{\mathrm{2}}$D manifolds. The next strongest resonances are the six 1s$\to $4p excitations. Each pair $^{\mathrm{2}}$S, $^{\mathrm{2}}$P and $^{\mathrm{2}}$D n$=$4 resonances interact so that their separation is not the splitting of the $^{\mathrm{3}}$P and $^{\mathrm{1}}$P 1s ionization thresholds, and their quantum defects are very much larger than the asymptotic values and for the n$=$4, they are about 1.6 for the $^{\mathrm{2}}$P and $^{\mathrm{2}}$D while for the $^{\mathrm{2}}$S they are about 1.8, reflecting the fact that the n$=$4 $^{\mathrm{2}}$S resonances are also strongly affected by the 1s3p$^{\mathrm{6}}$ resonance; the higher resonances in all series exhibit quantum defects of about 0.9. [1] W. C. Stolte, \textit{et al}, Phys. Rev. A \textbf{88}, 053425 (2013). Work supported by U.S. DOE.