Inner-shell Photoionization of Atomic Chlorine: Experiment and Theory

Relative partial cross sections have been measured following photoabsorption by atomic chlorine in the vicinity of the Cl 2$p$ and 1$s $ionization thresholds including the charge state fractions of the residual Cl ions. In addition, Breit-Pauli R-Matrix calculations have been performed in the vicinity of the 2$p$ thresholds which show reasonably good agreement with experiment. Including spin-orbit interactions, there are ten 2$p^{5}$3$s^{2}$3$p^{5}$ thresholds of Cl$^{+}$, and a total of 64 resonance series leading up to these thresholds from the ground J$=$3/2 state of the Cl atom; the results show two groups of resonances, broad in connection with the higher-energy thresholds, and narrow for the lower-energy Cl$^{+}$ thresholds. This is explained in terms of the angular momentum geometry of the situation which demonstrates that the wide transitions can decay \textit{via} the monopole term in the expansion of the inter-electron Coulomb interaction, while for the narrow resonances, the leading term is the (significantly smaller) dipole term. In the vicinity of the 1$s $thresholds, the 1$s \to $3$p$ resonance is seen clearly; otherwise the spectrum is quite similar to Ar.