Elucidating Anomalous Intensity Ratios in Chlorine L-Edge X-ray Absorption Spectroscopy: Multiplet Effects and Core Rydberg Transitions.

A relativistic core-valence-separated equation-of-motion coupled cluster (CVS-EOM-CC) study of chlorine L_2,3-edge X-ray absorption near-edge structure (XANES) spectra using CH₃Cl and CH₂ICl as representative molecules is reported. The nearly identical intensity for the main features in the L₂- and L₃-edge XANES spectra is attributed to multiplet effects and the overlap between core-valence and core Rydberg transitions. The multiplet effects originating from the interaction between the core hole and the C-Cl σ* orbitals account for around half of the deviation of the L₃ and L₂ intensity ratio from the 2:1 ratio of the numbers of 2p_3/2 and 2p_1/2 electrons. The 2p_3/2 → 4s core Rydberg transitions are shown to overlap with the 2p_1/2 → σ* transitions and contribute to the other half of the intensity anomaly. We demonstrate that triple excitations in CVS-EOM-CC calculations play important roles in accurate simulation of the overlap between the 2p_1/2 → σ* and 2p_3/2 → 4s transitions.