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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2024-10, Vol.128 (39), p.8373-8383
• Main Authors: Lin, Zhe, Liu, Junzi, Zhang, Chaoqun, Zheng, Xuechen, Cheng, Lan
• Format: Article
• Language: English
• Subjects: A: Structure, Spectroscopy, and Reactivity of Molecules and Clusters; Chemical calculations; Chlorine; Energy; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Phase transitions; Relativistic effects; Spin-orbit coupling; X-ray absorption near edge spectroscopy; X-ray spectroscopy
• ISSN: 1089-5639; 1520-5215; 1520-5215
• DOI: 10.1021/acs.jpca.4c04089

A relativistic core–valence-separated equation-of-motion coupled cluster (CVS-EOM-CC) study of chlorine L2,3-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 2p3/2 and 2p1/2 electrons. The 2p3/2 → 4s core Rydberg transitions are shown to overlap with the 2p1/2 → 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 2p1/2 → σ* and 2p3/2 → 4s transitions.