Ab-initio investigation of Br-3d core-excited electronic structures of HBr and HBr+

Ultrafast X-ray absorption spectroscopy is a powerful tool for real-time probing of chemical dynamics. Interpretation of the time-resolved absorption spectra requires knowledge of core-excited potentials, which necessitates assistance from high-level electronic-structure computations. In this study, we investigate Br-3d core-excited electronic structures of hydrogen bromide (HBr) using the spin-orbit general multiconfigurational quasidegenerate perturbation theory (SO-GMC-QDPT). Core-to-valence excitation energies and transition dipole moments are computed as functions of the internuclear distance for five electronic states of HBr (1Σ0+ , 3Π1, 1Π1, 3Π0+ , 3Π1) and two electronic states of HBr+ (2Π3/2, 2Σ1/2). The results illustrate the capabilities of the Br-3d edge probing to capture transitions of electronic-state symmetry as well as nonadiabatic dissociation processes evolving across avoided crossings. Furthermore, core-to-valence absorption spectra are simulated from the neutral and ionic ground states by numerically solving the time-dependent Schrodinger equation, which exhibit an excellent agreement with an experimental spectrum. The calculated comprehensive and quantitative picture of the core-excited potentials allows for transparent analyses of the core-to-valence absorption signals, filling the gap in the theoretical understanding of the Br-3d absorption spectra.