Signatures of the Bromine Atom and Open-Shell Spin Coupling in the X‑ray Spectrum of the Bromobenzene Cation

Tabletop X-ray spectroscopy measurements at the carbon K-edge complemented by ab initio calculations are used to investigate the influence of the bromine atom on the carbon core–valence transitions in the bromobenzene cation (BrBz⁺). The electronic ground state of the cation is prepared by resonance-enhanced two-photon ionization of neutral bromobenzene (BrBz) and probed by X-rays produced by high-harmonic generation (HHG). Replacing one of the hydrogen atoms in benzene with a bromine atom shifts the transition from the 1s_C* orbital of the carbon atom (C*) bonded to bromine by ∼1 eV to higher energy in the X-ray spectrum compared to the other carbon atoms (C). Moreover, in BrBz⁺, the X-ray spectrum is dominated by two relatively intense transitions, 1s_C→π* and 1s_C*→σ*(C*–Br), where the second transition is enhanced relative to the neutral BrBz. In addition, a doublet peak shape for these two transitions is observed in the experiment. The 1s_C→π* doublet peak shape arises due to the spin coupling of the unpaired electron in the partially vacant π orbital (from ionization) with the two other unpaired electrons resulting from the transition from the 1s_C core orbital to the fully vacant π* orbitals. The 1s_C*→σ* doublet peak shape results from several transitions involving σ* and vibrational C*–Br mode activations following the UV ionization, which demonstrates the impact of the C*–Br bond length on the core–valence transition as well as on the relaxation geometry of BrBz⁺.