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 1sC* orbital of the carbon atom (C*) bonded to bromine by similar to 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, 1sC -> x* and 1sC*-> a*(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 1sC -> x* doublet peak shape arises due to the spin coupling of the unpaired electron in the partially vacant x orbital (from ionization) with the two other unpaired electrons resulting from the transition from the 1sC core orbital to the fully vacant x* orbitals. The 1sC*-> a* doublet peak shape results from several transitions involving a* 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+.