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

Table-top 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 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 ~1 eV to higher energy in the x-ray spectrum compared to the other carbon atoms (C). The x-ray absorption spectra of neutral BrBz reveals the influence of the bromine atom on the x-ray absorption spectrum due to the 1sC* orbital, while the influence of the bromine atom on the cation spectrum is even more prominent. The x-ray spectrum of the cation is dominated by two relatively intense transitions, the 1sC→ π* and the 1sC*→ 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 1sC→ 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 1sC core orbital to the fully vacant π* orbitals. The 1sC*→ σ* doublet peak shape results from several transitions involving σ* and vibrational C*-Br mode activation following the UV ionization, which demonstrates the impact of the C*-Br bond length on the core-valence transition as well as the relaxation geometry of BrBz+.