Signatures of the bromine atom and open-shell spin-coupling in the x-ray spectrum of bromobenzene cation

M. Epstein, B.N. Cabral Tenorio, M.L. Vidal, V. Scutelnic, Z. Yang, T. Xue, A.I. Krylov, S. Coriani, and S.R. Leone
J. Am. Chem. Soc.  145, 3554 – 3560 (2023)

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->pi* and the 1sC*->sigma*(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->pi* 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 pi* orbitals. The 1sC*->sigma* 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+.

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