Table-Top X-ray Spectroscopy of Benzene Radical Cation

Ultrafast table-top X-ray spectroscopy at the carbon K-edge is used to measure the X-ray spectral features of benzene radical cations (Bz+). The ground state of the cation is prepared selectively by two-photon ionization of neutral benzene, and the X-ray spectra are probed at early times after the ionization by transient absorption using X-rays produced by high harmonic generation (HHG). Bz+ is well-known to undergo Jahn-Teller distortion, leading to a lower symmetry and splitting of the πorbitals. Comparison of the X-ray absorption spectra of the neutral and the cation reveals a splitting of the two degenerate π∗ orbitals as well as an appearance of a new peak due to excitation to the partially occupied π-subshell. The π∗ orbital splitting of the cation, elucidated on the basis of high-level calculations in a companion theoretical paper [Vidal et al. J. Phys. Chem. A. http://dx.doi.org/10.1021/acs.jpca.0c08732], is discovered to be due to both the symmetry distortion and even more dominant spin coupling of the unpaired electron in the partially vacant πorbital (from ionization) with the unpaired electrons resulting from the transition from the 1sC core orbital to the fully vacant π∗ orbitals.