Vibrational spectra of halobenzene cations in the ground and $B̃ 2B2$ electronic states obtained by one-photon mass-analyzed threshold ionization spectrometry

Vibrational spectra in the ground electronic states, $X̃ 2B1,$ of fluoro-, chloro-, bromo-, and iodobenzene cations and in the excited electronic states, $B̃ 2B2,$ of chloro- and bromobenzene cations have been measured by mass-analyzed threshold ionization (MATI) via one-photon excitation with vacuum ultraviolet light. The spectra consist mostly of strong 0–0 bands and fundamentals, overtones, and combinations of totally symmetric $a1$ modes. In particular, the observed vibrational bands in the $B̃$ states have been assigned by referring to the assignments for the ionic ground states and those for the neutral molecules. From the MATI spectra, the ionization energies of fluoro-, chloro-, bromo-, and iodobenzene to the ionic ground states have been determined to be $74 229±5,$ $73 177±5,$ $72 570±5,$ and $70 638±5 cm−1,$ respectively. The ionization energies to the $B̃$ states have also been determined to be $91 404±5$ and $85 822±5 cm−1$ for chloro- and bromobenzene, respectively.