Vibrational spectra of halobenzene cations in the ground and B 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.