Interpreting vibrationally resolved spectra of molecular dications (doubly positively charged molecules): HCl²⁺

Vibrationally resolved spectra of HCl2+ appear to show five vibrational levels for the X3Σ− ground electronic state, whereas calculations of vibrational levels supported by ab initio potential energy curves have been able to locate only three vibrational levels below the barrier; this discrepancy is resolved by considering vibrational states that the potential function supports in the continuum above the barrier maximum. A low resolution spectrum produced from first principles is compared with a spectrum obtained with threshold photoelectrons in coincidence (TPEsCO) spectroscopy, with agreement sufficient to suggest that care must be taken in the inversion of vibrational spectroscopic data for molecular dications to avoid generating potential functions that are too strongly bound.