The effect of vibrational and electronic autoionization on the CO⁺ A²Π vibrational populations in the wavelength range 618-750 Å

Vibrationally resolved relative fluorescence cross sections have been measured for the CO⁺ A²? ? X²S⁺ transition using monochromated synchrotron radiation between 618 and 750 Å. In this wavelength region the photoionization spectrum is dominated by intense autoionizing resonances belonging to the Rydberg series converging onto the various vibrational levels of the CO⁺ A²? or the B²S⁺ ionization thresholds. Autoionization from these Rydberg states into the CO⁺ A²? state causes significant variations to occur in the A²? ? X²S⁺ fluorescence cross sections and the CO⁺ A²? vibrational branching ratios. The results indicate that, in most cases, vibrational autoionization favours the ionization channel representing the minimum change in the vibrational state of the ion core. However, in one wavelength region, the experimental data appear to disagree with the theoretical propensity rule for vibrational autoionization. The effects of electronic autoionization from the Rydberg series converging onto the B²S⁺ threshold are compared with simple Franck-Condon predictions and theoretical cross sections calculated using a two-step multichannel quantum defect approach.