Radiative and radiationless vibronic deactivation rates in selectively excited CO⁺

A tunable dye laser has been used to selectively excite particular (ν = 0-3) vibrational levels of A ²Π CO⁺. The CO⁺ is produced in a He bath by Penning ionization of CO by He metastables. By studying the time resolved emission of CO⁺ from selected vibrational levels as a function of He pressure, values have been obtained for the radiative decay rates and the velocity averaged cross-sections for CO⁺ vibrational relaxation upon He collision. The cross sections are all rather large (<0.1 Ų) and increase considerably with increasing ν. Time resolved spectra of the vibrationally relaxed emission require the postulation of a three-level system to fit their form. It is proposed that both of these effects are consistent with a vibrational relaxation mechanism wherein the A ²Π CO⁺ vibrational levels relax via intermediate, highly excited, vibrational levels of the ground X²Σ state of CO⁺.