Collisional deactivation of selectively excited N₂⁺

Individual vibrational levels of the A ²Π_u state of the N₂⁺ molecular ion are excited by a tunable dye laser. Detection of an optical-optical double resonance fluorescence signal produced by a second laser demonstrates that the ions originally pumped into the A ²Π_u state are rapidly deactivated by collisions with He into high vibrational levels of the ground X ²∑ _g⁺ state. Time and wavelength resolved studies show clearly that the population of a given high vibrational level of the X ²∑_g⁺ state returns into lower A ²Π_u state levels giving rise, effectively, to vibrational relaxation within the A state. Rate constants and cross sections are derived for these collisional, internal-conversion processes. In homonuclear N₂⁺, where symmetry considerations prohibit X and A state perturbations, these processes proceed as rapidly as in heteronuclear species where perturbations are known to be present in the free molecule.