Collisional deactivation of selectively excited N+2

Individual vibrational levels of the A 2Πu state of the N+2 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 2Πu state are rapidly deactivated by collisions with He into high vibrational levels of the ground X 2Σ+g state. Time and wavelength resolved studies show clearly that the population of a given high vibrational level of the X 2Σ+g state returns into lower A 2Πu 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 N+2, 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.