COLLISION-INDUCED TRANSITIONS FROM N2(A' 5-SIGMA-G+) TO N2(B 3-PI-G) VIA THE GATEWAY MECHANISM

A beam containing N2 in long-lived states was allowed to interact with target particles in a collision cell. Intramolecular transitions A --> B and W --> B are induced, such as were studied earlier by us by means of the subsequent B --> A emission [R. Bachmann, X. Li, Ch. Ottinger, and A. F. Vilesov, J. Chem. Phys. 96, 5151 (1992)]. In the present work the product emission was observed under high resolution (1 angstrom FWHM). Most of the B --> A bands show the typical quasithermal rotational contours. However, in the emissions from the B state vibrational level upsilon = 10 sharp superimposed features were observed. They were assigned to transitions from the rotation/fine structure/LAMBDA sublevel 3PI2e(12). This particular level is perturbed by the A' 5SIGMA(g)+ state, serving as a so-called gateway to allow the otherwise spin-forbidden 5SIGMA(g)+ --> 3PI(g) collision-induced transition. According to this mechanism, the collisions scramble only the levels within the A' state, while the A' --> B transition occurs spontaneously through S/O coupling. A similar, less well characterized perturbation was also found for the B, upsilon = 12, 3PI0e(16) level. Apart from the gateway mechanism, any direct collisional coupling between the A' and B states appears to be insignificant. This result is contrary to one of the long-standing assumptions on the origin of the Lewis-Rayleigh nitrogen afterglow.