Collision-induced transitions from N2(A′ 5Σ+g) to N2(B 3Πg) 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 Å FWHM). Most of the B→A bands show the typical quasithermal rotational contours. However, in the emissions from the B state vibrational level v=10 sharp superimposed features were observed. They were assigned to transitions from the rotation/fine structure/Λ sublevel 3Πe2(12). This particular level is perturbed by the A′ 5Σ+g state, serving as a so-called gateway to allow the otherwise spin-forbidden 5Σ+g→3Πg 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,v=12, 3Πe0(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.