Rotational energy transfer within the B 3Πg v=3 manifold of molecular nitrogen

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Abstract
An optical–optical double resonance experiment has been carried out to study rotationally inelastic collisions of N2 B 3Π g v=3 by argon with initial and final state resolution. Nitrogen molecules in the metastable A state are generated by collisional excitation transfer from metastable argon atoms in a flow system. Specific B state rotational levels are prepared by pulsed pump laser excitation of isolated rotational lines in the B 3Π g –A 3Σ+ u (3,0) band near 688 nm. After a short delay, a probe laser interrogates the rotational populations in the B state by fluorescence excitation in the C 3Π u –B 3Π g (0,3) band near 406 nm. Collisional transfer from incident levels in all three spin–orbit manifolds of the B state was investigated. For molecules initially in the F 1 (Ω=0) manifold, a preference for conservation of fine‐structure label with even ΔJ changes was observed. This propensity is very pronounced for the J=0 level but is considerably relaxed for the higher levels investigated. By contrast, inelastic collisions involving the F 2 (Ω=1) and F 3 (Ω=2) manifolds do not exhibit a significant propensity to conserve fine‐structure label. A slight residual preference for even ΔJ changes is observed in collisional transitions within the F 2manifold. These experimental results are compared to the propensity rules expected for homonuclear 3Π rotationally inelastic collisions, both in the case (a) and (b) limits. The reduction of the predicted propensities by the transition to intermediate case coupling and ‘‘orbital‐correlated’’ scattering is discussed. The latter term refers to the difference potential for the N2–Ar interaction when the N2 unfilled π orbital is in or perpendicular to the triatomic plane. An unsuccessful attempt to detect collisional interelectronic transfer from the B state to the W 3Δ u state by G–W laser fluorescence excitation is also reported.