Rotational-state resolved coupling of CH A 2Δ and B 2Σ- in collisions with CO2

Abstract

Inelastic collisions of selected rovibronic levels of the CH A2Δ and B2Σ− states with CO2 have been investigated experimentally. Initial levels in A2Δ, = 1 and B2Σ−, = 0 were prepared by selective laser excitation. Time-gated emission from the initial and product levels was dispersed at rotational resolution for the first time. Conditions were established where 60–80% of the population remained in the initially populated rovibronic level. The rotational state propensities were established for electronically inelastic collisional transfer from A2Δ, = 1 to B2Σ−, = 0. The reverse transfer is complicated by the spectral overlap of the A–X(1,1) and (0,0) bands. A high N′ component can unambiguously be assigned to A2Δ, = 0. The distributions of ΔN for inter-electronic state transfer are generally broader than for pure rotational energy transfer within either state, particularly at higher N where the results are more clear-cut. For these levels the redistribution of rotational population during electronic transfer is, however, less complete than implied by a statistical prior distribution. This suggests that the electronic state-changing collisions sample more strongly interacting regions of the CH···CO2 excited state potentials than are required to cause pure rotational relaxation. These interactions are not sufficiently strong, however, to promote complete rotational (or vibrational) energy redistribution during the collision.