Rotational-state resolved coupling of CH A2Δ and B2Σ− 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.