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

Inelastic collisions of selected rovibronic levels of the CH A ²? and B ²?^- states with CO₂ have been investigated experimentally. Initial levels in A ²?, ? = 1 and B ²?^-, ?= 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 A ²?, ?= 1 to B ²?^-, ?= 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 A ²?, ?= 0. The distributions of ?N 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.