Relaxation processes in excited CO states. Part III: Inelastic cross-relaxation and radiative transitions caused by weak perturbations

Abstract

An analysis has been carried out of the pathways by which low lying vibrational levels of the d3Δ and e3Σ−- states of CO are collisionally populated from higher levels produced by resonance absorption of CO resonance radiation. It has been established that a principal source of the d3Δν=3 and d3Δν=4 levels is energy transfer between ground state CO and an e3Σν=4− molecule. The experiments suggest that transfer of the entire electronic energy of the e3Σν=4− molecule occurs. The highly populated d3Δν=5 level is a less important source of the lower 3Δ levels than is e3Σν=4−. In the presence of several torr Ar, cross relaxation between the A1Π state and the triplet states becomes very important, and appears to take place elastically. For example, A1Πν=0 is a source of d3Δν=4, but not of d3Δν=3. Whereas earlier work had demonstrated that the strong spin-orbit perturbations of d3Δν=5 by A1Πν=1 caused the d3Δν=5 − X1Σ+ transition to be intense enough to observe resonance radiation effects, we have now shown that similar effects, although considerably weaker, may be observed with d3Δν=4 and d3Δν=3, where in the latter case the perturbing levels of A1Πν=0 lie 750 cm−1 higher. For the ν′ = 3, 4, and 5 bands of the d3Δ-X1Σ+ transition, the expected ratio of the CO absorption strengths can be calculated, based on the relevant A1Π-X1Σ+ Franck-Condon factors. The relative values are in reasonable agreement, and also agree with the data obtained for the A1Π ← X1Σ+ transition. Observations have been made on the e3Σ− → a3Π (1−ν″) bands, which have not previously been observed, and reason why transitions from the strongly perturbed e3Σν=1− level are not a more prominent feature in this system are discussed.