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 d³Δ_ν=3 and d³Δ_ν=4 levels is energy transfer between ground state CO and an $e^3{\Sigma }_{\text{ν=4}}^{-}$ molecule. The experiments suggest that transfer of the entire electronic energy of the $e^3{\Sigma }_{\text{ν=4}}^{-}$ molecule occurs. The highly populated d³Δ_ν=5 level is a less important source of the lower ³Δ levels than is $e^3{\Sigma }_{\text{ν=4}}^{-}$ . 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, A¹Π_ν=0 is a source of d³Δ_ν=4, but not of d³Δ_ν=3. Whereas earlier work had demonstrated that the strong spin‐orbit perturbations of d³Δ_ν=5 by A¹Π_ν=1 caused the d³Δ_ν=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 d³Δ_ν=4 and d³Δ_ν=3, where in the latter case the perturbing levels of A¹Π_ν=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 $e^3{\Sigma }_{\text{ν=1}}^{-}$ level are not a more prominent feature in this system are discussed.