Model studies of the kinetics of collisional population transfer between dark and radiating excited electronic states: CaO(A′ 1Π)+N2O⇄CaO(A 1Σ+)+N2O

Abstract

In a previous article [J. Chem. Phys. 76, 429 (1982)] we presented a model for collisional energy transfer between dark (A′ 1Π, a 3Π) and radiating (A 1Σ+) excited electronic states of the alkaline earth oxides. The inelastic transitions result from coupling between the electric dipole of the collision partner and a transition dipole in the alkaline earth oxide, which arises from the non-Born–Oppenheimer coupling between the rovibronic manifolds of two different electronic states. Here we use the rate constants reported in the previous article to investigate population flow from the nominally v = 6 manifold of the A′ 1Π state of CaO into the nominally v = 0 manifold of the A 1Σ+ state, induced by collisions with N2O. The master equation is solved in the steady state limit. The resulting populations are then used to simulate the pressure dependence of the (0–0) band of the CaO A 1Σ+→X 1Σ+ spectrum, and investigate the variation of the predicted spectral features with respect to changing the conditions which characterize both the rate of formation of the excited states as well as the rate of translational loss out of the zone of observation. At low to moderate target gas pressure the major effect of collisions is A′→A population transfer in the region of the largest coupling between the two rotational manifolds. The concomitant intensity increase and the spectral variation in the A state emission are qualitatively similar to features seen in experimental spectra of Irvin and Dagdigian, which we present here. The pressure dependent changes in the emission spectra are extremely sensitive to the assumed rate of translational loss, which must be taken into account in any comparison between experiments carried out under flame and molecular beam conditions. Finally, we demonstrate that although it is possible to fit with a simple two-level kinetic model the observed pressure dependence of the total A state emission, summed over rotational lines, the resulting kinetic parameters which characterize this fit may bear little relation to those which characterize the master equation for the coupling between the underlying rotational manifolds.