State resolved measurements of $aC1H2$ removal confirm predictions of the gateway model for electronic quenching Available to Purchase

Collisional quenching of electronically excited states by inert gases is a fundamental physical process. For reactive excited species such as singlet methylene, $C1H2$⁠, the competition between relaxation and reaction has important implications in practical systems such as combustion. The gateway model has previously been applied to the relaxation of $C1H2$ by inert gases [U. Bley and F. Temps, J. Chem. Phys. 98, 1058 (1993)]. In this model, gateway states with mixed singlet and triplet character allow conversion between the two electronic states. The gateway model makes very specific predictions about the relative relaxation rates of ortho and para quantum states of methylene at low temperatures; relaxation from para gateway states leads to faster deactivation independent of the nature of the collision partner. Experimental data are reported here which for the first time confirm these predictions at low temperatures for helium. However, it was found that in contrast with the model predictions, the magnitude of the effect decreases with increasing size of the collision partner. It is proposed that the attractive potential energy surface for larger colliders allows alternative gateway states to contribute to relaxation removing the dominance of the para gateway states.