Non-adiabatic interactions in excitedC ₂ H molecules and their relationship toC ₂ formation in comets

Abstract

A unified picture of the photodissociation of theC ₂ H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC ₂ are formed during the photodissociation of theC ₂ H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC ₂ radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC ₂ fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC ₂ H radical. The photochemically active states are the 2²∑₊, 2²II, 3²II, and 3²∑₊, with photodissociation rate constants of 1.0×10⁻⁶, 4.0×10⁻⁶, 0.7×10⁻⁶, and 1.3×10⁻⁶s⁻¹, respectively. These rate constants lead to a total photochemical lifetime of 1.4×10⁵ s.