NONADIABATIC INTERACTIONS IN EXCITED C2H MOLECULES AND THEIR RELATIONSHIP TO C-2 FORMATION IN COMETS

A unified picture of the photodissociation of the C2H radical has been developed using the results from the latest experimental and theoreti cal work. This picture shows that a variety of electronic states of C- 2 are formed during the photodissociation of the C2H radical even if p hotoexcitation accesses only one excited state. This is because the ex cited states have many avoided crossings 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 on e electronic state to another and access new final electronic states o f the C-2 radical. The complexity of the excited state surfaces also e xplains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate t hrough a direct path are limited by dynamics to produce C-2 fragments with a modest amount of rotational energy, whereas those that dissocia te by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finall y, 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 the C2H radical. The pho tochemically active states are the 2(2) Sigma(+), 2(2) Pi, 3(2) Pi, an d 3(2) Sigma(+), with photodissociation rate constants of 1.0 x 10(-6) , 4.0 x 10(-6), 0.7 x 10(-6), and 1.3 x 10(-6) s(-1), respectively. Th ese rate constants lead to a total photochemical lifetime of 1.4 x 10( 5) s.