Photodissociation dynamics of free radicals

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Photochemical reaction of free radicals plays an important role in the atmospheric and interstellar chemical reactions. The dissociation dynamics of C_nH_m have been studied by using two dimensional ion imaging method.In the photoexcitation into (pi, pi<@D1**@>D1) state of vinylchloride dissociates into vinyl radical and chlorine atom. The translational energy distribution consists of two components ; one is a Gaussian-like translational energy distribution with high anisotropy, beta=1.0(]SY.+-。[)0.2, while the other is a statistical distribution with low anisotropy, beta=0.3 (]SY.+-。[) 0.2. The dissociation rate constants for the low translational energy component is estimated to be 10<@D111@>D1- 10<@D112@>D1 s<@D1-1@>D1.In the case of ultraviolet photodissociation of dichloroethylenes, there exists secondary dissociation of C_2H_2Cl radical following to chlorine atom and acetylene. At 193 nm photoexcitation, about 100% of C_2H_2Cl radical undergo to the secondary dissociation.