TIME-RESOLVED GAS ELECTRON-DIFFRACTION STUDY OF THE 193-NM PHOTOLYSISOF 1,2-DICHLOROETHENES

We have employed time-resolved gas electron diffraction. using an exci te-and-probe technique developed on the basis of various precedents,1- 2 for study of the 193-nm photodissociation of isomeric 1,2-dichloroet henes. The procedure3 involves a pulsed electron beam synchronous with photoexcitation and on-line electron diffraction intensity recording with a photodiode array. When the time delay between excitation and di agnostic electron scattering is 15 ns, only fragmentation and no cis-t rans somerization is observed. When the time delay is several millisec onds, cis-trans isomerization is prevalent. This finding confirms the thesis that, because of faster intervening dissociative processes, cis -trans isomerization is not a primary photochemically driven unimolecu lar rearrangement of 193-nm irradiated dichloroethenes but hinges upon intermolecular collisions.