Multiphoton Photodissociation of Several Halocarbons.

The 2 x 193 nm photodissociations of CHCl$_{3 }$, CFCl$_{3}$, CF$_{3}$CCl$_{3}$, CCl$_{4}$, CH$_{3}$-CCl$_{3}$ and CH$_{3}$CH$_{2}$-CCl$_{3 }$have been examined using dispersed fluorescence. It was found that the initial photodissociation of CHCl$_{3}$ forms large amounts of CH(A$^{2}\Delta )$ while the photodissociation of CCl$_{4}$ forms lesser but still significant amounts of CX(A$^{2}\Delta )$ The photodissociations of CH$_{3}$-CCl$_{3}$ and CH$_{3}$CH$_{2}$-CCl$_{3 }$ produce CH$_{3}$-C and CH$_{3}$CH$_{2}$-C presumably in the A$^{2}\Delta $ state. The exact photoproducts of the fluorinated species are currently unknown. Fluorescence rise time measurements show that the CH(A$^{2}\Delta )$ and CCl(A$^{2}\Delta )$ photoproducts quickly react to form C$_{2 }$(d$^{3}\Pi _{g})$.~However, formation of C$_{2 }$(d$^{3}\Pi _{g})$ is attenuated when the primary photoproducts are CF$_{3}$C, CH$_{3}$-C and CH$_{3}$CH$_{2}$-C and disappears completely when it is CF. In addition, the atomic and molecular halogen photoproduct channels were investigated using ab initio calculations. Intrinsic Reaction Coordinate calculations were performed at the MP2 level of theory using the LANL2DZ basis set in order to characterize the dissociation pathways for all of species investigation.. The results of the calculations show the presence of three transition states and an ion-pair isomer intermediate for all molecules. The broken symmetry structure of the transition states for the formation of molecular bromine is in agreement with the first step of the addition mechanism proposed by Cain and co-workers for CX$_{2 }$+Y$_{2}$ reactions. ~ ~ ~ ~