Fragmentation of the valence electronic states of CF3Cl+ and CF3Br+ studied by threshold photoelectron-photoion coincidence spectroscopy

Using tunable vacuum-UV radiation from a synchrotron source, fragmentation studies have been performed on the valence states of CF3Cl+ and CF3Br+ in the energy range 10-25 eV both with and without energy state selectivity of the photoelectrons. In the latter case, thresholds and yield curves are obtained for the parent and fragment ions, and the results agree well with complementary studies of other groups. In the former case, threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy is used to measure the decay pathways of individual valence states of CF(3)X(+) (X = Cl, Br). TPEPICO spectra are recorded both continuously as a function of photon energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained, and at a fixed photon energy with good time resolution, allowing mean translational kinetic energy releases, [KE](t), to be measured. [KE](t) values are measured at the Franck-Condon maxima of the valence states of CF(3)X(+), substantially extending the earlier work of Powis (Mel. Phys. 1980, 39, 311). The CF(3)X molecules are not big enough to belong to the 'large molecule' limit. By comparing the [KE](t) values with those predicted for the limiting extremes of a statistical and an impulsive dissociation process, some information on the nature of the photodissociation dynamics can be inferred. All the first four excited states ((A) over tilde (2)A(1), (B) over tilde (2)A(2), (C) over tilde (2)E, and (D) over tilde 2E) of CF(3)X(+), where dissociation occurs by loss of an F or X atom, show evidence of isolated-state behavior, and the [KE](t) values suggest that there is a relationship between the part of the parent molecule where ionization occurs and the bond that breaks to form the fragment ion + atom products. Statistical values of [KE](t) are most likely when ionization occurs at a part of the molecule furthest away from the bond that breaks. Conversely, impulsive (and hence larger) values of [KE](t) are more likely when the breaking bond lies close to the part of the molecule where ionization occurs.