FRAGMENTATION OF THE VALENCE ELECTRONIC STATES OF CF3CL-PHOTOION COINCIDENCE SPECTROSCOPY( AND CF3BR+ STUDIED BY THRESHOLD PHOTOELECTRON)

Using tunable vacuum-UV radiation from a synchrotron source, fragmenta tion studies have been performed on the valence states of CF3Cl+ and C F3Br+ 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 re sults agree well with complementary studies of other groups. In the fo rmer case, threshold photoelectron-photoion coincidence (TPEPICO) spec troscopy is used to measure the decay pathways of individual valence s tates of CF(3)X(+) (X = Cl, Br). TPEPICO spectra are recorded both con tinuously as a function of photon energy, allowing threshold photoelec tron spectra and yields of the fragment ions to be obtained, and at a fixed photon energy with good time resolution, allowing mean translati onal 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. 1 980, 39, 311). The CF(3)X molecules are not big enough to belong to th e 'large molecule' limit. By comparing the [KE](t) values with those p redicted for the limiting extremes of a statistical and an impulsive d issociation process, some information on the nature of the photodissoc iation 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 t he 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 furth est away from the bond that breaks. Conversely, impulsive (and hence l arger) values of [KE](t) are more likely when the breaking bond lies c lose to the part of the molecule where ionization occurs.