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Using tunable vacuum-UV radiation from a synchrotron source, fragmentation studies have been performed on the valence states of CF 3 Cl + and CF 3 Br + 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 (Mol. 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 (Ã 2 A 1 , B 2 A 2 , C 2 E, and D 2 E) 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. X Abstract published in AdVance ACS Abstracts, February 15, 1996. 4350

A new design of threshold electron analyser is presented which takes advantage of chromatic aberrations in electron lenses. Calculations show it to be capable of both high resolution ( approximately 3.5 meV half-width) and high collection efficiency (100% for zero energy electrons) even though a high extraction field (20 V Cm-1) is used in order to extract ion fragments with equally high efficiency. It is therefore particularly suitable for use in electron-ion coincidence experiments. Representative results are shown for the photoionization of argon and oxygen by vacuum ultraviolet radiation from the Daresbury synchrotron radiation source. These demonstrate the potential of both the electron and ion analysers to be used for such coincidence experiments.

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