The use of threshold photoelectron-photoion coincidence spectroscopy to probe the spectroscopic and dynamic properties of the valence states of CCl₃F⁺, CCl₃H⁺ and CCl₃Br⁺

Using tunable vacuum-ultraviolet radiation from a synchrotron source in the range 10–25 eV, threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to determine the decay pathways of the valence electronic states of CCl3X+ (X=F, H, Br). TPEPICO spectra are recorded continuously as a function of photon energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are also recorded with improved time resolution, allowing total mean translational kinetic energy releases, 〈KE〉t , into some dissociation channels to be determined. By comparing 〈KE〉t values for single bond-fission processes (i.e. cleavage of a C–Cl or C–X bond) with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. Excited states of all three parent cations show evidence for isolated-state behaviour, and the 〈KE〉t values suggest a relationship between the part of the molecule where ionisation occurs and the bond that breaks to form daughter ion plus neutral atom products; impulsive values of 〈KE〉t are more likely to be obtained when the breaking bond lies close to the part of the molecule where ionisation occurs, statistical values when ionisation occurs further away from the breaking bond. At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond. With CCl3F and CCl3Br, the appearance energies of the daughter ions are close to the thermochemical energy for production of that ion with isolated neutral atoms, suggesting strongly that these ions form by bond-fission processes only. With CCl3H, at certain energies some fragment ions can only form with molecular neutral fragments (e.g. CCl2++HCl), involving bond-breaking and bond-making processes. It is suggested that this phenomenon is related to the small size of the hydrogen atom, and hence less steric hindrance in a tightly constrained transition state along the reaction coordinate.