RESUMO
The threshold photoelectron spectrum and threshold photoelectron-photoion coincidence spectra of CHCl2F, CHClF2 and CH2ClF are reported in the range 11.3-24.8 eV. Tunable photoionizing radiation with a resolution of 0.3 nm is provided from a synchrotron source with a vacuum-UV monochromator. The coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Energetic comparisons suggest that the major products of the titled molecules dissociate in a similar manner at low photon energy, with the parent and first fragment ion, corresponding to cleavage of the weakest bond, appearing at their thermochemical thresholds. The second major ion, corresponding to cleavage of the second weakest bond, is formed ca. 1 eV higher than its predicted threshold, this disparity implying state-selected dissociation. CHCl2F and CHClF2 fragment in a similar manner at higher photon energies, with minor ions formed by the cleavage of three bonds possessing lower appearance energies than fragment ions formed by the cleavage of two bonds. CH2ClF displays the more expected behaviour, namely sequential bond cleavage as the photon energy increases. These observations can be rationalised in terms of the height of the barrier on the exit channel, as determined by the steric bulk of the leaving group. For the three titled molecules, mean translational kinetic energy releases have also been measured into the channels involving C-F or C-Cl bond fission. These data infer that impulsive dissociations occur at lower energy, with a trend towards statistical behaviour with increasing photon energy. Competition between statistical and impulsive processes is observed, for example C-Cl vs. C-F bond cleavage in CHCl2F+ and CHClF2+.