Fragmentation of the valence states of CF2Cl2+, CF2H2+, and CF2Br2+ studied by threshold photoelectron–photoion coincidence spectroscopy

Abstract

Using tunable vacuum-ultraviolet radiation from a synchrotron, the decay pathways of the valence electronic states of CF 2 X 2 + (X=Cl,H,Br) At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond.With CF 2 Cl 2 and CF 2 Br 2 , the appearance energies of the fragment ions are close to the thermochemical energy for production of that ion with neutral atoms, suggesting that these ions form by bond-fission processes only. With CF 2 H 2 , the one ion unambiguously assigned, CFH + , can only form at certain energies with molecular neutral fragments (i.e. CFH + + HF), involving simultaneous bond-breaking and bond-making processes. The <KE> t values for cleavage of a single C−F or C−X bond suggest a relationship between the part of the molecule where ionisation occurs and the bond that breaks ; impulsive values of <KE> t are more likely to be obtained when the breaking bond lies close to the part of the molecule from which ionisation occurs, statistical values when ionisation occurs further away from the breaking bond. Furthermore, for all CF 2 X 2 + cations there is a trend from impulsive to statistical behaviour as the photon energy is increased.4

The photoionization dynamics of the three structural isomers of dichloroethene

“…For dissociation of the B state of C 2 H 2 Cl 2 + , the value of <f> t , 0.32-0.36, is very close to the pure-impulsive value of 0.40, whereas dissociation of higherlying valence states leads to <f> t values less than 0.10 and much closer to the statistical value of 0.08. This behaviour has been observed in previous studies by our group for similar-sized molecules [34], and is characteristic of behaviour based on the theories of intramolecular vibrational energy redistribution. That is, as the photon energy increases, successively more electronic and vibrational states of the parent ion can be accessed, so there is less chance of fragmentation occurring along a specific potential energy surface via a selective C−Cl bond dissociation.…”

Fragmentation of Energy-Selected SF5CF3+ Probed by Threshold Photoelectron Photoion Coincidence Spectroscopy: Bond Dissociation Energy of SF5-CF3 and Its Atmospheric Implications

“…This value of <KE> t corresponds to 21% of the available energy, in excellent agreement with the prediction of the pure-impulsive model. 23 This is to be expected, because the ground state of SF 5 CF 3 + at the Franck-Condon maximum lies over 1 eV higher in energy than the dissociative limit to CF 39,40 In all these cases, the ground state of the parent cation in the Franck-Condon region lies above the first dissociative ionisation energy.…”

Threshold photoelectron–photoion coincidence spectroscopy study of CHCl2F+, CHClF+2 and CH2ClF+: Steric influence of the chlorine, fluorine and hydrogen atoms

“…This trend has also been observed in the TPEPICO studies of the analogous parent ions CH 2 F 2 + and CH 2 Cl 2 + . 33,36 In the present work, the first and second ions observed are formed via C-Cl and C-F bond fission, respectively. This holds true for the whole of this study, regardless of the parent molecule.…”

“…These are assigned as vibrational structure within the X % state, very similar to previous TPEPICO data on CH 2 F 2 obtained by our group. 33 The structure in both molecules arises from excitation of the ν 2 bending mode of the CH 2 moiety, resulting primarily from the extensive decrease in the HCH bond angle upon ionization. 34 The observed spacing of 1290 cm -1 is in excellent agreement with ab initio and non-threshold photoelectron studies.…”