The photoionization dynamics of the three structural isomers of dichloroethene

Abstract

In honour of Professor John Brown's 65 th birthday festschrift __________________________________________ Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron spectrum, threshold photoelectron photoion coincidence spectrum and ion breakdown diagram of the 1,1, cis-1,2 and trans-1,2 isomers of C 2 H 2 Cl 2 have been recorded in the range 9−23 eV. The energies of the peaks in the threshold photoelectron spectrum are in good agreement with outer-valence Greens function caculations. The major difference between the isomers, both predicted and observed experimentally is that the F and G states of

Threshold photoelectron photoion coincidence spectroscopy of trichloroethene and tetrachloroethene

“…For tetrachloroethene, a fourth product is seen in which three chlorine atoms are lost. This is in agreement with our photoionisation results for the dichloroethenes [1]. Examination of thermochemistry and branching ratios from this and the dichloroethene study suggest that when two chlorine atoms are lost following photoionisation, they are lost as two Cl atoms for the three isomers of dichloroethene and trichloroethene, but as molecular Cl 2 from C 2 Cl 4 .…”

“…The available energy is shared between states, and hence is less likely to be localised in a vibrational mode that would lead to mode-specific dissociation of the C 2 H x Cl 4-x + ion. The same pattern was observed in our study of the three dichloroethenes [1]. This seems to confirm that the fragmentation of the chloroethene cations is statistical in nature at higher energies, but becomes more non-statistical as energies approach their threshold values.…”

“…At higher energies, for both molecules <f> t approaches 0.08, the value calculated as the lower statistical limit for fractional kinetic energy release. Such behaviour was also seen in previous studies by us on the dichloroethenes and similar-sized molecules [1,19]. The fact that <f> t decreases with increasing energy is easily reconciled with the theories of intramolecular energy redistribution.…”

“…For (E)-1,2-dichloroethene where the chlorine atoms are on opposite sides of the molecule, it harder to see this happening as the transition state will be highly constrained, unless upon ionisation the C=C bond becomes weak enough for pseudo-rotation to occur. For the three dichloroethenes [1] and trichloroethene, the consistency of the onset for this channel suggests that the same process must be occurring. However, for tetrachloroethene this product is formed at 12.52 eV, around 2 eV lower than the channel involving formation of two chlorine atoms but above the limit for the …”

“…We have studied their photoionisation dynamics using both threshold photoelectron photoion coincidence (TPEPICO) spectroscopy and a selected ion flow tube to measure the reactions of these molecules with small cations. Previously we have published results for photoionisation and the ion-molecule reactions of the three isomers of dichloroethene [1,2], where the principal aim was to look for examples of isomeric effects in the reactions. The photoionisation results for trichloroethene and tetrachloroethene are described in this paper, and the results for reactions of a series of cations with monochloroethene, trichloroethene and tetrachloroethene will be published elsewhere [3].…”