Shining new light on the multifaceted dissociative photoionisation dynamics of CCl4

Abstract

Internal energy selected carbon tetrachloride cations have been prepared by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet radiation. The threshold photoelectron spectrum shows a newly observed vibrational progression corresponding to the ν₂(e) scissors mode of CCl₄⁺ in the third, ²E band. Ab initio results on the first four doublet and lowest-lying quartet electronic states along the Cl₃C⁺–Cl dissociation coordinate show the state to be strongly bound, and support its relative longevity. The ²T₁ and à ²T₂ cationic states, on the other hand, are barely bound and dissociate promptly. The ²T₂ state may intersystem cross to the quartet ã state, which dissociates to a triplet state of the CCl₃⁺ fragment ion. This path is unique among analogous MX₄⁺ (M = C, Si, Ge; X = F, Cl, Br) systems, among which several have been shown to have long-lived states, which decay by fluorescence. The breakdown diagram, recorded here for the first time for the complete valence photoionisation energy range of CCl₄, is interpreted in the context of literature based and CBS-QB3, G4, and W1U computed dissociative photoionisation energies. No Cl₂-loss channel is observed in association with the CCl₂⁺ or CCl⁺ fragments below the 2 or 3 Cl-loss reaction energies, and Cl₂ loss is unlikely to be a major channel above them. The breakdown diagram is modelled based on the calculated dissociative photoionisation onsets and assuming a statistical redistribution of the excess energy. The model indicates that dissociation is not impulsive at higher energies, and confirms that the ²T₂ state of CCl₄⁺ forms triplet-state CCl₃⁺ fragments with some of the excess energy trapped as electronic excitation energy in CCl₃⁺.