Shining new light on the multifaceted dissociative photoionisation dynamics of CCl4

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+.