Shining new light on the multifaceted dissociative photoionisation dynamics of CCl4.

Jonelle Harvey,Richard P Tuckett,Andras Bodi

doi:10.1039/c4cp03009e

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, B̃(2)E band. Ab initio results on the first four doublet and lowest-lying quartet electronic states along the Cl3C(+)-Cl dissociation coordinate show the B̃ state to be strongly bound, and support its relative longevity. The X̃(2)T1 and Ã(2)T2 cationic states, on the other hand, are barely bound and dissociate promptly. The C̃(2)T2 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 C̃ 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 C̃(2)T2 state of CCl4(+) forms triplet-state CCl3(+) fragments with some of the excess energy trapped as electronic excitation energy in CCl3(+).

Highlights

Internal energy selected carbon tetrachloride cations have been prepared by imaging photoelectron photoion coincidence spectroscopy using synchrotron vacuum ultraviolet radiation
The model indicates that dissociation is not impulsive at higher energies, and confirms that the C 2T2 state of CCl4+ forms triplet-state CCl3+ fragments with some of the excess energy trapped as electronic excitation energy in CCl3+
Excitation of non-totally-symmetric vibrations such as n2 of e symmetry are formally forbidden in photoelectron spectroscopy unless the point group of the molecule changes

Summary

Introduction

Internal energy selected carbon tetrachloride cations have been prepared by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet radiation. Photoionisation and tunable vacuum ultraviolet (VUV) light.[4] Their unimolecular dissociation dynamics can be studied by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy at the VUV beamline of the Swiss Light Source.[5,6] Dissociative photoionisation thresholds and measured dissociation rates in the 103–107 sÀ1 range can routinely be interpreted by statistical theory,[7] to derive thermochemical data with accuracy sometimes below 1 kJ molÀ1.8,9 Statistical theory assumes that the complete phase space is explored by the system, and the dissociation rate essentially corresponds to the probability that sufficient kinetic energy is present in the reaction coordinate. In mono-, di-, and trifluoroethene, it was shown that C-state cations can undergo internal conversion to the Bstate, either on a bound or a repulsive domain along the C–F bond stretch coordinate Should the latter take place, the product Bstate cation suffers non-statistical F-loss promptly. While the dissociation is impulsive close to the onset of ionisation, it takes on a statistical character as coupling to the repulsive ground state of the parent cation gets weaker with higher electronic excitation

Methods

Results

Conclusion