Vacuum-UV and electron dissociative ionisation studies of SF 5Cl

Abstract

A complementary study of the interaction of SF 5Cl in the gas phase with vacuum-UV photons and low-energy electrons from the onset of ionisation, ca. 12 eV, up to 20 eV is presented. The photon-induced experiments have used tunable vacuum-UV radiation from a synchrotron and threshold photoelectron photoion coincidence spectroscopy for product ion detection, the electron-induced experiments a trochoidal electron monochromator and a quadrupole mass spectrometer. The strengths and limitations of both experiments are contrasted, the main difference being the absence of state selectivity in the electron-induced study. The parent cation is not observed in either study, suggesting that its ground electronic state is repulsive following Franck–Condon vertical excitation. The fragment cations SF 5 +, SF 4Cl +, SF 4 + and SF 3 + have been observed in both studies, with reasonable agreement in their threshold appearance energies. Using a variant of threshold photoelectron photoion coincidence spectroscopy applicable when the ground state of the parent cation is repulsive, the first dissociative ionisation energy of SF 5Cl is determined to be 12.3 ± 0.2 eV, leading to a value for the adiabatic ionisation energy for the SF 5 radical of 9.92 ± 0.28 eV. The electron-induced experiment is sensitive to ion-pair production, and onsets for F + and Cl + production have been observed which are only possible energetically if the accompanying fragments are the anions SF 4Cl − and SF 5 −, respectively. A lower limit for the electron affinity of the SF 4Cl radical of 4.88 eV is determined, a value confirmed by ab initio calculations. The electron-induced experiment is very sensitive to gas impurities, and the effects of minute quantities of SF 4, FCl, Cl 2 and possibly SF 2 in the gas sample are observed.