On the adiabatic ionisation energy of the CF₃ free radical

The different methods that have been used to measure the adiabatic ionisation energy of the CF3 free radical in the gas phase are reviewed, with values covering the wide range 8.6–9.1 eV. In a study of vacuum-UV photofragmentation of perfluoropropane, C3F8, using the threshold photoelectron–photoion coincidence technique, our value for the appearance energy of CF3+ at 298 K, 13.0±0.1 eV, leads to an upper limit for the adiabatic ionisation energy of CF3 of 8.8±0.2 eV, in good agreement both with that deduced from a low-temperature photoionisation mass spectrometric (PIMS) study of CF3Br and with values obtained from three ion–molecule bimolecular studies. It is lower, however, than the value obtained indirectly from a PIMS study of C2F4 and from ab initio calculations.