Comprehensive vacuum ultraviolet photoionization study of the CF₃^• trifluoromethyl radical using synchrotron radiation

The trifluoromethyl radical, CF₃^•, is studied for the first time by means of threshold photoelectron spectroscopy (TPES). The radical is produced in the gas phase using the flash-pyrolysis technique from hexafluoroethane as a precursor. CF₃⁺ total ion yield and mass-selected TPES of the radical are recorded using a spectrometer based upon velocity map imaging and Wiley-McLaren time-of-flight coupled to the synchrotron radiation. The high resolution of the instrument and of the photons allows the observation of rich vibrational progressions in the TPES of CF₃^•. By using Franck-Condon factors computed by Bowman and coworkers, we have been able to simulate the TPES. The initial vibrational temperature of the radical beam has been evaluated at 350 ± 70 K. The structures have been identified as transitions between (n₁,n₂) and (n₁⁺,n₂⁺) vibrational levels of CF₃ and CF₃⁺ with small excitation of the breathing mode, ν₁⁺_, and large excitation (n₂⁺ = 10–26) of the umbrella mode, ν₂⁺, in the cation. From the energy separation between the two resolved peaks of each band, a value of 994 ± 16 cm⁻¹ has been derived for the ν₁⁺ breathing frequency of CF₃⁺. For the high-lying n₂⁺ levels, the apparent ν₂⁺ umbrella spacing, 820 ± 14 cm⁻¹, is fairly constant. Taking into account the ν₂⁺ anharmonicity calculated by Bowman and coworkers, we have deduced ν₂⁺ = 809 ± 14 cm⁻¹, and semi-empirical estimations of the adiabatic ionization energy IE_ad.(CF₃^•) are proposed in good agreement with most of previous works. A value of the vertical ionization potential, IE_vert.(CF₃^•) = 11.02 eV, has been derived from the observation of a photoelectron spectrum recorded at a fixed photon energy of 12 eV.