Fragmentation of valence electronic states of CHF 2CF 3+ studied by threshold photoelectron-photoion coincidence (TPEPICO) techniques in the photon energy range 12-25 eV

Vacuum ultraviolet synchrotron radiation and threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy have been used to study the decay dynamics of the valence electronic states of CHF ₂CF ₃⁺. The threshold photoelectron spectrum (TPES) and ion yield curves of the observed fragments have been recorded in the photon energy range 12-25 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight (TOF) mass spectrometry, respectively. Using a combination of the measured TPES and ab initio molecular orbital calculations, we conclude that the CHF ₂CF ₃⁺ cation adopts a staggered C _s geometry, the X∼ ²A^' ground state being formed by electron removal from the 18a ^' σ-bonding orbital of CHF ₂CF ₃. Upon ionisation, large geometry changes and broad spectral bands are both predicted and observed. The next outer-valence orbitals of CHF ₂CF ₃, 17a ^' and 11a ^'', are predominantly associated with fluorine 2p orbitals located on the CHF ₂ group. Translational kinetic energy releases are determined from fixed-energy TPEPICO-TOF spectra. The ground state of CHF ₂CF ₃⁺ dissociates through C-C bond cleavage with a relatively small release of energy. By contrast the à and B∼ states dissociate rapidly by an impulsive mechanism with a larger fractional release of energy to CHFCF ₃⁺+F. Upper limits for the ionisation threshold of CHF ₂CF ₃ (12.70±0.05 eV) and the enthalpy of formation of CHFCF ₃⁺ at 298 K ( 299±7 kJ mol ^-1) are determined.