Fragmentation of valence electronic states of <f>CHF2CF3+</f> 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 <f>CHF2CF3+</f>. 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 <f>CHF2CF3+</f> cation adopts a staggered <f>Cs</f> geometry, the <f>X˜ 2A′</f> ground state being formed by electron removal from the <f>18a′</f> <f>σ</f>-bonding orbital of <f>CHF2CF3</f>. Upon ionisation, large geometry changes and broad spectral bands are both predicted and observed. The next outer-valence orbitals of <f>CHF2CF3</f>, <f>17a′</f> and <f>11a′′</f>, are predominantly associated with fluorine 2p orbitals located on the <f>CHF2</f> group. Translational kinetic energy releases are determined from fixed-energy TPEPICO-TOF spectra. The ground state of <f>CHF2CF3+</f> dissociates through C–C bond cleavage with a relatively small release of energy. By contrast the <f>A˜</f> and <f>B˜</f> states dissociate rapidly by an impulsive mechanism with a larger fractional release of energy to <f>CHFCF3++F</f>. Upper limits for the ionisation threshold of <f>CHF2CF3</f> (<f>12.70±0.05</f> eV) and the enthalpy of formation of <f>CHFCF3+</f> at 298 K (<f>299±7 kJ mol−1</f>) are determined. [Copyright &y& Elsevier]