DETERMINATION OF THE FIRST DISSOCIATIVE IONISATION ENERGY OF POLYATOMIC MOLECULES BY THRESHOLD PHOTOELECTRON PHOTOION COINCIDENCE (TPEPICO) SPECTROSCOPY: APPLICATION TO CF₄, SF₆, SeF₆, TeF₆ AND SF₅CF₃

A recent paper (Science 2000, 289, 611) has suggested that an anthropogenic greenhouse gas, SF5CF3, recently detected in the atmosphere, has the highest radiative forcing of any gas-phase molecule. The ground state of SF5CF+ 3 is repulsive in the Franck–Condon region, the parent ion is not observed, and the onset of ionisation can only give an upper limit to the energy of the first dissociative ionisation pathway of SF5CF3 (to CF+ 3 + SF5 + e−). Using a variation of threshold photoelectron photoion spectroscopy, we have determined the kinetic energy released into the two fragments over a range of energies. Using an impulsive model, the data has been extrapolated to zero kinetic energy to obtain a value for the first dissociative ionisation energy for SF5CF3 of 12.9 ± 0.4 eV. The enthalpy of formation at 0 K of SF5CF3 is determined to be −1770 ± 47 kJ mol−1, and the dissociation energy of the SF5–CF3 bond at 0 K to be 392 ± 48 kJ mol−1 or 4.06 ± 0.45 eV. The implication of this bond strength is that SF5CF3 is very unlikely to be broken down by UV radiation in the stratosphere. A similar experiment for CF4 (to CF+ 3 +F+e−), SF6 (to SF+ 5 + F+e−), SeF6 (to SeF+ 5 +F+e−) and TeF6 (to TeF+ 5 +F+e−) yielded values for their dissociative ionisation energies of 14.45 ± 0.20, 13.6 ± 0.1, 14.1 ± 0.5 and 14.5 ± 0.6 eV, respectively. The first two results agree with previous data on the CF3 and SF5 free radicals. The final two results yield 0 K enthalpies of formation of SeF+ 5 and TeF+ 5 to be 166 ± 52 and 4 ± 62 kJ mol−1, respectively.