DETERMINATION OF THE FIRST DISSOCIATIVE IONISATION ENERGY OF POLYATOMIC MOLECULES BY THRESHOLD PHOTOELECTRON PHOTOION COINCIDENCE (TPEPICO) SPECTROSCOPY: APPLICATION TOCF4,SF6,SeF6,TeF6andSF5CF3

A recent paper (Science2000,289, 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${\rm SF}_5{\rm CF}_3^+$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${\rm SF}_5{\rm CF}_3~({\rm to~ CF}_3^+ + {\rm SF}_5 + {\rm e}^-)$. 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 forSF5CF3of 12.9 ± 0.4 eV. The enthalpy of formation at 0 K ofSF5CF3is determined to be-1770 ± 47kJ mol-1, and the dissociation energy of theSF5–CF3bond at 0 K to be392 ± 48kJ mol-1or 4.06 ± 0.45 eV. The implication of this bond strength is thatSF5CF3is very unlikely to be broken down by UV radiation in the stratosphere. A similar experiment for${\rm CF_4~(to~CF_3^+ + {\rm F} + {\rm e}^-)$,${\rm SF}_6~({\rm to~SF}_5^+ + {\rm F} + {\rm e}^-)$,${\rm SeF}_6~({\rm SeF}_5^+ + {\rm F} + {\rm e}^-)$and${\rm TeF}_6~{\rm (to~TeF}_5^+ + {\rm F} + {\rm e}^-)$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 theCF3andSF5free radicals. The final two results yield 0 K enthalpies of formation of${\rm SeF}_5^+$and${\rm TeF}_5^+$to be 166 ± 52 and4 ± 62kJ mol-1, respectively.