Fragmentation of energy-selected SF5CF3+ probed by threshold photoelectron photoion coincidence spectroscopy: bond dissociation energy of SF5-CF3, and its atmospheric implications

摘要

Using tunable vacuum-UV radiation from a synchrotron in the range 12-26 eV, we have measured the threshold photoelectron and threshold photoelectron – photoion coincidence spectrum of SF\(_5\)CF\(_3\), a new anthropogenic greenhouse gas. The ground state of SF\(_5\)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 SF\(_5\)CF\(_3\), to CF\(_3^+\) + SF\(_5\) + e\(^-\). We have determined the kinetic energy released into the two fragments over a range of photon energies in the Franck-Condon region of the ground state of SF\(_5\)CF\(_3^+\). Using an impulsive model, the data has been extrapolated to zero kinetic energy to obtain a value for the first dissociative ionisation energy for SF\(_5\)CF\(_3\) of 12.9 \(\pm\) 0.4 eV. A similar experiment for CF\(_4\) to CF\(_3^+\) + F + e\(^-\)) and SF\(_6\) (to SF\(_5^+\) + F + e\(^-\)) yielded values for their dissociative ionisation energies of 14.45 \(\pm\) 0.20 and 13.6 \(\pm\) 0.1 eV, respectively, in agreement with previous data on the CF\(_3\) and SF\(_5\) free radicals. The enthalpy of formation at 0 K of SF\(_5\)CF\(_3\) is determined to be -1770 \(\pm\) 47 kJ mol\(^{-1}\), and the dissociation energy of the SF\(_5\)-CF\(_3\) bond at 0 K to be 392 \(\pm\) 48 kJ mol\(^{-1}\) or 4.06 \(\pm\) 0.45 eV. The implication of this bond strength is that SF\(_5\)CF\(_3\) is very unlikely to be broken down by UV radiation in the stratosphere. In addition, over the complete energy range of 12-26 eV, coincidence ion yields of SF\(_5\)CF\(_3\) have been determined. CF\(_3^+\) and SF\(_3^+\) are the most intense fragment ions, with SF\(_5^+\), SF\(_4^+\) and CF\(_2^+\) observed very weakly. Energetic constraints require that SF\(_3^+\), SF\(_4^+\) and CF\(_2^+\) can only form with CF\(_4\) + F, CF\(_4\) and SF\(_6\), respectively, so that fragmentation of SF\(_5\)CF\(_3^+\) to these ions involves migration of a fluorine atom across the S-C bond.