R. A. Kennedy

The vacuum-UV absorption spectrum of SF5CF3; implications for its lifetime in the earth’s atmosphere

Abstract Using vacuum-UV radiation from a synchrotron source, the absorption spectrum of SF5CF3 has been measured in the range 50–150 nm at a resolution of 0.12 nm. The cross-section at the Lyman-α wavelength of 121.6 nm is 1.5±0.3×10 −17 cm 2 molecule −1 . The loss of SF5CF3 on a molecular basis from the earth’s atmosphere is dominated, not by photon-induced dissociation, but by electron attachment in the mesosphere above 60 km, yielding SF5−. The lifetime of SF5CF3 in the earth’s atmosphere, however, is determined primarily by the meterological conditions that transport it from the earth’s surface to the mesosphere, and not by processes that occur in that region. By comparison with data for SF6, a lifetime of ca. 1000 years for SF5CF3 is estimated.

A study of low energy electron attachment to trifluoromethyl sulphur pentafluoride, SF5CF3: atmospheric implications

An investigation of electron attachment to the potent greenhouse gas SF5CF3 has been carried out in atmospheric pressure nitrogen and argon buffer gases at 300 K. The experiments were conducted under nonthermal electron-swarm conditions, using an instrument that combines a drift tube with a quadrupole mass spectrometer. Electron attachment rate constants, ka, have been determined as a function of mean electron energy (e = 0.04–1.9 eV). ka decreases as e increases. The estimated thermal electron attachment rate constant is kth(SF5CF3) ≈ (7.7 ± 0.6) × 10−8 cm3 molecule−1 s−1. The only observed anion product is SF5−. Free electron attachment destroys SF5CF3. This places an upper limit on the atmospheric lifetime of SF5CF3 of the order of 1000 years.

Charge transfer from neutral perfluorocarbons to various cations : long-range versus short-range reaction mechanisms

The bimolecular reactions of the high recombination energy cations Ar+, F+, and Ne+ with four fully saturated (CF4, C2F6, C3F8, and n-C4F10) and three unsaturated (C2F4, C3F6, and 2-C4F8) perfluorocarbons (PFCs) are reported. The cation branching ratios obtained from these reactions, and from the reactions with O2+, H2O+, N2O+, O+, CO2+, CO+, N+, and N2+ [reported by us, Jarvis et al., J. Phys. Chem. 100 (1996) 17166], are compared with those determined from the threshold photoelectron–photoion coincidence spectra of the PFCs at the recombination energies of the reagent cations. This comparison provides information that helps to interpret the dynamics of charge transfer, and whether it occurs via a long-range or a short-range mechanism. Energy resonance and good Franck-Condon factors connecting the ground electronic state of a reactant neutral molecule to one of its ionic states, at the recombination energy of the reagent cation, are generally considered to be sufficient criteria for long-range charge transfer to occur. However, the results from this study imply that good Franck-Condon factors are not critical in determining the efficiency of a long-range charge transfer. Instead, the results suggest that, in addition to the requirement for energy resonance, the electron taking part in the charge-transfer process must be removed from a molecular orbital which is unshielded from the approaching reagent cation. This enables the cation to exert an influence on the electron at large impact parameters.

Investigations of low energy electron attachment to ground state group 6B hexafluorides (SF6, SeF6, and TeF6) using an electron-swarm mass spectrometric technique

Abstract Studies of low energy electron attachment to SF6, SeF6, and TeF6 have been carried out in an atmospheric pressure nitrogen buffer gas (number density N) at 300 K. The experiments are conducted under nonthermal electron-swarm conditions, using an instrument that combines an atmospheric pressure drift tube, with a quadrupole mass spectrometer. Details of the design, construction and operation of the drift tube and the associated fast electron gate are presented. Electron drift times can be measured, and mean electron drift velocities in N2 as a function of the density reduced electric field strength E/N are reported. Density normalised electron attachment coefficients, α, and electron attachment rate constants, ka, together with product anion branching ratios (for SeF6 and TeF6) are determined as a function of E/N. The studies presented here cover the range E/N = (0.4–17) × 10−18 V cm2, corresponding to mean electron energies of 0.04–0.6 eV. For all three molecules, ka decreases as E/N increases. SF6 attaches electrons much more rapidly than either SeF6 or TeF6. The ratios ka(SF6):ka(SeF6):ka(TeF6) ≈ 3000:10:1 are found not to vary with E/N. The estimated thermal (300 K) electron attachment rate constants are kth(SF6) ≈ (2.5 ± 0.3) × 10−7 cm3 s−1, kth(SeF6) ≈ (8.0 ± 1.2) × 10−10 cm3 s−1, and kth(TeF6) ≈ (8.2 ± 1.1) × 10−11 cm3 s−1. For all three molecules, attachment is dominated by the capture of near-zero-energy electrons. In each case the dominant anion product is XF6− (X = S, Se, Te), accompanied by XF5−. No other anion products directly arising from electron attachment to XF6 are observed. Extrapolation of the relative product anion intensities to zero attaching gas concentration yields the following branching ratios for attachment under swarm conditions: SeF6–SeF5− (20%), SeF6− (80%); and TeF6–TeF5− (3%), TeF6− (97%). These ratios are found to be independent of E/N. The observation of SeF6− and TeF6− as the dominant anions from SeF6 and TeF6 is ascribed to stabilisation of the initial anion formed by electron capture through collisions with the nitrogen buffer gas. For SF6, the observed proportion of SF5− decreases from 8% to 1% over the E/N range of this study, whereas an increase in the SF5− branching ratio with E/N is anticipated from previous low-pressure, electron beam investigations.

Selected ion flow tube study of the reactions of O− and O2− with CHC12F, CHC1F2, CHF3, CH2C1F, CH2F2, CH3F, CHF2CHF2, CH2FCF3, and CH3CHF2

Abstract In this paper we report the reactions of O− and O2− with several saturated hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) in a 0.5 Torr helium buffer gas at about 300 K using a selected ion flow tube. The reaction rate coefficients and branching ratios determined for the large number of reactions studied are presented. O− is found to react with all the molecules studied, and several distinct reaction processes are observed. These include nucleophilic attack on chlorine, nucleophilic attack on carbon, and hydrogen, proton, and H2+ abstraction. Nucleophilic attack on carbon is found to dominate the O2− reactions with the HCFCs. Bimolecular reactions of O2− with the HFCs do not occur.

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 (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 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 . 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 , , and 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 and to be 166 ± 52 and 4 ± 62 kJ mol-1, respectively.