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 SF 5 CF 3 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 x 10 -17 cm 2 molecule -1 . The loss of SF 5 CF 3 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 SF 5 -. The lifetime of SF 5 CF 3 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 SF 6 , a lifetime of ca. 1000 years for SF 5 CF 3 is estimated.3

VUV photon induced fluorescence study of SF5CF3

“…A similar behaviour has also been observed in the fragmentation of the ground and excited states of SF 5 CF 3 + [ 14 ]. The parent ion preferentially dissociates to CF 3 + + SF 5 and not to SF 5 + + CF 3 , as can be concluded from the CF 3 + ion yield which mirrors the threhold photoelectron spectrum of SF 5 CF 3 [ 15 ]. Such a property is typical of small molecules, dissociating only on one repulsive potential energy surface with no curve crossing.…”

“…In that case, the determined SF 5 −CF 3 bond dissociation energy would be too high. Our result yields a slightly lower value than, but within error limits, for the SF 5 −CF 3 bond dissociation energy from that determined by threshold photoelectron -photoion coincidence spectroscopy, 4.06 ± 0.45 eV [ 5 ]. The calculated SF 5 −CF 3 bond dissociation energy is slightly lower, 3.50 eV [ 13 ].…”

“…These measurements show that photon absorption in the band with the maximum at 11.4 eV leads to the dissociation of the molecule. At excitation energies of 13.51 eV and higher, an emission band centred around 300 nm is observed in the fluorescence spectra of SF 5 As mentioned above, the electron-impact induced fluorescence of SF 6 showed emission around 300 nm for electron energies above 13.5 ± 1.5 eV [ 28 ]. The emission was tentatively assigned to SF 5 , SF 4 or SF 3 fragments but we believe that it may come directly from SF 6 .…”

“…This corresponds to the dissociation CF 3 X→ CF 3 * + X. For SF 5 CF 3 , the lowest energy to produce a CF 3 fragment is that for the reaction SF 5 CF 3 → CF 3 + SF 5 . So if we begin to observe the excited CF 3 fragment at 10.2 ± 0.2 eV (Fig.…”

“…These systems, particularly the former one, could in principle contribute to the observed spectra. However, the creation of the CF fragment from the SF 5 The higher flux of the Gas Phase beam line and the better spectral resolution of the spectrometer at Elettra allowed us to investigate the intensity behaviour of the CF 3 and CF 2 fluorescence with excitation energy. Two fluorescence regions, one from 550 to 650 nm (CF 3 emission) and the other from 250 to 310 nm (CF 2 emission), were selected and measured in the photon energy range 13.6 − 27.0 eV in steps of 0.1 eV.…”