J. Phys. Chem. C 2007, 111, 18271-18278
18271
Mechanism for Electron-Induced SF5CF3 Formation in Condensed Molecular Films Sergey Solovev,† Adam Palmentieri, N. D. Potekhina,† and Theodore E. Madey* Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers, The State UniVersity of New Jersey, Piscataway, New Jersey 08854 ReceiVed: August 16, 2007
A potent greenhouse gas, trifluoromethyl sulfur pentafluoride (SF5CF3), has been discovered in the Earth’s upper atmosphere. There are no known natural sources of SF5CF3, and it has been reported that high-energy electron irradiation (5keV) of condensed mixtures of SF6 and CF4 forms SF5CF3. This paper focuses on a search for the electron-induced formation of SF5CF3 at low electron energies and for the mechanism of formation of SF5CF3. Temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD) are used to search for products formed under electron irradiation of ultrathin condensed films (∼3 ML) of SF6 and CF4. The major evidence for the formation of SF5CF3 via irradiation is the observation in ESD of ionic fragments containing S-C bonds, which are also seen in the mass spectra of SF5CF3. The maximum yield of SF5CF3 products in the condensed film is induced by ∼8 eV electron irradiation; the mechanism is associated with low-energy dissociative electron attachment (DEA) of SF6 and CF4 followed by radical reactions to form the product molecules.
I. Introduction Molecular trifluoromethyl sulfur pentafluoride (SF5CF3) was first detected in the upper atmosphere in 2000 by Sturges et al.1 It is one of the most powerful greenhouse gases with a global warming potential that is approximately 20 000 times greater than that of CO2. In 2000, the concentration was observed to be 0.12 parts per trillion; since then it has been increasing at a rate of 6% per year. Until recently, there has been no definitive information about the source of SF5CF3, except that it has an anthropogenic nature. In the upper atmosphere, the concentration of SF5CF3 is directly proportional to the concentration of SF6, and it has been suggested that molecular reactions with the breakdown products of SF6 may cause the formation of SF5CF3.2 SF6 is widely used as an electron scavenger to suppress breakdown in high-voltage applications, and one hypothesis suggests that SF5CF3 forms in a gas-phase reaction involving SF6. When SF6 molecules break down or are activated they may react with fluoropolymer molecules that are a source of CF3 radicals formed by highvoltage discharges. Thus, high-voltage equipment using SF6 as a gas dielectric may be a potential source of SF5CF3. In 2005, an experiment was designed to test this hypothesis in a study of CF4, C2F6, C3F8, and SF6 gas mixtures under corona discharge conditions.3 Under the conditions of these experiments, no traces of formation of SF5CF3 were observed. It was concluded that the formation of trifluoromethyl sulfur pentafluoride in electrical devices is not likely, and another explanation for the formation of SF5CF3 is necessary. An experiment reported very recently addressed the issue whether electron irradiation of a condensed ice mixture of SF6 and CF4 will form SF5CF3. In these studies, SF6 and CF4 were coadsorbed on a silver mirror at 12 K. Then, the ices were irradiated with high-energy electrons (5 keV) and beam currents * Author for correspondence. E-mail:
[email protected]. † Permanent address: A.F.Ioffe Physico-Technical Institute, St. Petersburg, Russia.
of 10 and 100 nA. Formation of SF5CF3 was inferred from infrared absorption spectra that were taken before and after electron irradiation. Peaks associated with SF5CF3 were in the infrared absorption spectra at 1160 and 846 cm-1. The authors suggest that SF5CF3 forms via radical-radical combination of SF5 and CF3 radicals and theorize that SF5CF3 molecules form in the atmosphere via these radical-radical combinations in the gas phase or assisted by aerosol particles.4 The main motivation for the present investigation is a desire to improve the understanding of the radiation-induced mechanisms of chemical reactions in condensed layers. The details of the process by which at very low electron impact energies (
