Mechanism for Electron-Induced SF5CF3Formation in Condensed Molecular Films

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 SF6and CF4forms SF5CF3. This paper focuses on a search for the electron-induced formation of SF5CF3at 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 SF6and CF4.The major evidence for the formation of SF5CF3via 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 SF5CF3products in the condensed film is induced by ∼8 eV electron irradiation; the mechanism is associated with low-energy dissociative electron attachment (DEA) of SF6and CF4followed by radical reactions to form the product molecules.