Kinetics of the reactions involving CF2 and CF in a pure tetrafluoromethane plasma: I. Production of CF2 and CF via electron-impact dissociation

The kinetics of the production and loss of CF2 and CF radicals in a glow discharge in pure CF4 is investigated by the laser-induced fluorescence method. The effective rate constants for electron-impact dissociation of CF4 molecules along the pathways toward CF2 and CF radicals are determined within a wide range of the reduced electric field (80-250 Td). It is shown that, along with the direct electron-impact dissociation of CF4, the radicals are also produced via the dissociation of the CxFy polymer fluorocarbon particles that form in the plasma. A detailed analysis of the kinetics of the radical production and loss in a modulated discharge made it possible to evaluate the contribution of the electron-impact dissociation of CF4 to the production of radicals and, consequently, to determine the dissociation rate constants kCF2 and kCF . A comparison of the obtained kCF2 and kCF values with the results of calculations by the Monte Carlo method and the literature data on the cross sections for electron-impact dissociation of CF4 molecules enabled the normalization of these cross sections in the threshold region and the construction of the model cross sections for the electron-impact dissociation of CF4 into neutral products. The calculated cross sections allow a satisfactory description of the experimental results throughout the entire range of E/N under study. A significant scatter (up to 100%) in the experimental data on kCF2 and kCF at low values of E/N is related to the considerable contribution of the CxFy polymer molecules (and, probably, CxFy+ ions and fluorocarbon grains) to the production of CF2 and CF radicals both in the plasma volume and on the surface of a fluorocarbon film covering the discharge tube wall