Electron disappearance in pulse irradiated fluorocarbon gases

Abstract

Rate constants for electron attachment to some highly reactive fluorocarbon gases have been determined by microwave conductometric methods. Initial ionization of the gas was by pulse radiolysis with 2.8 MeV electrons. To facilitate measurements on very reactive compounds a method has been developed which is applicable when the electron half-life is < 1 μsec. This method uses the amplitude of the steady-state response of the detector during the pulse and the rate of production of electrons based on measurements on the nonattaching buffer gas alone. This steady-state method was tested on mixtures of CF2Cl2 or SF6 in C3H8 and was shown to agree with direct measurements of the decay of the electron concentration. With propane as the buffer gas the method is applicable where the electron half-life is greater than about 0.5 μsec at 1 torr total pressure. At higher pressures of attaching gas decreases in apparent rate constant were observed which could be attributed to electron reaction before complete thermalization. Values for thermal attachment rate constants (cm3 molecule−1 · sec−1) at 25 °C found in this work are 1.3 × 10−9 for CF2Cl2; 1.2 × 10−8 for cyclo-C4F8, 1.4 × 10−7 for cyclo-C4F6, and 4.9 × 10−8 for C4F8-2. In comparison to these values that for cyclo-C3F6 was determined to be < 3 × 10−14. Perfluoropropylene was found to undergo a transition from three- to two-body behavior at high pressures of buffer gas, and the rate constant values imply a negative ion lifetime near 1 μsec. The limiting value of the two-body rate constant was found to be 3 × 10−10 cm3 molecule−1 · sec−1.