Ionization and temperature dependent attachment cross section measurements in C3F8 and C2H3Cl

Abstract

Total ionization and attachment cross sections have been measured in C3F8 at 330 K using an electron beam and a total ion collection technique, calibrated by similar measurements on N2O and Xe. Our total ionization cross section is similar in general shape to a previous measurement of this type, but with typically half the magnitude. The ionization threshold cannot be accurately derived from these measurements, due to severe upward curvature immediately above threshold. The positive-ion signal rises above the background at 13.0±0.1 eV, to be regarded as a lower limit to the true threshold. An overall ionization cross section with a threshold at 13.3 eV is recommended, based on threshold data from photoelectron spectroscopy and the present data between 14 and 80 eV. The room temperature total attachment cross section peaks at 2.8 eV with a value of 1.75×10−17 cm2. This is 14 times smaller than the only other measurement of this type we are aware of. There is much better agreement with two more recently reported values unfolded from swarm experiments. The temperature dependence of the predominant dissociative attachment process, involving F− production, was studied in a different apparatus using a mass filter and ion pulse counting. At 730 K the peak cross section has increased by ∼60% and the threshold is lower by 1.1 eV. This second type of measurement was used to study the predominant dissociative attachment process in C2H3Cl, involving Cl− production. At 290 K this has a threshold at 0.85 eV and a peak at 1.35 eV of 3.2×10−17 cm2, in good agreement with recent work elsewhere. At 850 K the cross section at the peak is 2.6 larger, and lower in energy by 0.33 eV, while at 0 eV it has reached 6×10−18 cm2. At higher temperatures effects ascribed to thermal dissociation of the C2H3Cl were observed. The implications of the present results regarding the use of these gases in diffuse discharge switches are discussed.