Thermal electron attachment to NF 3, PF 3, and PF 5

Abstract

A flowing-afterglow Langmuir-probe apparatus was used to measure rate constants ( k a) for electron attachment to NF 3 and PF 5 over the temperature range T = 300–550 K. Electron attachment to NF 3 is dissociative and produces only F − ionic product in the temperature range studied. At room temperature, k a(NF 3) = 7 ± 4 × 10 −12 cm 3 s −1. The temperature dependence of k a(NF 3) above 340 K is characterized by an activation energy of 0.30 ± 0.06 eV. Attachment to PF 5 is nondissociative in a helium buffer at pressures in the range 53–160 Pa (0.4–1.2 Torr). The rate constant k a(PF 5) is 1.0 ± 0.4 × 10 −10 cm 3 s −1 at 300 K and is approximately temperature independent over much of the temperature range studied. PF 3 does not attach electrons in this temperature range. Upper limits to k a(PF 3) were determined (and attributed to impurities): k a < 1 × 10 −12 cm 3 s −1 at 296 K and k a < 1 × 10 −10 cm 3 s −1 at 550 K. The electron attachment rate constants measured in the present work are so small that corrections were required to account for electron/ion recombination contribution to the observed decay of the electron density in the plasma.