Electron attachment and detachment: C6F5Cl, C6F5Br, and C6F5I and the electron affinity of C6F5Cl

Measurements are reported of rate constants for electron attachment to C6F5X (X=Cl,Br,I) and thermal electron detachment from C6F5Cl- over the temperature range 300-550 K in 133 Pa of He gas in a flowing-afterglow Langmuir-probe apparatus. This is the first case we know of where the parent anion has sufficiently low electron detachment energy that detachment (from C6F5Cl- in this case) has been observed in competition with a channel for dissociative electron attachment yielding a thermally stable anion (here, Cl-). Because of this competition, it is shown that a simple mass spectrometric determination of the product branching fractions at long times will lead to erroneous results at elevated temperatures. The electron density profiles provide evidence for a new plasma decay process involving the detaching and nondetaching anions trapped in the space charge field of the positive ions. Electron attachment rate constants were found to be 1.0x10-7, 1.1x10-7, and 2.0x10-7 cm3 s-1, at 300 K, for C6F5Cl, C6F5Br, and C6F5I, respectively, estimated accurate to ±25% except for C6F5I, where there is ±30% uncertainty. Rate constants for C6F5Cl changed little over our temperature range, while those for C6F5Br, and C6F5I increased with temperature. Electron detachment occurred only for C6F5Cl- in our temperature range. Detachment rate constants were immeasurable at room temperature but approached 4000 s-1 at 550 K. From these data the electron affinity (EA) for C6F5Cl was determined, EA (C6F5Cl)=0.75±0.08 eV. G3(MP2) calculations (based on Moeller-Plesset perturbation theory) were carried out for the neutral and anion and yielded EA(C6F5Cl)=0.728 eV