Electron attachment to halomethanes at high temperatures

We have modified our high-temperature flowing-afterglow apparatus to include a movable Langmuir probe, a 4-needle reactant gas inlet, and a microwave discharge plasma source for the purpose of measuring electron attachment rate constants at high temperatures. We have focused initially on molecules which have very small attachment rate constants, k$_{a}$, at room temperature to see if their behavior at high temperatures can be described in Arrhenius fashion. We have reported k$_{a}$ for CH$_{3}$Cl, but only above 600 K, because the value at 600 K was quite small: 5.8 $\times $10$^{-12}$ cm$^{3}$ s$^{-1}$. The Arrhenius plot for these data imply k$_{a}$ = 10$^{-17}$ cm$^{3}$ s$^{-1}$ at 300 K, a value that is so small as to be immeasurable with any current apparatus. We now have k$_{a}$ for other halomethanes, CF$_{3}$Cl, CF$_{2}$Cl$_{2}$, and CH$_{2}$Cl$_{2}$. The halomethane data cover seven orders-of-magnitude in k$_{a}$. Electron attachment to CF$_{3}$Cl is endothermic by 143 meV at 300 K, but our measurements indicate that there is a barrier of about 400 meV, probably related to the energy at which the anion surface crosses that of the neutral. The reactions for CH$_{3}$Cl, CF$_{2}$Cl$_{2}$, and CH$_{2}$Cl$_{2}$ are exothermic, but our data again indicate large barriers to attachment which accounts for the extremely slow attachment at 300 K. From these data and literature measurements at 300 K, one can make educated guesses as to the behavior of k$_{a}$ for other halomethanes.