Dissociative recombination in electron-beam excited argon at high pressures

We report studies of dissociative recombination in electron-beam excited argon at pressures from 200 to 12 000 Torr. In these experiments, we measure the decay in absolute intensity of the fluorescence from Ar (3p5 4p) dissociative products. By summing over all significant product fluorescence, we determine the total effective recombination rate for electrons, the electron density, and the branching fraction for production of several individual states by recombination. For pressures above three atmospheres, similar quantities are observed by monitoring the Ar*2 excimer emission. At low pressures, we find that the rates are in good agreement with previous workers. At high pressures, we observe a factor of 6 increase in the recombination coefficient over that measured at 200 Torr. At a constant electron beam current, the electron density rises as the argon density increases, reaches a maximum at ∼5000 Torr, and decreases after 7000 Torr. A discussion of possible models to explain the pressure dependence of the recombination rate is presented.