Electron-ion recombination in an ambient molecular gas

Three-body electron-ion recombination in a molecular ambient gas is investigated theoretically. The limited success attained by a classical treatment is lost when necessary account is taken of the discreteness of the negative energy levels of the recombining electron. Thus the three-body recombination coefficients calculated in carbon dioxide and in water vapour differ from those measured by Warman et al. (1979) and Sennhauser et al. (1980) being much lower and falling off far more steeply with increase in temperature. It is noted that the ions involved in the laboratory study, (CO 2 + ) n and H 3 O + (H 2 O) n , are open to dissociative recombination and it is pointed out that this process and three-body recombination supplement one another with striking consequences. The mechanism is in two stages: firstly, collisions between the electrons and the ambient gas molecules populate the Rydberg levels; secondly, the electrons in the Rydberg levels participate in radiationless transitions leading to dissociation in the same way as free electrons. A suitably descriptive name for the mechanism is collisional dissociative recombination. Detailed calculations give gas-density-dependent recombination coefficients in quite good agreement with those measured.