Excitation and dissociation of hydrogen by an electron swarm

Abstract

Electrons moving in a uniform electric field ionize and excite gas molecules and, for feeble swarm currents, are known to multiply exponentially with distance. The rate of quantum production can therefore be expressed by an excitation coefficient analogous to Town send’s ionization coefficient. Excitation rates must be known to calculate secondary effects by photo-electric emission from a cathode. A novel method of intensity measurements by counting quanta vacuum ultra-violet radiation has been developed, using a fluorescent converter, photo -multiplier and scaler, to obtain the absolute magnitude of the excitation coefficient as a function of the electric field. The absolute in tensity of the radiation was found by means of a photo-chemical reaction of known quantum efficiency. With electron currents up to 10 -8 A it was shown that the emitted flux of ultra-violet quanta was proportional to the current and varied exponentially with distance from the cathode, in agreement with theory. The rate of dissociation of molecular hydrogen in an electrodeless discharge was measured by absorbing the atomic hydrogen formed and observing the rate of decrease of gas pressure. From this and electric data a dissociation coefficient was calculated showing that dissociation losses account for a large part of the energy balance. It seems that dissociation proceeds principally by electronic excitation from the ground state 1 1 ∑ g to the lowest repulsive state 1 3 ∑ u . An average cross-section of 0.2 ╥a 2 0 has been obtained for this transition in moderate electric fields.