Dissociation of hydrogen in high-frequency discharges

Abstract

The collision processes which influence the yield of hydrogen atoms from a microwave discharge source have been considered. For hydrogen gas pressures in the range 0.02-1.5 Torr it is shown that H atom production is determined mainly by direct electron impact dissociation of H2 and by an excitation transfer process involving collisions of H (n=2) excited atoms with H2, leading to the formation of the repulsive b3 Sigma u state. Atom loss by three-body recombination is found to be small compared with wall recombination. A study of the balance between atom production and loss allows the dependence of the dissociation fraction on source pressure and input microwave power to be predicted. These predictions are found to be in satisfactory accord with the measured characteristics of a 2.45 GHz microwave discharge source previously developed in the authors' laboratory.