  Selective H-atom line broadening was found to be present throughout the
volume (13.5 cm ID x 38 cm length) of RF generated H2O plasmas in a GEC cell.
Notably, at low pressures (ca. <0.08 Torr), a significant fraction (ca. 20%) of
the atomic hydrogen was 'hot' with energies greater than 40 eV with a pressure
dependence, but only a weak power dependence. The degree of broadening was
virtually independent of the position studied within the GEC cell, similar to
the recent finding for He/H2 and Ar/H2 plasmas in the same GEC cell. In
contrast to the atomic hydrogen lines, no broadening was observed in oxygen
species lines at low pressures. Also, in control Xe/H2 plasmas run in the same
cell at similar pressures and adsorbed power, no significant broadening of
atomic hydrogen, Xe, or any other lines was observed. Stark broadening or
acceleration of charged species due to high electric fields can not explain the
results since (i) the electron density was insufficient by orders of magnitude,
(ii) the RF field was essentially confined to the cathode fall region in
contrast to the broadening that was independent of position, and (iii) only the
atomic hydrogen lines were broadened. Rather, all of the data is consistent
with a model that claims specific, predicted, species can act catalytically
through a resonant energy transfer mechanism to create hot hydrogen atoms in
plasmas.
