Evidence of the production of hot hydrogen atoms in RF plasmas by catalytic reactions between hydrogen and oxygen species

TL;DR

This study provides evidence that catalytic reactions between hydrogen and oxygen species in RF plasmas produce hot hydrogen atoms with energies over 40 eV, independent of electric fields or charged particle acceleration.

Contribution

It demonstrates that specific catalytic reactions can generate hot hydrogen atoms in RF plasmas, supported by experimental data and a resonant energy transfer model.

Findings

20% of hydrogen atoms are 'hot' with >40 eV energy at low pressure

Hot hydrogen atoms are uniformly distributed throughout the plasma volume

No broadening observed in oxygen lines or in control plasmas without catalytic conditions

Abstract

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.