Evidence of an energy transfer reaction between atomic hydrogen and argon II or helium II as the source of excessively hot H atoms in radio-frequency plasmas
From the width of the emitted 656.3 nm Balmer alpha line, it was found that low-pressure (0.5 torr) capacitively coupled radio-frequency (RF) He/H_2 (95/5%) as well as Ar/H_2 (95/5%) plasmas showed excess broadening throughout the volume (13.5 cm ID × 38 cm length) of a General Electronics Conference (GEC)-type cell; not merely in the vicinity of the electrodes of Ar/H_2 plasmas as reported by several groups. About 50% of the hydrogen of the He/H_2 was ‘hot’ with an average hydrogen atom energy of 40 50 eV, compared to ̃1 eV for pure hydrogen, whereas Ar/H_2 showed a single fast or ‘hot’ 40 50 eV population. The broadening was undiminished at 15 cm from the powered electrode, independent of power over a substantial range, but dependent on the hydrogen concentration. In contrast to the atomic hydrogen lines, no broadening was observed in helium or argon lines. Also, in ‘control’ Xe/H_2 plasmas run in the same cell at similar pressures and absorbed 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 cannot 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, the data are consistent with a model wherein He(+) and Ar(+) act catalytically through a resonant energy transfer mechanism to create ‘hot’ hydrogen atoms in plasmas.