Ion–Molecule Reactions in a 50-MHz Discharge

Abstract

Extensive investigations of the dependence of the relative ionic composition of a high‐frequency plasma on the discharge conditions (particularly the pressure) and of the relative ionic composition of the plasma as a function of the translational energy of the ions issuing from the orifice of the sampling probe at constant pressure were made by the use of a quadrupole mass spectrometer employing a retarding‐potential grid system. The results indicate that the ions collected and observed are formed in a narrow region in the plasma (approximately 1 mm in depth) in front of the probe orifice and that these ions gain most of their energy from the electric field of the sheath in a distance that is much less than a reaction mean‐free path (∼0.01 mm). The average time available for reaction (reaction time) was a few tenths of a microsecond and independent of the vessel geometry and dimensions. The reaction time was relatively insensitive to changes in pressure and was reasonably constant for ions in several different gases. An explanation is given in terms of a distrubance to the electric fields of the plasma in the vicinity of the probe aperture.