Radial distribution of molecular ions in the positive column of dc glow discharges using infrared diode-laser spectroscopy.

Abstract

The highly sensitive technique of infrared ion spectroscopy using frequency-tunable diode lasers has been introduced to study radial distribution of molecular ions in the positive column of dc glow discharges. Compared with previous techniques of plasma diagnostics, this method has the advantage that it is in situ and nonintrusive, has a high spatial resolution, and is sensitive to quantum states and velocities of molecular ions. ${\mathrm{ArH}}^{+}$ in Ar or He discharge and ${\mathrm{H}}_{3}$${\mathrm{}}^{+}$ in an ${\mathrm{H}}_{2}$ discharge have been used because of the strong infrared absorption and relative simplicity of the plasma chemistry. Our observations show that, while under certain discharge conditions where the radial distribution of molecular ions has the form of the zeroth-order Bessel function ${J}_{0}$(\ensuremath{\surd}a/D r) predicted by Schottky, it deviates markedly from the Schottky form in most plasma conditions. In particular, for ${\mathrm{ArH}}^{+}$ in an Ar plasma under high-current (1.25-A), high-pressure (10-torr) conditions, a large center depletion of the ion is noticed in the observed radial intensity variation of the absorption line. The observed intensity variation has been converted to radial concentration using vibrational and kinetic temperatures determined from relative intensities and widths of spectral lines. The result shows a large center depletion in molecular-ion concentration. Possible causes of such an effect are discussed. The variation of molecular-ion density at the center of the positive column as a function of discharge current has also been measured for ${\mathrm{ArH}}^{+}$ and ${\mathrm{H}}_{3}$${\mathrm{}}^{+}$.