Ionization measurements in optically pumped discharges

Abstract

The kinetics of ionization and electron removal in opticallypumped non-equilibrium plasmas sustained by a CO laser are studied usingnon-self-sustained dc and RF electric discharges. Experiments in opticallypumped CO/Ar/N2 mixtures doped with O2 and NOdemonstrated that associative ionization of CO produces free electrons at arate up to S = 1015 cm-3 s-1. The ionization ratecoefficient, inferred from the CO vibrational population measurements, iskion = (1.1-1.8)×10-13 cm3 s-1. It isshown that excited NO and possibly O2 molecules also contribute tothe vibrationally stimulated ionization process. In a CO/Ar plasma,applying a dc bias to the cell electrodes resulted in the rapidaccumulation of a deposit on the negative electrode due to a large clusterion current. The average mass of an ion in this plasma, estimated bymeasuring the mass of the deposit, is m≅250 amu, which isconsistent with the mass spectrometer analysis of the deposit. The depositdid not accumulate when small amounts of O2 and NO were added tothe CO/Ar plasma, which presumably indicates the destruction of the clusterions.It is demonstrated that adding small amounts of O2 to theoptically pumped CO/Ar plasmas significantly increases the electrondensity, from ne = (4-7)×109 cm-3 tone = (1-2)×1011 cm-3. This effect occurs at anearly constant (within 50%) electron production rate S, indicating asubstantial reduction in the overall electron removal rate. This reductioncan be qualitatively interpreted as the destruction of rapidly recombiningcluster ions in the presence of the O2 additive, and theirreplacement by monomer ions with a slower recombination rate. Furtherstudies of the ion composition in optically pumped plasmas are suggested.