Ionization measurements in optically pumped discharges

The kinetics of ionization and electron removal in optically pumped non-equilibrium plasmas sustained by a CO laser are studied using non-self-sustained dc and RF electric discharges. Experiments in optically pumped CO/Ar/N 2 mixtures doped with O 2 and NO demonstrated that associative ionization of CO produces free electrons at a rate up to S = 10 15 cm -3 s -1 . The ionization rate coefficient, inferred from the CO vibrational population measurements, is k ion = (1.1-1.8) × 10 -13 cm 3 s -1 . It is shown that excited NO and possibly O 2 molecules also contribute to the vibrationally stimulated ionization process. In a CO/Ar plasma, applying a dc bias to the cell electrodes resulted in the rapid accumulation of a deposit on the negative electrode due to a large cluster ion current. The average mass of an ion in this plasma, estimated by measuring the mass of the deposit, is m ≅250 amu, which is consistent with the mass spectrometer analysis of the deposit. The deposit did not accumulate when small amounts of O 2 and NO were added to the CO/Ar plasma, which presumably indicates the destruction of the cluster ions. It is demonstrated that adding small amounts of O 2 to the optically pumped CO/Ar plasmas significantly increases the electron density, from n e = (4-7) × 10 9 cm -3 to n e = (1-2) × 10 11 cm -3 . This effect occurs at a nearly constant (within 50%) electron production rate S , indicating a substantial reduction in the overall electron removal rate. This reduction can be qualitatively interpreted as the destruction of rapidly recombining cluster ions in the presence of the O 2 additive, and their replacement by monomer ions with a slower recombination rate. Further studies of the ion composition in optically pumped plasmas are suggested.