Ionization measurements in optically pumped discharges

Abstract

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/N2 mixtures doped with O2 and NO demonstrated that associative ionization of CO produces free electrons at a rate up to S = 1015 cm-3 s-1. The ionization rate coefficient, inferred from the CO vibrational population measurements, is kion = (1.1-1.8)×10-13 cm3 s-1. It is shown that excited NO and possibly O2 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 O2 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 O2 to the optically pumped CO/Ar plasmas significantly increases the electron density, from ne = (4-7)×109 cm-3 to ne = (1-2)×1011 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 O2 additive, and their replacement by monomer ions with a slower recombination rate. Further studies of the ion composition in optically pumped plasmas are suggested.

Radio frequency energy coupling to high-pressure optically pumped nonequilibrium plasmas

“…Basically, ionization is produced in collisions of two highly vibrationally excited molecules when the sum of their vibrational energies exceeds the ionization energy. Ionization by this mechanism has been previously observed in CO-Ar and CO-N 2 gas mixtures optically pumped by resonance absorption of CO laser radiation at pressures of Pϭ0.1-1.0 atm and temperatures of Tϭ300-700 K. 7,[9][10][11][12] In these optically pumped nonequilibrium plasmas, where high vibrational levels of CO are populated by near-resonance V-V exchange, a gas temperature rise results in rapid relaxation of the upper vibrational level populations because of the exponential rise of the vibration-translation (V-T) relaxation rates with temperature. 13 In other words, ionization by mechanism ͑1͒ can be limited and even terminated by the gas heating.…”

“…Ionization of carbon monoxide by this mechanism has been previously observed in CO-Ar and CO-N 2 gas mixtures optically pumped by resonance absorption of CO laser radiation. 7,[9][10][11][12] The calculated 9-11 and measured 7,12 steady-state electron density sustained by a 10 W CO laser in these optically pumped plasmas is in the range of n e ϳ10 10 -10 11 cm Ϫ3 . Two 3 cm diameter brass plate electrodes are placed in the cell as shown in Fig.…”

“…Triggering the rf power coupling to the vibrational modes of the cell gases requires the initial electron density, n e , to exceed a certain threshold value. Our recent studies of associative ionization in CO laser pumped plasmas 7,11,12 showed that the electron density in these plasmas,…”

“…On the other hand, it suggests that a discharge system sustained by an external source with a negative feedback between gas heating and ionization rate, and using a sub-breakdown rf field to draw the discharge current, might be unconditionally stable. 7 An ionization process that satisfies this condition is the associative ionization in collisions of two highly vibrationally excited molecules, [7][8][9][10][11][12] AB͑v ͒ϩAB͑ w ͒→͑ AB͒ 2…”

Large Volume Atmospheric Pressure Glow Discharges

“…Alternatively, electron removal rates can be reduced. Recent results obtained in a plasma optically pumped by a CO laser suggest that the production of metastable species may reduce both dissociative recombination and electron attachment rates [29].…”