Kinetics of Metastable Argon Optical Excitation and Gain in Ar/He Microplasmas.

The optically pumped rare-gas metastable laser is capable of high-intensity lasing on a broad range of near-infrared transitions for excited-state rare gas atoms (Ar*, Kr*, Ne*, Xe*) diluted in flowing He. The lasing action is generated by photoexcitation of the metastable atom to an upper state, followed by collisional energy transfer with He to a neighboring state and lasing back to the metastable state. The metastables are generated in a high-efficiency electric discharge at pressures of ∼0.4 to 1 atm. The diode-pumped rare-gas laser (DPRGL) is a chemically inert analogue to diode-pumped alkali laser (DPAL) systems, with similar optical and power scaling characteristics for high-energy laser applications. We used a continuous-wave linear microplasma array in Ar/He mixtures to produce Ar(1s5) (Paschen notation) metastables at number densities exceeding 1013 cm-3. The gain medium was optically pumped by both a narrow-line 1 W titanium-sapphire laser and a 30 W diode laser. Tunable diode laser absorption and gain spectroscopy determined Ar(1s5) number densities and small-signal gains up to ∼2.5 cm-1. Continuous-wave lasing was observed using the diode pump laser. The results were analyzed with a steady-state kinetics model relating the gain and the Ar(1s5) number density.

Production and loss of O2(1Δ g ) at atmospheric pressure using microwave-driven microplasmas.

We have used arrays of microwave-generated microplasmas operating at atmospheric pressure to generate high concentrations of singlet molecular oxygen, O2(1Δ g ), which is of interest for biomedical applications. The discharge is sustained by a pair of microstrip-based microwave resonator arrays which force helium/oxygen gas mixtures through a narrow plasma channel. We have demonstrated the efficacy of both NO and less-hazardous N2O additives for suppression of ozone and associated enhancement of the O2(1Δ g ) yield. Quenching of O2(1Δ g ) by ozone is sufficiently suppressed such that quenching by ground state molecular oxygen becomes the dominant loss mechanism in the post-discharge outflow. We verified the absence of other significant gas-phase quenching mechanisms by measuring the O2(1Δ g ) decay along a quartz flow tube. These measurements indicated a first-order rate constant of (1.2 ± 0.3) × 10-24 m3 s-1, slightly slower than but consistent with prior measurements of singlet oxygen quenching on ground state oxygen. The discharge-initiated reaction mechanisms and data analysis are discussed in terms of a chemical kinetics model of the system.