Semiconductor laser absorption diagnostics of atomic oxygen in an atmospheric-pressure plasma

Abstract

Narrow-bandwidth semiconductor (GaAlAs) lasers have been used to record spectrally resolved atomic oxygen (7772 Å) and argon (8425 Å) lineshapes, corresponding to the 3 s 5 S 0 2→3 p 5 P 3 and 4 s 3 P 1→4 p 3 D 2 transitions, in an atmospheric pressure, 1.4- kW, 27-MHz inductively coupled argon-oxygen (12% O 2/argon) plasma. Electron number density and kinetic temperature values were inferred from the Stark- and Doppler-broadening components of the absorption lineshapes, respectively. The ionization temperature was calculated from the measured electron number density assuming ionization (Saha) equilibrium. Values of excited- state species number density and population temperature were determined from the frequency- integrated absorption coefficient for each transition. The difference between the ionization and population temperatures reflects the presence of a suprathermal electron number density in the flowfield. In addition, the excellent agreement between the kinetic and population temperatures suggests that the population in the oxygen and argon lowest excited states may be described by a Boltzmann distribution at the kinetic temperature. The methods presented extend effectively the range of semiconductor-laser diagnostics to mixed-gas plasmas and flowfields containing atomic oxygen.