Ultraviolet laser scattering in atmospheric microdischarges

Spatially resolved temperature measurements within a microdischarge in atmospheric pressure air have been conducted using Rayleigh and Raman scattering of a pulsed ultraviolet laser. The Rayleigh scattering analyses utilized spatially resolved time-gated imaging of the interaction region of the microdischarge and laser. Optical filtering of the collected Rayleigh signal minimized noise due to the elastic background component, including stray reflections, which often hinder such imaging. A single Rayleigh intensity image across the dc discharge produced a radial profile of translational temperature, with the analysis based on the ideal gas inverse relationship of temperature with the gas target density. The alternative technique using the vibrational Raman shifted signal benefited from being spectrally removed from the laser source background scatter noise, but the signal was several orders of magnitude less in intensity than Rayleigh scattering. The Raman analysis method involved monitoring Q-branch signals originating from multiple N₂(X) vibrational states populated in the microdischarge, which allowed assignment of translational and vibrational temperatures. Rotational temperature was analyzed from the structure of the S and O branches. The Raman technique also was capable of generating a spatially resolved temperature profile with multiple spectra by translating the laser through the microdischarge. Both Rayleigh and Raman scattering results are compared to passive optical emission spectral analyses of the N₂ second positive system from the microdischarge where the rotational and vibrational temperatures of the N₂(C) excited state were calculated. A comparison of the N₂(X) and N₂(C) temperatures derived from laser scattering and emission spectroscopy, respectively, is presented.