AbstractsChemistry

Laser light scattering diagnostic techniques, for characterization of weakly ionized molecular plasmas, are developed and demonstrated in high pressure, low temperature air-like molecular plasmas sustained by a continuous wave CO laser. Highly non-Boltzmann vibrational distribution functions (VDFs) of neutral species in the plasmas, which is created by anharmonic vibration-to-vibration up-pumping, are measured by spontaneous Raman scattering. Vibrational levels of all molecular species are significantly overpopulated compared to a Boltzmann distribution, and corresponding vibrational temperatures of molecular species (> 2000 K) are much higher than the translational/rotational temperature (< 500 K). Spatial distributions of the VDFs are also measured, and compared to master equation calculations. An instrumental system for filtered Thomson/rotational Raman scattering is developed. A single mode pulsed titanium:sapphire laser in combination with a rubidium vapor filter is employed in order to completely attenuate strong Rayleigh scattering/stray light, which causes severe interference because of the weak intensity and small wavelength shift of Thomson/rotational Raman scattering. The spectral purity of the titanium:sapphire laser has been increased to over 0.99999 using injection seeding, active cavity locking and a stimulated Brillouin scattering cell, greatly reducing the broadband residual component of the laser. When the wavelength of the laser is precisely tuned to the strong absorption line of rubidium at 780nm, the Rayleigh/stray light signal is absorbed essentially completely by the filter, placed in the detection path, while the Doppler broadened/wavelength shifted signal is transmitted. In order to optimize the instrument, the spectral purity as a function of circulating seed power in the laser cavity, as well as the effect of seed wavelength – cavity wavelength mismatch, is also studied. The utility of the instrument is demonstrated in measurements of electron temperature, electron number density and heavy species rotational/translational temperature in plasmas. Electron temperature and number density are measured by filtered Thomson scattering in an argon constricted dc glow discharge. The measurement of heavy species rotational/translational temperature is demonstrated by filtered rotational Raman scattering in a weakly ionized molecular plasma.