Coherent anti-Stokes Raman scattering performed on expanding thermal arc plasmas

The expanding plasma emanating from a thermal arc plasma source that can be used for deposition of thin films is studied using laser spectroscopic techniques. The argon-hydrogen plasma is characterized by very fast recombination that cannot be explained by atomic processes. To explore this phenomenon, which has been related to wall association of hydrogen atoms and recirculation, CARS (coherent anti-Stokes Raman scattering) is performed on (argon-)hydrogen plasmas. The periphery of the plasma appears to be rich in hydrogen molecules, in accordance with the recirculation model. No highly rovibrationally excited states are detected in the periphery, in spite of the spectrometer's very good sensitivity (0.1 Pa H₂ at 300 K). For the plasma, rotational and vibrational temperatures as well as absolute H₂ densities are measured. A simple model for the observed (non-Boltzmann) rotational populations is developed. {} 1996 The American Physical Society.