H₂ Mole Fraction Measurements in a Microwave Plasma Using Coherent Anti-Stokes Raman Scattering Spectroscopy

In an effort to provide insights into the thermochemical composition of a microwave plasma chemical vapor deposition (MPCVD) reactor, the mole fraction of H₂ is measured at various positions in the plasma sheath, at pressures of 10 and 30 Torr, and at plasma powers ranging from 300 to 700 W. A technique is developed by comparing the Q(1)₀₁ transition of experimental and theoretical spectra aided by the Sandia CARSFT fitting routine. Results reveal that the mole fraction of H₂ does not vary significantly from its theoretical mixture at the parametric conditions examined. Furthermore, the $ν″=1→ν′=2$ vibrational hot band was searched, but no transitions were found. An analytical explanation for the increase in the temperature of H₂ with the introduction of N₂ and CH₄ is also presented. Finally, because the mole fraction of H₂ does not appear to deviate from the theoretical composition, the rotational and translational modes of H₂ are shown to be approximately in equilibrium, and therefore, the rotational temperatures may be used to estimate the translational temperatures of H₂.