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.