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A joint investigation has been undertaken of the gas-phase chemistry taking place in a hot-filament chemical vapor-deposition (HFCVD) process for diamond synthesis on silica surfaces by a detailed comparison of numerical modeling and experimental results. Molecular beam sampling using quadrupole mass spectroscopy and resonance-enhanced multiphoton ionization time of flight mass spectroscopy (REMPI-TOF-MS) has been used to determine absolute concentrations of stable hydrocarbons and radicals. Resulting species of a CH4/H2, a CH4/D2 (both 0.5%/99.5%) and a C2H2/H2 (0.25%/99.75%) feedgas mixture were investigated for varying filament and substrate temperatures. Spatially resolved temperature profiles at various substrate temperatures, obtained from coherent anti-Stokes Raman spectroscopy (CARS) of hydrogen, are used as input parameters for the numerical code to reproduce hydrogen atom, methyl radical, methane, acetylene, and ethylene concentration profiles in the boundary layer of the substrate. In addition, the concentration of vibrationally excited hydrogen is determined by CARS. Results reveal only qualitative agreement between measured data and simulations, concerning concentrations of stable species and radicals probed near the surface, on filament and substrate temperature dependence, respectively. Hydrogen and deuterium experiments show similar behaviour for all species. In the case of CH4 as feedgas the model describes measured concentration profiles of CH3, CH4, and C2H2 qualitatively well. Large differences between model and experiment occur for hydrogen atoms (factor of 2) and C2H4 (factor of 3). For acetylene as feedgas the model is not able to give any predictions because no conversion of C2H2 is seen in the model in contrast to the experiment.

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Using coherent anti-Stokes Raman spectroscopy ͑CARS͒ the spectral shift and width of the collisionally narrowed Q-branch structures of nitrogen and the 1 symmetric stretch vibration in methane were investigated at high densities. The gas samples either contained the pure substance or, for the case of nitrogen and methane, were diluted with argon, methane and carbon monoxide or argon and nitrogen, respectively, in the pressure range 50-2000 bar and at temperatures between 300 and 700 K. The simultaneous recording of spectra at ambient conditions ensured a frequency measurement accuracy of 0.07 cm Ϫ1 . Contributions to the line shapes and frequency shifts are determined that originate from narrowing of the rotational structure and from vibrational dephasing in nitrogen, methane, and its mixtures. The results are compared with quasiclassical calculations of the band shape and shift to determine thermally averaged collision cross sections for energy relaxation and vibrational dephasing as a function of temperature. In the investigated density regime, for nitrogen the band shape is dominated by collisional narrowing. The peak position of the band does not strongly depend on composition of the sample and the maximum red shift of the Raman frequency diminishes with increasing temperature. For methane at densities above 50 amagat effects from rotational relaxation are no longer detectable and dephasing collisions are dominant. In addition to vibration-translation relaxation, vibrational energy transfer is an important process for line broadening at high densities. The frequency shift of the Q-band strongly depends on mixture composition and temperature.

Vibrational‐rotational CARS spectra of molecular nitrogen and oxygen at high densities and temperatures are recorded using a scanning CARS‐spectrometer. Measurements are performed at pressures up to 2500 bar and temperatures up to 850 K. The spectral line position and halfwidth of the fully collapsed Q‐branch are determined for pure N2, O2, N2/O2 mixtures and Ar/O2 mixtures. Comparisons of the experimental data with different theoretical models for the rotational energy transfer are also presented. Three quantum mechanical approaches (MEG‐, ECS‐, PEG‐laws) are compared with a classical description of the relaxation process treating the angular momentum as a continuous variable.

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