Excited state density distributions of H, C, C2, and CH by spatially resolved optical emission in a diamond depositing dc-arcjet reactor

Spatially resolved optical emission spectroscopy is used to investigate excited species in a dc-arcjet diamond depositing reactor. Temperature measurements indicate a cold plasma with electrons, excited states, and gas in nonthermal equilibrium. The H, C, C2, and CH excited state number densities decrease exponentially with the distance from the nozzle and have a pronounced increase in the shock structure above the substrate. The H emission increases throughout the boundary layer to the substrate surface, whereas emission from other species has a maximum in the boundary layer and then decreases again towards the substrate. The reconstructed radial distribution of excited state concentrations are Gaussian, with the C and C2 distributions broader than the H and CH ones. The optical emission is calibrated with either Rayleigh scattering or laser-induced fluorescence to furnish absolute number densities. We find all the excited species to be present in concentrations two or more orders of magnitude smaller than the corresponding ground states measured in the same reactor and conditions. We find that C2(d-a) emission intensity correlates well with laser-induced fluorescence measurements of C2(a) concentration in the arcjet plume. Ground state concentrations of the other species do not vary as their emission intensity except near the substrate, where the variations of CH(A-X), CH(B-X), and C2(d-a) emission intensities are good monitors of the corresponding concentration changes.