Number density and temperature of acetylene in hot-filament and arc-jet activated CH 4/H 2 gas mixtures measured using diode laser cavity ring-down absorption spectroscopy

Abstract

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) with an external cavity diode laser has been used to make diagnostic measurements of acetylene absorption in hot-filament and DC-arc plasma jet reactors operating with 1% CH 4 in H 2 and ≤0.8% CH 4/H 2 in excess Ar, respectively, and used for diamond chemical vapour deposition. The acetylene was widely distributed throughout the reactor, extending well beyond the regions of highest thermal activation as a result of diffusion processes. The average acetylene concentration was determined as (2.9±0.6)×10 13 molecules cm −3 in the hot filament reactor and (1.2±0.2)×10 14 molecules cm −3 in the DC arc jet reactor, accounting for ∼1.7% and 40% of the total C budget in the respective cases. The average acetylene gas temperature along the viewing column was 550±150 K in both environments. Variations in the concentration and temperature of acetylene with the gas feed composition and input power processing conditions for the two reactors are presented. The experiments demonstrate the effectiveness of near-IR diode laser CRDS as a probe of highly luminous environments.