Measurements of C2 and CH concentrations and temperatures in a dc arc jet using cavity ring-down spectroscopy

Abstract

Cavity ring-down spectroscopy has been used to measure the absorbances, concentrations and temperatures of C2(a3Πu v=0) and CH(X2Π v=0) radicals in a ⩽10 kW dc arc jet used for chemical vapor deposition (CVD) of polycrystalline diamond films and operated with CH4/H2/Ar gas mixtures. Gas temperatures derived from the intensities and widths of rotationally resolved C2 d3Πg−a3Πu spectral lines are 3300±200 K in the free plume, rising to ∼4800 K close to the substrate on which the diamond film is grown. For an input power of ∼6 kW, a 3.3% CH4/H2 ratio with excess Ar and a pressure of 50 Torr, the conditions typically employed for diamond film CVD, concentrations of C2(a) are between 6.0±0.2×1012 and 1.5±0.2×1013 cm−3 in the free plume at distances >5 mm from the substrate. These values are derived assuming a 1 cm column length as implied by spatially resolved studies of C2(d−a) optical emission; the spread reflects the decline in performance of the arc jet torch heads over time. The concentration of CH(X) in the free plume is 7.0±1.3×1012 cm−3 under similar operating conditions. Within 5 mm of the substrate, the measured absorption by both radicals rises steeply. The concentrations of C2(a) and CH(X) increase with added CH4 at fixed powers of 5.5 and 5.8 kW, but for mixing ratios in excess of 5% CH4/H2, the concentrations of both radicals become invariant. Measured C2(a) absorbance also increases with power input to the arc jet, but the CH absorbance is independent of this operating parameter.