Measurements of C 2 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 C 2(a 3Π uv= 0) and CH(X 2Πv=0) radicals in a ≤10 kW dc arc jet used for chemical vapor deposition (CVD) of polycrystalline diamond films and operated with CH 4/H 2/Ar gas mixtures. Gas temperatures derived from the intensities and widths of rotationally resolved C 2 d 3Π g-a 3Π u spectral lines are 3300±200K 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% CH 4/H 2 ratio with excess Ar and a pressure of 50 Torr, the conditions typically employed for diamond film CVD, concentrations of C 2(a) are between 6.0±0.2×10 12 and 1.5±0.2×10 13cm -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 C 2(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±1.3×10 12cm -3 under similar operating conditions. Within 5 mm of the substrate, the measured absorption by both radicals rises steeply. The concentrations of C 2(a) and CH(X) increase with added CH 4 at fixed powers of 5.5 and 5.8 kW, but for mixing ratios in excess of 5% CH 4/H 2, the concentrations of both radicals become invariant. Measured C 2(a) absorbance also increases with power input to the arc jet, but the CH absorbance is independent of this operating parameter. © 2002 American Institute of Physics.