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

Cavity ring-down spectroscopy has been used to measure the absorbances, concentrations and temperatures of C-2(a(3)Pi(u) v=0) and CH(X(2)Pi v=0) radicals in a less than or equal to10 kW dc arc jet used for chemical vapor deposition (CVD) of polycrystalline diamond films and operated with CH4/H-2/Ar gas mixtures. Gas temperatures derived from the intensities and widths of rotationally resolved C-2 d(3)Pi(g)-a(3)Pi(u) spectral lines are 3300+/-200 K in the free plume, rising to similar to4800 K close to the substrate on which the diamond film is grown. For an input power of similar to6 kW, a 3.3% CH4/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+/-0.2x10(12) and 1.5+/-0.2x10(13) 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 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+/-1.3x10(12) cm(-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 CH4 at fixed powers of 5.5 and 5.8 kW, but for mixing ratios in excess of 5% CH4/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. (C) 2002 American Institute of Physics.