Cavity ring-down spectroscopy measurements of the concentrations of C2(X1Σg+) radicals in a DC arc jet reactor used for chemical vapour deposition of diamond films

Absorption spectroscopy measurements are reported of the absolute number density (3.0 ± 0.9 × 1012 cm−3) and vibrational temperature (3000 ± 500 K) of C2(X1Σg+) radicals in a DC arc activated chemical vapour deposition reactor under the standard operating conditions (3.3% CH4 in H2 and 6.4 kW discharge) used to grow polycrystalline diamond films. The ratio of number densities of C2 radicals in their ground (X1Σg+) and low-lying electronically excited (a3Πu) states is close to that expected from a Boltzmann distribution at the measured temperature, despite the more rapid rates of reaction of C2(X) with H2 and hydrocarbon molecules.