C2 swan band emission intensity as a function of C2 density

We report the systematic comparison of the optical emission intensity of the d(3) Pi --> a(3) Pi (0, 0) vibrational band of the C-2 Swan system with the absolute C-2 concentration in Ar/H-2/CH4 and Ar/H-2/C-60 microwave plasmas used in the deposition of nanocrystalline diamond. The absolute C-2 concentration is obtained using white-light absorption spectroscopy. Emission intensity correlates linearly with C-2 density for variations of several plasma parameters and across two decades of species concentration. Although optical emission intensity generally is not an accurate quantitative diagnostic for gas phase species concentrations, these results confirm the reliability of the (0,0) Swan band for relative determination of Cp density with high sensitivity under conditions used for hydrogen-deficient plasma-enhanced chemical vapour deposition of diamond.