C2 column densities in H2/Ar/CH4 microwave plasmas

We report the observation of the d 3Πg→a 3Πu Swan bands of the C2 molecule both in absorption and emission in a H2/Ar/CH4 microwave discharge plasma. The input mole fraction of methane is varied from 1% to 33%. From the observed absorptions, we calculate the column density of gas phase C2. The calculated concentration of C2 is higher in discharges containing large fractions of argon than in discharges containing large fractions of hydrogen. These observations are useful in understanding the contribution of the C2 molecule to the gas phase chemistry of the microwave plasma-assisted chemical vapor deposition of diamond.