AbstractA highly sensitive multi‐element optical absorption technique is used to measure the absolute column density of both CH3and CH radicals in a dc hollow cathode plasma‐assisted chemical vapor deposition (CVD) system with CH4and H2used as the input gases. The plasma gas temperature is determined at different spatial points using the H2emission spectrum near 460 nm. The spatial maps of the temperature and the radical densities provide information on the chemical processes taking place in the discharge. The CH3and CH radical density measurements made in the dc discharge system are compared with similar measurements made in a hot filament CVD system. The [H]/[H2] ratio is derived from the CH3and CH densities using an analysis based on partial thermodynamic equilibrium of the single carbon hydrocarbon abstraction reacti