The ground state atomic hydrogen density in a hydrogen/acetylene discharge is compared with the diamondlike carbon (DLC) film quality as determined frominsituellipsometry measurement of the refractive index. Two‐photon laser‐induced fluorescence (LIF) was employed for the detection of the ground state atomic hydrogen in the rf generated discharge. Absolute atomic hydrogen number densities were determined by calibrating the LIF detection system using a transfer standard. A direct correlation was found between the level of molecular hydrogen dissociation in the plasma and the refractive index of the DLC film. ©1995 American Institute of Physics.