The deposition process of amorphous hydrogenated carbon (a-C:H) in a 13.56 MHz hydrocarbon glow discharge system is investigated. Mass spectrometry was used to measure the relative abundance of the different ionic species. While there is a strong dependence of the mass spectra on the type of hydrocarbon gas (e.g., benzene, hexane, methane), the film properties are independent of the precursor. Spatially resolved spectroscopy of the optical emission was applied to study the interaction between energetic ions and substrate surface. Strong emission from small fragments, such as C, C2, and CH in the vicinity of the negatively self-biased cathode shows the importance of impact-induced fragmentation and explains the independence of film properties of the hydrocarbon process gas. With an electrically heated Langmuir probe we measured the potential, the density and the electron temperature of the plasma. In situ determination of the properties of the growing film, such as refractive index, thickness, absorption, and optical gap, was achieved by recording the reflectivity of the substrate during deposition.