A supersonically expanding cascaded arc plasma in argon is analyzed experimetally by emission spectroscopy. The thermal cascaded arc plasma is allowed to expand through a conically shaped nozzle in the arc anode into the vacuum vessel. In the nozzle monomers (C(x)H(y)) are injected. The monomers are dissociated and ionized by the argon carrier plasma, and transported toward a substrate by means of the expansion. Emission spectroscopy is used to obtain temperatures and particle densities. By varying external parameters (eg., arc power, gas flow rates) plasma parameters can be linked with film parameters (e.g., refractive index).