The chemistry of an argon/nitrogen thermal plasma expanding through a graphite nozzle is investigated in order to unravel the role of plasma species in the deposition (1-3 nm s^-1 rate) of non-hydrogenated amorphous carbon nitride (a-C:N) films. The spectral emission of plasma species is studied and is used in combination with mass spectrometry and nozzle temperature measurements to distinguish possible mechanisms of species production and excitation.