High-resolution optical emission spectra of the CN(B(2)Sigma(+)-X(2)Sigma(+)) and CH(A(2)Delta-X(2)Pi) transitions were observed in the dissociative excitation reaction of CH3CN with the microwave-discharge flow of Ar. The H2O molecules contained in the starting materials and/or adsorbed on the wall of the apparatus were removed by using P2O5 as a desiccant. The pressure of Ar, P-Ar, was in the range of 0.1-01.8 Torr. From the simulation analysis of the observed spectra, the ratio of the concentrations of the CH(A(2)Delta) and CN(B(2)Sigma(+)) states, N-CH(A)/N-CN(B), was determined as 0.09-0.41. It was indicated that the CN(B(2)Sigma(+)) state was formed via the ion-electron recombination as well as the energy transfer from the metastable state of Ar. Based on the correlation between the N-CH(A)/N-CN(B) and [N]/([N]+[C]) ratios reported in the system without desiccation [Jpn. J. Appl. Phys. 40, 332 (2001)], the [N]/([N]+[C]) ratio in the desiccated system was predicted to be approximate to0.18. The hydrogenated-amorphous carbon nitride films prepared under the conditions of P-Ar = 0.1, 0.4, 0.6, and 0.8 Torr were characterized by the Rutherford backscattering (RBS) analysis and the Fourier transform infrared (FTIR) spectroscopy. The observed [N]/([N]+[C]) ratios of the films were in the range of 0.17-0.21, being in good agreement with the above prediction. The structure of the films was independent of P-Ar. The, observed correlation between the N-CH(A)/N-CN(B) and [N]/([N] + [C]) ratios can be rationalized by the consideration that the relative concentrations of the CH(A(2)Delta) and CN(B(2)Sigma(+)) states and those of the precursor free radicals of the films are supposed to originate commonly to the relative concentrations of the active species of the discharge flow of Ar. (C) 2004 American Vacuum Society.