The dissociative excitation reaction of C2H2 with the electron-cyclotron resonance plasma of Ar was investigated based on the electrostatic-probe measurements and on the optical emission spectroscopy of the CH(A(2)Delta-X-2 Pi) transition. The density, n(e), and the temperature, T-e, of free electrons were controlled by adding H2O molecules externally into the reaction region, and the dependence of the CH(A(2)Delta-X-2 Pi) emission intensity on the addition of H2O was observed to compare with the evaluated dependencies based on n(e) and T-e. The mechanism of production of CH(A(2)Delta) was found, predominantly, to be the electron impact with the contribution of 10-20% of the electron-impact dissociation of C2H radicals; the contribution of the ion-electron recombination was negligible. Hydrogenated amorphous carbon films were fabricated using the same reaction system. The atomic compositions, Raman spectra, and the hardness of films were discussed in terms of the variations of n(e) and T-e upon the addition of H2O molecules.