Mechanisms of dissociative excitation of BrCN in electron cyclotron resonance plasmas of Ar and He

The dissociative excitation reactions of BrCN induced by the electron cyclotron resonance (ECR) plasmas of Ar and He were studied based on the electrostatic-probe measurements and on the optical emission spectra of the B 2Σ+-X 2Σ+ transition of CN radicals. The density, ne, and the temperature, Te, of free electrons were controlled by adding H2O molecules externally into the reaction region, and the dependence of the CN(B 2Σ+-X 2Σ+) emission intensity on the addition of H2O was observed. The partial pressures of Ar (or He) and BrCN were 3 mTorr and 1 mTorr, respectively, and the partial pressure of H2O, PH2O, was varied in the range 0.0-0.6 mTorr. The CN(B 2Σ+-X 2Σ+) emission intensity showed a negative dependence on PH2O. By comparing the observed PH2O dependence with the evaluated dependences based on ne and Te, the mechanism of dissociation of BrCN was found, predominantly, to be the electron impact in the Ar plasma and ion-electron recombination in the He plasma. The difference in the dissociation mechanism is interpreted in terms of that of the magnitudes of the rate constants for the charge transfers from Ar+ and He+