Drastic effects of N2 addition in the chemical composition of C2H2/Ar glow discharges

The effects of nitrogen on the physicochemical properties of cold acetylene plasmas have been experimentally studied in C2H2/Ar capacitive radio frequency (RF) discharges. Two discharges containing respectively 0.8% and 6.3% N-2 in the initial gas mixture were investigated under conditions of incipient polymerization, using mass spectrometry, light scattering, and optical emission spectroscopy. During the initial transient, dust particles, small polyynes, and C-2 radicals were found to form and decay in parallel with a steep drop in the C2H2 concentration and an increase in the concentration of hydrogen cyanide, which was more prominent for the 6.3% N-2 mixture, and participated actively in the plasma chemistry. Over this time interval, relevant plasma parameters including the electron density and excitation temperature, the self-bias voltage, and the density of electronic excited states of various plasma species were found to undergo sharp variations. After this initial transient, lasting for about 20 s, a steady state was reached with stable plasma properties. Ion distributions were measured in the steady state, where no dust particles remained. The distributions of positive ions were dominated by species with an even number of carbon atoms, reflecting the prevailing polymerization mechanisms. The increase in the N-2 concentration from 0.8% to 6.3% led to a decrease in the global cation signal and to a marked growth in the intensity of detected anions. The cation distributions did not change much, but in the anion distributions, the peaks corresponding to the masses of the C2n-1N- (n = 1-4) ions grew by orders of magnitude, in contrast with those of the adjacent C2nH- peaks, which showed comparatively modest changes. These results could help identify anionic polymerization routes. Note that the two anion families mentioned correspond to the negative ions observed thus far in interstellar and circumstellar media.