Spectroscopic investigation of molecular formation in laterally colliding laser-produced carbon plasmas

We report detailed spectroscopic investigation of carbon dimer production in the stagnation layer formed due to laterally colliding plasma plumes under vacuum and N₂ ambience. Two spatially separated Nd:YAG laser pulses are used to generate plasma plumes in close proximity to a solid graphite target. A comparative optical emission spectroscopic study of a stagnation layer and a single carbon plasma plume is carried out in order to find the optimum experimental conditions for carbon dimer (C₂) production. Significant enhancement in the C₂ production is observed in the stagnation layer as compared to that observed in the single plume. Further, a substantial increase in carbon dimer formation is monitored by introducing N₂ gas. Along with carbon dimer, carbon nitride (CN) band emission is also observed in the emission spectra. Spatiotemporal evolution of C₂ and CN emission reveals that the yield of carbon dimer production is at a maximum close to the target surface, and decreases with an increase in time as well as spatial position. On the contrary, CN emission increases with the target distance. The dominant pathways for CN formation are suggested, and it is demonstrated that C₂ acts as a precursor to CN formation. The above observation is correlated with cooling of ejected carbon species in the collisional regime and the estimated vibrational temperature of C₂.