Atomic and Molecular Species Post-2 μs Dynamics in Laser-Induced Carbon Plasmas in Air

Abstract

Laser-induced carbon plasma in air undergoes various physicochemical processes that affect the kinetic chemistry of species of the plasma plume. We report the time- and space-resolved characterization of carbon plasma produced by infrared nanosecond laser into air at atmospheric pressure. Investigating the laser fluence effect highlights dissociation for fluences >40 J cm⁻², and recombination processes in the fluence range of 10–40 J cm⁻². Emission intensities of C₂ and CN molecules undergo an enhancement at specific spatiotemporal locations in the laser-induced plasma. At a value of 27 J/cm² and 0.8 mm from the plasma ignition, molecular band formation is favored for the specific temperature and density values of 1.7 × 10¹⁵ cm⁻³ and 9502 K. The vibrational temperatures of molecules are determined using nonlinear spectral data fitting program. The shock front between laser-induced carbon plasma and air may lead to a significant shock wave that affects the occurrence of molecular CN and C₂ formation. This can be explained by the distinct temperatures exhibited by CN and C₂ molecules with laser fluence. The atomic carbon travels farther to react and form C₂, where the ionization–recombination process plays a significant role in its formation. Collisions of C with N neutrals and N₂ molecules are the plausible origin of CN generation. Moreover, the density of CN in the plasma depends on C₂ molecules.