Gas-kinetic simulation of carbon vapour molecular composition at nanosecond laser ablation of graphite in vacuum

Chemical reactions between small carbon molecules up to C-5 in rapidly expanding pure carbon vapour are considered under the assumption of translational equilibrium. The equilibrium between translational and internal degrees of freedom of molecules (vibrations and rotations) is not implied. To take into account the difference between the translational and internal temperatures, the conventional equations of chemical kinetics are complemented with energy balances. Numerical calculations are made for the expansion dynamics, typical for graphite ablation by 25 ns laser pulse at the target temperatures from 2500 to 6000 K. The translational part of internal energy of vapour rapidly decreases at expansion and a large gap between translational and internal temperatures arises after about 10(-6)-10(-5) s. Translational temperature becomes very low and internal temperatures of C-2 to C-5 molecules remain as high as 2000-3000 K. The molecular composition calculated at the target temperature of 2500 K qualitatively agrees with experiments and is characterized by a high fraction of C-3 molecules. In the range 4000-6000 K, molecular composition depends weakly on the temperature. The fraction of C-3 remains high. The fraction of C is less and the fraction of C-5 is significantly higher compared with the case of 2500 K.