Modelling chemical kinetics of small clusters after nanosecond laser ablation

Nanoclusters of various materials have recently been obtained by laser ablation. Strong evaporation of a condensed phase caused by laser irradiation is well known to generate an overcooled vapour. Further expansion thereof increases the oversaturation degree and facilitates homogeneous nucleation and cluster growth. To investigate homogeneous nucleation at very high expansion rates attained at nanosecond laser ablation, kinetic equations are applied describing all the possible gas-phase chemical reactions of dissociation and coalescing between small clusters. Additional cooling due to thermal emission by clusters is taken into account. Twenty smallest carbon molecules are considered. The model is applied to nanosecond laser ablation of graphite in vacuum. The resulted vapour molecular composition is characterised by dominating molecules C₃ and C₅ and an exponential drop of heavier clusters concentrations with their mass. The growth of heavier clusters is controlled by the balance between liberating the latent heat of their formation and the energy losses by expansion and thermal emission.