Role of ambient gas and laser fluence in governing the dynamics of the plasma plumes produced by laser blow off of LiF-C thin film

摘要

The time- and space-resolved emission profiles of Li Ⅰ and Li Ⅱ emission lines from the laser-blow-off plumes of a multilayered LiF-C thin film have been studied using spectroscopic technique. The evolution features were analyzed in different ambient environments ranging from high vacuum to 3 mbars of argon pressures and at various fluences of the ablating laser. During the evolution of the plume, a transition region was found to exist between 4 and 6 mm. Here, the plume dynamics changed from free expansion to collisional regime, where the plume experienced viscous force of the medium. The enhancement observed in neutral lines, in comparison with ionic lines, is explained in terms of the yield difference in electron impact excitation and ionization processes. Substantial difference in the arrival time distribution of the plume species was observed for Li Ⅰ and Li Ⅱ lines at high ambient pressures. Three expansion models are invoked to explain the evolution of the plume in different ambient conditions. The laser fluence was found to control the ratio of ions and neutrals.