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

Abstract

The time- and space-resolved emission profiles of LiI and LiII 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 3mbars 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 6mm. 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 LiI and LiII 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.

Effect of ambient gas on the expansion dynamics of plasma plume formed by laser blow off of thin film

“…At 10 −2 Torr pressure and time delay t > 400 ns, the plume expands in the collisional regime where the estimated plume dimension (∼7 mm) is greater than the mean free path of the ejected species [8]. In this regime, interpenetration of the plasma plume into the ambient gas occurs, which causes an increase in collisions between the plume species and ambient gas atoms.…”

“…A detailed description of experimental setup has been reported in our previous papers [8,9]. Only the main features and additional part of the setup are summarized here.…”

“…Hence, the understanding of structural formation in the ejected plume species and its spatial and temporal variations with different experimental condition are essential for using these techniques in applied research. Further, the comparative analysis of the plume expansion in vacuum and ambient environment can provide the knowledge of many physical processes, such as deceleration, attenuation/enhancement of emission, hydrodynamic instabilities, formation of shock waves and plume oscillations [8,11]. It is worth mentioning that neutral species are the main constituents of the LBO plumes although it also consists of the ions mostly in singly ionised state [8].…”

“…The optical emission spectroscopic technique of LBO plume [8][9][10] has revealed that expansion feature of plume is highly dependent on the experimental conditions and varies according to the ambient environment and laser fluence. But this technique cannot provide information regarding the influence of experimental parameters on the structure of the plasma plume.…”

“…Further, the comparative analysis of the plume expansion in vacuum and ambient environment can provide the knowledge of many physical processes, such as deceleration, attenuation/enhancement of emission, hydrodynamic instabilities, formation of shock waves and plume oscillations [8,11]. It is worth mentioning that neutral species are the main constituents of the LBO plumes although it also consists of the ions mostly in singly ionised state [8]. Hence, it will be worthwhile to study the interaction between background gas and neutral rich plume with regard to the formation of structures in plume due to hydrodynamic instability and reactive scattering.…”