The gas dynamics and atomic kinetics of a laser ablation plume of lithium, expanding adiabatically in vacuum, are included in a numerical model, using isothermal and isentropic self-similar analytical solutions and steady-state collisional radiative equations, respectively. Measurements of plume expansion dynamics using ultrafast imaging for various laser wavelengths (266–1064nm), fluences (2–6.5Jcm−2), and spot sizes (50–1000μm) are performed to provide input parameters for the model and, thereby, study the influence of laser spot size, wavelength, and fluence, respectively, on both the plume expansion dynamics and atomic kinetics. Target recoil pressure, which clearly affects plume dynamics, is included in the model. The effects of laser wavelength and spot size on plume dynamics are discussed in terms of plasma absorption of laser light. A transition from isothermal to isentropic behavior for spot sizes greater than 50μm is clearly evidenced. Equilibrium conditions are found to exist only up to 300ns after the plume creation, while complete local thermodynamic equilibrium is found to be confined to the very early parts of the expansion.
Optical emission spectroscopy is used to investigate the spatial evolution of the electron temperature (Te) and electron density (Ne) in the plasma generated by laser ablation in a vacuum of a wide-band-gap material, such as LiF, with a pulsed 193 nm excimer laser operating at a fluence of 1.5Jcm−2 close to the threshold. It is found that, whereas Ne (in the range of 1016cm−3) decreases by a factor of 2 as the distance to the target increases, Te exhibits a sharp decrease (from 1.85 eV to 0.66 eV) between 1 and 2 mm from the target and it remains practically constant for longer distances from the target. These results provide direct measurements of the electron temperature and density during nanosecond laser ablation of LiF.
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