G. Cristoforetti; E. Tognoni; L.A. Gizzi
Applications, present and potential, of laser-induced plasmas are today widespread in a large range of fields and continue to extend each day because of the advancement of laser science and technology. These circumstances call for a better knowledge and characterization of physical and chemical processes occurring in a plasma. The spectrum of the radiation emitted by the plasmas is a mine of information about the plasma state and therefore spectroscopy remains one of the key techniques for its investigation. The interpretation of emission spectra, however, requires a deep knowledge of the elementary processes determining the atomic state population and the fractional ion densities, and of their balance. The transient character of laser-induced plasmas and the presence of spatial gradients introduce time-dependent and non-local effects in the energy level population, and thus increase the complexity of the plasma modeling. The thermodynamic approach, describing the atomic and ionic state population by statistical distributions, is the easiest and the most widely used way to model the plasma state but does not account for non-local and non-steady-state effects. It is therefore in many cases unfit for this purpose. As a consequence, kinetic modeling of the plasma is often needed, which must be in many cases integrated by hydrodynamic modeling of plasma expansion and appropriate equations describing the transport of radiation and charged particles into the plasma.
In this complex framework, this paper aims at giving a concise description of theoretical issues, redirecting to the key literature for more details, and to delineate possible scenarios occurring in the wide range of laser-induced plasmas. Examples of different classes of laser-induced plasmas are reported, including some experimental results for completeness. Special attention is devoted to the case of 'cold' or thermal plasmas, in particular those produced in laser-induced breakdown spectroscopy. The occurrence of local thermodynamic equilibrium is, in that case, discussed as well as the relevance of phenomena leading the system out of equilibrium, such as radiative, transient and diffusive processes. The most important directions for future work, in particular regarding non-stationary and diffusive effects, are also suggested.
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