In this chapter we discuss plasma morphology and its relation to LIBS analysis. We limit the presentation to studies of the past decade, and only a few earlier results, which are of special importance, are mentioned. More attention is given to recent publications, of the past five years. We also limit our discussion to those studies that are relevant to chemical analysis. Many other studies, which were focused on plasma physics or on the laser ablation processes and were not related to LIBS, are not included.
The term plasma morphology refers to spatial characterization of the plasma produced by a laser pulse. The information may be either one-, two-, or three-dimensional and is usually time dependent. Moreover, the spatial data may be related to various plasma characteristics, such as its density, temperature, and compositional distribution. Spectral information is often of interest, since it indicates the presence of atomic species at various locations in the plasma andallows for calculation of temperature and pressure distributions.
The morphological information is directly related to plasma dynamics. It is generally considered that the mechanism for laser-induced breakdown involves three consecutive processes: firstly, a multiphoton absorption, which leads to the ionization and the establishment of free electrons; secondly, a nonresonant continuum absorption of the laser radiation by these free electrons and plasma charges (inverse brehmsstrahlung); and, finally, electron collisions that lead to further ionization of the gas. The last stage results in an increased electron density, heating, and expansion of the gas.