Basic mechanisms of signal enhancement in ns double-pulse laser-induced breakdown spectroscopy in a gas environment
Understanding the mechanisms of the signal enhancement in double-pulse laser-induced breakdown spectroscopy is highly desirable. It is evident, however, that it is not possible to fit a unique general model to the observations obtained in collinear or in orthogonal geometries, in pre-spark or in re-heating schemes, using ns or fs pulses, in a gas or liquid environment. We believe, instead, that by considering separately the specific experimental cases (for example: a given irradiation geometry, a given pulse timing, etc.) the comprehension of the processes occurring during double-pulse experiments might become easier. We focus on one specific experimental case, namely ns double-pulse irradiation of solid targets in a gaseous environment, and classify the experiments according to the irradiation mode (orthogonal pre-spark, collinear and orthogonal re-heating). Then, we propose a description of the processes occurring in the different cases, on the basis of data and interpretations that are available in the literature.