Laser-induced resonance fluorescence as a diagnostic technique in non-thermal equilibrium plasmas

Laser-induced fluorescence (LIF) techniques currently used as an optical diagnostic to study and characterize non-thermal equilibrium plasmas are reviewed. The general mechanisms occurring in one- or two-photon resonant absorption-induced fluorescence when used for the detection of species in their fundamental state are analysed. We emphasize the techniques based on the simultaneous absorption of two photons for the detection of light atoms and molecules and, particularly, two-photon absorption laser-induced fluorescence (TALIF) and derivative techniques, such as resonant enhanced multi-photo-ionization, two-photon absorption laser-induced stimulated emission (TALISE) and photo-fragment translational spectroscopy, in which molecules are photo-dissociated and the ejected atomic fragments are simultaneously detected by TALIF. The kinetics of one- and two-photon absorption are treated in detail including, for the latter, a modelling, which indicates the conditions of emergence of TALISE. The main calibration methods allowing one to obtain the absolute density of species detected by LIF and some examples of applications demonstrating the interest of these diagnostic techniques in plasma processing are presented.