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

Abstract

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.

Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

“…This is a well-known problem in the application of the LIF technique in light emitting media like those encountered in plasma and combustion research, which is usually solved by gating the detection. The Hamamatsu PMT R928 is equipped with a gate unit that enables triggered operation of the PMT during a short time interval with a preset duration between 0.8 and 10 s. This further reduces the continuous background by a factor of 2ϫ10 4 , in principle enabling operation of the PMT with maximum gain in all circumstances. In the case of the Kr LIF measurements ͑see Sec.…”

“…Consequently, the technique has found widespread application in many research areas including lowtemperature plasma research. 4 When applying LIF ͑and any other active optical/ spectroscopic technique͒ for the detection of ground-state atoms, one has to deal with the large energy spacing between the ground state and any excited electronic state in these atoms. In the case of atomic hydrogen, the energy difference between the nϭ1 ground state and the nϭ2 state is 10.2 eV, corresponding to a 121.6 nm Lyman-␣ photon, which is well in the vacuum ultraviolet ͑VUV͒ wavelength range.…”

Mesure de la concentration absolue de SiO par spectroscopie d'absorption UV

“…La connaissance de concentrations absolues d'espèces actives comme sioX, SiHX, durant la phase plasma représente un paramètre essentiel dans la connaissance et l'optimisation du procédé industriel de dépôt d'oxydes. Parmi les diagnostics optiques développés pour la mesure de concentrations d'espèces, la fluorescence induite par laser apparaît comme la technique la plus pertinente [1][2][3], mais cette méthode ne semble pas être adaptée à un environnement industriel et peut représenter des coûts d'investissement et de fonctionnement non négligeables. Cette présente étude a été motivée par la mesure de la concentration absolue résolue en temps de SiO par absorption résonante ou par auto-absorption dans un plasma atmosphérique non thermique.…”

Calculation of the spatial resolution in two-photon absorption spectroscopy applied to plasma diagnosis

“…Calculation of the spatial resolution in two-photon absorption spectroscopy applied to plasma diagnosis M. Garcia-Lechuga, 1,2 L. M. Fuentes, 3 K. Gr€ utzmacher, 1 C. P erez, 1,a) We report a detailed characterization of the spatial resolution provided by two-photon absorption spectroscopy suited for plasma diagnosis via the 1S-2S transition of atomic hydrogen for optogalvanic detection and laser induced fluorescence (LIF). A precise knowledge of the spatial resolution is crucial for a correct interpretation of measurements, if the plasma parameters to be analysed undergo strong spatial variations.…”

“…The commonly applied detection scheme was direct or stepwise laser induced fluorescence (LIF), i.e., Ref. 3 and the references therein.…”