Temperature evaluation by simultaneous emission and saturated fluorescence measurements: A critical theoretical and experimental appraisal of the approach

Abstract

Temperature is one of the most important physical parameters of plasmas induced by a focused laser beam on solid targets, and its experimental evaluation has received considerable attention. An intriguing approach, first proposed by Kunze (H.-J. Kunze, Experimental check of local thermodynamic equilibrium in discharges, Appl. Opt., 25 (1986) 13–13.) as a check of the existence of local thermodynamic equilibrium, is based upon the simultaneous measurement of the thermal emission and the optically saturated fluorescence of the same selected atomic transition. The approach, whose appealing feature is that neither the calibration of the set-up nor the spontaneous radiative probability of the transitions is needed, has not yet been applied, to our knowledge, to analytical flames and plasmas. A critical discussion of the basic requirements for the application of the method, its advantages, and its experimental limitations, is therefore presented here. For our study, Ba+ transitions in a plasma formed by focusing a pulsed Nd:YAG laser (1064nm) on a glass sample containing BaO are selected. At various delay times from the plasma initiation, a pulsed, excimer-pumped dye laser tuned at the center of two Ba transitions (6s 2S1/2→6p 2P°3/2; 455.403nm and 6p 2P°1/2→6d 2S1/2; 452.493nm) is used to enhance the populations of the excited levels (6p 2P°3/2 and 6d 2S1/2) above their thermal values. The measured ratio of the emission and direct line fluorescence signals observed at 614.171nm (6p 2P°3/2→5d 2D5/2) and 489.997nm (6d 2S1/2→6p 2P°3/2) is then related to the excitation temperature of the plasma. Our conclusion is that the approach, despite being indeed attractive and clever, does not seem to be easily applicable to flames and plasmas, in particular to transient and inhomogeneous plasmas such as those induced by lasers on solids.