The relationship between the fluorescence yield and the underpopulation of doublet excited states

Abstract

The influence of radiative non-equilibrium on the occupation of excited metal states in flames can be investigated by measuring the deviation Δ T of line reversal (or excitation) temperature T L from “true” (or translation) temperature T t of the flame as a function of metal concentration. In this paper it is shown that, under certain conditions concerning the rates of the de-excitation processes, the measurements of Δ T for atoms with close lying upper levels (such as the alkali atoms) may be interpreted by assuming a single excitation level. In that case, the fluorescence yield Y D , obtained when both lines are used in excitation and measured in fluorescence, occurs as a parameter in the expressions which describe the behavior of Δ T for low and high metal densities. The theoretical relationships were checked by comparing the experimental values of Y D and Δ T for the yellow sodium doublet in a typical hydrogen-oxygen-argon flame. Satisfactory agreement was found. In order to determine more reliably the low density asymptote of the excitation temperature, the variation of Δ T with atomic density was theoretically analysed to first order in the density. Fluorescence yield factors were again measured for several alkali lines with an improved experimental set-up and agreed with our previous results, (1,2,3) except for the sodium doublet. The effective cross sections (see Table 1) derived from these yield factors for the quenching of excited sodium (3P 1 2 , 3 2 ) , potassium (4P 1 2 , 3 2 ) and rubidium (5P 1 2 , 3 2 ) atoms by various flame molecules, are in satisfactory agreement with the results of Jenkins' flame experiments. (4,5)