Determination of overall decay and collisional rate constants in flames by laser-excited atomic fluorescence spectroscopy

Abstract

The use of laser-excited fluorescence spectroscopy has generated interest as a technique for the measurement of atomic trace concentrations in heterogeneous flame systems. Under saturation of the radiatively excited level and for a two-level electronic system, a direct relationship, independent of laser intensity and collisional conditions, exists between the observed fluorescence intensity and the total analyte concentration in the flame. In practice, such relationship is not as simplistic because most species are best described by three- or multi-level electronic systems for which the fluorescence radiance exhibits a dependence on the overall decay and collisional rate constants governed by the experimental system used. In the present study, the numerical methods of reiterative convolution and multiple linear regression are employed to analyze fluorescence signals and to approximate the values of the overall decay and collisional rate constants. The experiments are performed on indium and thallium in the H₂-O₂-Ar, C₂H₂-O₂-Ar, and N₂-H₂-O₂-Ar flames. For these systems, the overall decay rate constants evaluated are for the radiatively excited level and for a thermally excited level, the collisional rate constants are those controlling the population transfer between these two levels. The validity of these rate constants is assessed by employing their values in the determination of the total concentrations of the analytes in the three flames. These concentration results are then compared to theoretically calculated analyte concentrations. These comparisons also help in understanding the significance of the rate constants in the accurate calculation of the analyte concentrations. In addition, the overall decay and collisional rate constant results are used in the determination of the fluorescence quantum efficiency, and the saturated spectral irradiance. The results from these determinations help in the characterization of the environmental system, and in the prediction on the ease of saturation for different excitation transitions.