Abstract
This paper is part of a more general study aimed to the determination of the best experimental procedures for reliable quantitative measurements of Fe–Mn alloys by LIBS. In this work, attention is pointed on the self-absorption processes, whose effect deeply influences the LIBS measurements, reflecting in non-linear calibration curves. The effect of self-absorption on the line intensity can be quantified by defining a self-absorption coefficient, that measures the deviation of the line intensity from the linear extrapolation of the curve of growth in the optically thin regime. The authors demonstrated in a previous paper that self-absorption coefficients could be calculated once the electron density of the plasma is known and the Stark coefficients of the lines are available. However, when the Stark coefficients of the lines of interest are not known, a different approach is needed. In this work a new method for evaluation of self-absorption coefficients in LIBS measurements is presented, which does not require the knowledge of Stark coefficients. In order to understand the basic principles and setting out the theoretical tools that will be used for the analysis of the alloys, a preliminary study was done on pure Mn; LIBS spectra were acquired in different experimental conditions, at different laser energies and different delays after the laser irradiation of the sample. Moreover, collinear double pulse measurements were also performed. Analytical relations were derived and experimental procedures devised for evaluation of the self-absorption coefficients of several Mn lines, which are important for characterization and control of the experimental conditions in which the analysis is performed.
Highlights
In recent years, the Laser-Induced Breakdown Spectroscopy (LIBS) technique affirmed as a powerful analytic tool for the determination o f the elemental composition o f materials [1-4 ]
In this paper, starting from the theoretical treatment o f these effects, we propose a method for quantifying the influence o f selfabsorption on the measured intensity o f the lines considered, at the same time providing a quick way o f determining, among the different emission lines o f a given element, the ones more appropriate for the calculation o f plasma parameters as w ell as for LIBS analytical measurements
From Eqs. (13), (14) and (15) we can immediately see that the dependence o f the line intensity ratio on the plasma parameters is quite complex, since both the absolute values o f the fc(A0) coefficients and their ratio depend on plasma temperature; both the absolute values o f the self-absorption coefficients and their ratio depend on the plasma dimensions / and, through the parameter n, on the plasma electron density n e
Summary
The Laser-Induced Breakdown Spectroscopy (LIBS) technique affirmed as a powerful analytic tool for the determination o f the elemental composition o f materials [1-4 ]. The method here proposed is based on the measurement o f the intensity ratio o f two lines o f the same ionization stage and on the comparison o f the experimental to the theoretically predicted value; this approach is well known in the literature as a direct check o f the absence o f important self-absorption effects in spectroscopic measurements [34]. In this paper, starting from the theoretical treatment o f these effects, we propose a method for quantifying the influence o f selfabsorption on the measured intensity o f the lines considered, at the same time providing a quick way o f determining, among the different emission lines o f a given element, the ones more appropriate for the calculation o f plasma parameters (temperature and electron density) as w ell as for LIBS analytical measurements. The k (A) coefficient is the absorption coefficient (cm- 1 ), which takes into account both the absorption by lower level
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