Schemes for excitation of rare earth monoxides in flame laser-induced molecular ionization spectrometry

Abstract

Laser-induced ionization spectra of PrO, TbO, and CeO molecules in a flame were recorded in the wavelength ranges 440-480 nm and 535-575 nm. Based on study of the spectra, we propose a two-step scheme for excitation of PrO and CeO molecules in which in the first step, the molecule goes to the excited electronic state, while the excitation energy in the second step is selected so that the total energy imparted to the molecule corresponds to its ionization potential.Introduction. The laser-induced ionization spectrometry method is based on laser excitation followed by ionization of atoms or molecules in flames. Detection limits have been achieved so far for more than 40 elements (including rare earths) that are significantly better than those obtained by other spectroscopic methods using flame atomization. The analytical form for determination of elements in flame laser-induced ionization spectrometry is generally atoms, formed when the analyte sample is injected into the flame. The fraction of free atoms depends on the nature of the element, the temperature of the analytical zone, the composition of the flame, and a number of other factors. In most work on laser-induced ionization spectrometry, widely available low-temperature flames have been used. However, use of such flames for determination of difficultly atomizable rare earths is not very effective, due to the small fraction of their free atoms. At the same time, the concentration of monoxide molecules for such elements in the flame is rather high. Therefore it seems logical to use specifically monoxide molecules as the analytical form. Laser-induced ionization spectra of monoxides of Sc, Y, and La in a flame were obtained for the first time in [1]. The use of monoxide molecules as the analytical form is described in [2], devoted to determination of phosphorus.Systematic studies and development of flame laser-induced molecular ionization spectrometry were begun in the mid-1990s [3]. For ionization of LaO molecules, it is suggested that two-step schemes be used [4] in which laser radiation in the first step takes the molecule to an excited electronic state, and in the second step to the ground state of the molecular ion, the energy of which corresponds to the ionization potential of the neutral molecule. This makes it possible to significantly increase the ionization signal compared with one-step excitation. The magnitude of the signal obtained depends on the choice of the excitation scheme for the monoxide molecules. It is not possible to choose the optimal excitation scheme a priori, since the molecules have a huge set of excited states participating in dissipation of the excitation energy. In addition, for a very large number of molecules no literature data are available on their radiation and energy characteristics.