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Lidia Smentek



Spectroscopy of Materials, Quantum Chemistry

The tools of quantum chemistry are used for the description of the spectroscopic properties of materials doped by lanthanide and actinide ions. The ions of these two series have a common electronic structure of equivalent electrons, 4f and 5f, respectively. Due to this property the theoretical description of such materials is formulated in the language of operator techniques of atomic spectroscopy.

Many materials become fluorescent when doped with the lanthanides. Using appropriate rare earth ions and particular host lattices it is possible to select the light emission with the desired wavelength from the visible and near infrared regions of spectra. For example, Er3 + has its dominant emission at 1.5 (m which is the preferred wavelength for long distance fiber-optic communication. This property of rare earth ions has caused an increasing interest in the application of rare earth semiconductors as active media for light emitting diodes, injection lasers and other optoelectronic devices.

The use of rare earths in clinical medicine (surgical tools), medical technology (noninvasive tests) and molecular biology (analytical tools) is growing at an ever increasing rate. Lasers are used practically in almost all possible aspects of life, but their impact upon the state of the art of medicine and surgery is one of the most revolutionary ones observed since the first laser beam was generated in 1960. Virtually all applications of rare earths in crystals in the medical field are based on their unusual spectroscopic properties. In spite of the complex structure of the lattice particles of the environment of the rare earth ion in a crystal, their spectra are characterized by very sharp, atomic in nature, lines while broad bands are typically expected in the solid state. Due to this particular property there is an explosion in the applications of these systems. At the same time, the theory of the spectroscopic properties of rare earth ions in crystals is not developed adequately.

Materials which contain the rare earth ions are not only recognized as good laser media, but they are also widely used in newly developed technologies such as semiconductor light-emitting devices and as dopants in optical fibers. Due to their favorable properties, rare-earth-doped fiber lasers offer rich possibilities in telecommunication applications, atmospheric pollution measurements, noninvasive medical diagnostic procedures and industrial monitoring and control.

The most impressive applications of the rare earth doped materials are in bioinorganic chemistry; the importance of these materials has increased since it was discovered in 1970 that rare earth ions are ideal probes in the investigations on binding sites in proteins. The most attractive role of the lanthanides is connected with the work on calcium binding in proteins. Calcium, as an inert system, is very difficult to examine directly, but at the same time it is a very important element in biochemistry. The chemical similarity of rare earth ions to calcium, and their ability to luminesce at room temperature, are the main properties due to which the lanthanides are so commonly used as probes.

Substitution of Ca by a rare earth ion has enabled one to establish the position of Ca in trypsin, an enzyme of the pancreatic juice. It was also found that admixing Tb3 + to another protein, pancreatic elastase, results in strong luminescence of Tb, and at the same time, under such conditions binding of calcium, magnesium and zinc is possible.

Magnetic resonance imagining of biological structure is the alternative technique to the light-based microscopy. The efficiency of this technique is not limited by the scattering of light to the layers of cells on the surface, and in addition it does not produce toxic substances as do the methods that use dyes and fluorochromes. This is an especially useful method to monitor cellular Ca2 + and its role in the physiology of a cell and internal biochemical processes. Recently, the lanthanide ion Gd3 + has been used as a contrast agent. This ion is characterized by the half-filled shell of equivalent 4f electrons, and it belongs to the group of the so-called S-state ions (their ground state is [8S]7/2). Gadolinium is a good contrast agent due to its high magnetic moment. To avoid toxicity of the aqua ion, all but one coordination site of Gd3 + are bound by a chelate. The remaining site is left for the water molecule that produces the signal in all imaging experiments.

The properties of LiGdF4:Eu, again due to the special properties of gadolinium ion, has led to a technological revolution of a new generation of highly efficient fluorescent lights. This new phosphor replaces conventional materials; it emits twice as many photons than it absorbs. One uv photon from Xe (instead of poisonous mercury in standard devices) is absorbed by Gd3 +, and then in two portions it is transferred to two doping Eu ions that act as luminescence centers.

These few examples demonstrate the wide range of applications of the lanthanide ions in various disciplines, and show the motivation for the theoretical research even if, at the first sight, there is no direct connection between basic research and the practical use of its results.

What is the mechanism of the energy transfer between the sensitizer and activator? What is the efficiency of the energy transfer and the sensitized luminescence? The answers to these questions are not known. At the same time expected high precision of the answers shows the importance of the basic research that has to be performed in the field of spectroscopy of materials doped by the rare earth ions; an adequate theoretical model of these processes is the object of presented research.

The research is based on perturbation theory. The properties of the materials are discussed in the terms of the effective tensor operators that are defined in the language of Racah algebra. The theory is verified by ab initio calculations performed by unique numerical programs. Advanced tools of quantum chemistry and a long list of very interesting phenomena to be described and understood are offered to potential students and collaborators.