Spectra And Energy Levels Of Rare Earth Ions In Crystals Pdf

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Applied Optics

This website uses cookies to deliver some of our products and services as well as for analytics and to provide you a more personalized experience. Click here to learn more. By continuing to use this site, you agree to our use of cookies. We've also updated our Privacy Notice. Click here to see what's new. The present status of our knowledge of the structure of the spectra of the doubly and triply ionized spectra of the rare earths is derived partly from experimental data of the emission spectra of the free ions which provide the energy level scheme in great detail but are difficult and laborious to analyze.

For the lower levels knowledge of the structure comes from the crystal absorption and fluoresence spectra. In all cases approximate theoretical calculations of the energies are essential. The rare earths, in common with the actinides, have the most complicated spectra of any of the elements. This is because the incomplete 4 f -shell produces a very large number of low-lying levels. The transitions between these give a many-line spectrum without any apparent regularities.

Even multiplets, which are such prominent features of the spectra of the transition elements, are in general unrecognizable. For these reasons the rare-earth spectra have long been neglected. Because of the growing importance of the rare earths in recent years, their spectra have received renewed attention.

In the present article we shall restrict ourselves to the spectra of the doubly and triply ionized elements for several reasons. Most atomic spectra are obtained by vaporizing the element, exciting the vapor by an electric discharge, and thus obtaining the emission spectrum of the free ions or neutral atoms.

The triply ionized rare earths, and to a lesser extent also the doubly ionized ones, can be incorporated into crystal lattices where they retain their sharp energy levels only slightly modified by the internal crystal field. This makes it possible to obtain some of the energy levels, particularly the important low ones, from the absorption and fluorescence spectra of crystals containing these rare earths.

An extensive program of the study of the spectra of doubly and triply ionized rare earths is under way at Johns Hopkins University and both the emission spectra of the free ions and the crystal spectra are investigated. One of the chief purposes of this study is the comparison of the levels of the free rare-earth ion with those of the same ion in a crystal lattice, in order to be able to obtain an exact evaluation of the influence of the crystal field. The present paper is a report of the work carried out or planned for the immediate future at Johns Hopkins University.

The free-ion spectra are obtained from a controlled spark in a rare-gas atmosphere at reduced pressure. The 3-A spectrum is that of a dc arc, and even at this low-excitation lines of the doubly ionized ion appear.

Above A there is no trace of the second singly ionized spectrum, and as the excitation is increased the third also gradually gives way to the fourth. The lower excitation of an arc or microwave discharge is also needed to eliminate the first and second spectra, but for our purposes it is not necessary to distinguish between the two. It is important for this separation that the pulsed discharge not be allowed to oscillate as this will superimpose a lower excitation discharge on the high and make the suppression of the second spectrum difficult.

Figure 2 shows a comparison of hot and mild sparks for various wavelength regions of the neodymium spectrum. In the upper section Nd iv lines predominate for the hot spark A although some very weak Nd iii may be found. A slightly longer wavelength region center is very rich in Nd iii.

Even longer wavelengths bottom show mainly ii and i in the mild spark B and very little in the hot. This separation by wavelength is also very significant for the analysis and is discussed further in Sec.

The use of large gratings is therefore most appropriate. In our work a The use of an interferometer would not contribute much because, particularly in the hot spark, the lines are not sharp enough for utilizing the greater inherent accuracy of the interferometer. The fact that thousands of lines must be measured in each spectrum makes the use of modern automatic measuring techniques imperative.

For the crystal absorption spectral light from a continuous source, usually a high-pressure mercury or xenon lamp, a tungsten ribbon lamp or a zirconium arc depending on the wavelength region, are passed through a properly oriented crystal immersed in liquid helium.

The Zeeman effect is often important, so the dewar vessel must be constructed so that it can fit between the pole pieces of a magnet. At low enough temperature the wavenumbers of the absorption lines directly represent the position of the excited energy levels. The number and spacing of the components depends on the symmetry and intensity of the crystal field. A sharp-line fluorescence spectrum is obtained, containing in some cases several hundred sharp lines, [4] by illuminating the crystal by an intense light source.

When the coupling between the electronic level and the crystal lattice is large the excitation energy is dissipated before fluorescence can take place.

This is the case for most hydrated salts [5] except for the central rare earths from Sm to Dy. In other lattices all rare earths fluoresce strongly but not from all levels.

The anhydrous chlorides and bromides, the garnets and the oxides are among the crystals that show general fluorescence. III, especially Fig. Absorption transitions occur between this lowest level and any higher level unless forbidden by selection rules.

All six appear in the left-hand spectrum of Fig. Very low levels, such as the higher Stark components of the ground state, for which the absorption spectrum is not in an accessible region, must be confirmed by additional fluorescing levels.

In complicated fluorescence spectra, single levels can be excited by monochromatic illumination, and this greatly helps in the analysis of such spectra.

The fluorescence spectra are extremely sensitive for very small amounts of impurities, and great care must be taken to identify such impurity lines which occur in even the purest available materials. For the crystal spectra the task of obtaining the energy level system from the observed wavelengths of the absorption and fluorescence lines is in general direct and simple. For the free-ion emission spectra this is a much more elaborate and difficult problem.

It can be solved much more expeditiously when one knows approximately what to expect. In fact, a satisfactory analysis of these complicated spectra would be virtually impossible without a thorough theoretical study of the structure of the energy levels.

Such a study is based on the general theory of atomic structure, as for instance set forth in the book by Condon and Shortley [8] or those by Slater, [9] systematized for the complicated rare ion cases by Racah.

It has been known since the early days of the Bohr theory that the 4 f , 5 d , and 6 s electrons have very nearly the same energy for the rare earths. This is true especially for the neutral and singly ionized atoms. Fortunately, the situation is much clearer for the divalent ions and even more so for the trivalent ones.

The reason for this is that because of the higher nuclear charge and the consolidation of the inner shells the screening is more perfect and the levels appear more nearly in the order of their principal quantum numbers.

For the fourth spectra the 4 f orbits always have the lowest energy, then come 5 d , 6 s , and 6 p. Figures 4 and 5 show for ions with three external electrons how this situation changes as we go to the lower stages of ionization.

For the second and first spectra, as Fig. It is, of course, not sufficient to know merely the position of the centers of the configurations as indicated in Fig. We must know their total width and the arrangement of the individual energy levels. The method for doing this can be briefly indicated as follows. The outer electrons are regarded first to be in a central field provided by the nucleus screened by the 54 electrons of the completed xenon-like shell.

After first considering the outer electrons as completely independent, their electrostatic repulsion and their spin-orbit interactions are introduced by the potentials. In performing these calculations interactions between states of only one configuration are taken into consideration, but no further restrictions are made. For the higher stages of ionization, other configurations of the same parity are far away for the lowest configurations and we make no fatal mistake by leaving out interconfiguration interactions, although they are by no means negligible.

The systematics of handling the very large number of individual states have been developed by Racah. This and other symmetry considerations greatly limit the number of matrix elements of H 1 and H 2 that have to be calculated and restrict the order of the interaction matrix that has to be diagonalized to the number of different states with the same J occurring in the configuration. The situation is simplest for the 4 f n configurations. This has been carried out for all the crystal spectra of the trivalent rare earths, and Fig.

For the higher ones crystal splittings also have been used for identifying the levels. It is seen that in the crowded regions the mere position of a level is not enough for identification. Here the levels are very sensitive for the particular choice of the parameters, and the fact that the calculations are only approximate ones puts a limit to the accuracy one can expect.

The situation is similar for the other trivalent rare earths. In all cases the first few multiplets have been identified beyond possibility of doubt. For the higher energies the situation is often so complicated that positive identifications cannot be made without a great deal of further study.

Figure 8 gives a summary of the results obtained so far. The energy levels obtained in this way are, of course, those of the ion in a particular crystal lattice. Using the center of the Stark components eliminates the influence of the crystal field in first order. This, however, is not sufficient to obtain the free-ion levels with any high degree of accuracy. Figures 10 and 11 give a survey of these configurations for the third and fourth spectra, respectively, obtained from preliminary empirical data and very approximate calculations.

Because of the particular couplings involved we must in general expect the strongest lines to be transitions from the bottom of one configuration to the bottom of the other one, from the middle to the middle, etc.

The situation for the third spectra is considerably different. For La iii the 4 f and 5 d levels must practically coincide and the transitions between them lie in the far infrared. For this reason they have not yet been found. Figure 10 shows that because of the varying width of the configurations the situation for the intermediate elements may be irregular. For Gd iii for instance the strong lines must again be expected in the infrared, and a search for them Callahan [25] has revealed that they certainly cannot lie in the visible or ultraviolet.

For the third spectra also the transitions between the other low configurations lie mostly in the visible or accessible ultraviolet where precise wavelength measurements can be made more easily. We may now come back to the crystal spectra of the divalent ions. The sharp-lined crystal spectra in the visible and adjacent regions of the trivalent and divalent ions are due to normally forbidden transitions between levels of the 4 f n configuration.

These are made possible, except for rare cases of magnetic dipole radiation, by the admixture through the crystal field of parts of opposite parity in the wave functions. We have seen that for the trivalent ions the latter lie usually in the vacuum ultraviolet which leaves the visible and near ultraviolet free for the crystal spectra.

Sm and Eu have long been known to form divalent ions. Recently it has been found by Kiss [27] and others that in crystals that contain trivalent rare-earth ions these can be converted into divalent ions by irradiation with gamma rays.

This makes it possible to obtain. The energy levels derived from the crystal spectra are obtained without ambiguity and their interpretation is now possible without difficulties.

Spectra and energy levels of rare earth ions in crystals

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Spectroscopy of Rare Earth Ions

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Spectroscopy of Rare Earth Ions

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Progress in crystal field theory is surveyed, an area which is basic to our understanding of the energy levels. The treatment of dynamical properties includes studies of coherence phenomena in isolated ions, energy transfer between ions and co-operative phenomena associated with ion-ion and ion-lattice interactions. In addition, the role of electron spins and nuclear spins is studied by light scattering and double resonance techniques. The presence of inhomogeneous broadening of spectral lines is observed and studied in many contexts, leading to new insights into general problems of the disordered state. Considerable attention is devoted to describing new experimental techniques whose development is of prime importance for progress in the spectroscopy of RE-activated solids.

This website uses cookies to deliver some of our products and services as well as for analytics and to provide you a more personalized experience. Click here to learn more. By continuing to use this site, you agree to our use of cookies. We've also updated our Privacy Notice. Click here to see what's new. The present status of our knowledge of the structure of the spectra of the doubly and triply ionized spectra of the rare earths is derived partly from experimental data of the emission spectra of the free ions which provide the energy level scheme in great detail but are difficult and laborious to analyze.

Optical Spectra of Transparent Rare Earth Compounds investigates the optical spectra of transparent rare earth RE compounds such as europium chalcogenides. Emphasis is placed on the underlying physics in selected examples, and theoretical results are usually presented without proof in a form that allows their application to the interpretation of experimental data. This book is comprised of 11 chapters and begins with an overview of the spectra of RE ions in ionic crystals, paying particular attention to the sharpness of many lines in the absorption and emission spectra. How these very narrow lines arise, what interactions determine their energy, and how they can be used to investigate particular properties of the solid state are explained in detail. Subsequent chapters explore the energy structure of RE free ions in solids; trivalent RE ions in a static crystal field and in a phonon field; magnetic interactions and hyperfine interactions; and Jahn-Teller systems.

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Rare-earth dopants are arguably one of the most studied optical centers in solids, with applications spanning from laser optoelectronics, biosensing, lighting to displays. Leveraging the state-of-the-art photonic technologies, on-chip rare-earth quantum devices functioning as quantum memories, single photon sources and transducers have emerged, often with potential performances unrivaled by other solid-state quantum technologies. These existing quantum devices, however, nearly exclusively rely on macroscopic bulk materials as substrates, which may limit future scalability and functionalities of such quantum systems. Thus, the development of new platforms beyond single crystal bulk materials has become an interesting approach. In this review article, we summarize the latest progress towards nanoscale, low-dimensional rare-earth doped materials for enabling next generation rare-earth quantum devices.

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