Optical studies under pressure

In the next section the rare-earth compounds that have been studied by optical means under pressure so far will be reviewed. Then, after a brief introduction of the most commonly used high pressure device, the diamond anvil cell, sect. 4 presents a discussion of the pressure-induced changes of the crystal-field levels and their interpretation. In sects. 5 and 6 some aspects of the dynamical effects under pressure are discussed. These include lifetime and intensity measurements, the influence due to excited configurations and charge transfer bands, and the electron-phonon coupling. 

Studies of the optical properties as a function of pressure and temperature can yield valuable clues to changes in the electronic structure of electronically conducting fluids. This is especially true if the dielectric dispersion is measured. But optical studies under these conditions pose an obvious technical problem. The cell, the furnace, and the high-pressure vessel all must include windows that are transparent over the widest possible wavelength range. The cell window, in particular, must be resistive to chemical attack since it is in contact with the fluid sample. 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

In the study of atmospheric aerosols several techniques have been used to determine optical constants from measurements on particulate samples. And there have been many such measurements. Yet under pressure from funding agencies and from those waiting at computer terminals for optical constants of the complicated mixture that is the atmospheric aerosol, comparatively little effort has been expended on evaluating these techniques by applying them to particles of solids with known optical constants. 

OPTICAL STUDIES OF NON-METALLIC COMPOUNDS UNDER PRESSURE... 

The most common pressure sensor for optical studies is ruby (AI2O3 Cr3+, Piermarini et al. (1975)), whose strong Ri and R2 luminescence line shifts under pressure have been calibrated up to 180 GPa at room temperature (Mao et al., 1978 Mao, 1989). At low temperatures the line position has to be corrected by a known temperature-induced shift (Noack and Holzapfel, 1979). Besides ruby also other sensors utilizing rare-earth ions have been proposed and discussed in literature (Shen et al., 1991). In most of these cases the pressure induced shifts of luminescence lines are used to determine the pressure (see sect. 4.5). 

By far the largest part of the experimental results presented in this work, has been obtained with the aid of optical studies. Only in exceptional cases other methods, like electron-spin resonance or neutron scattering, have been employed to get information about energy levels under pressure. Obviously, these methods must be used in the case of non-transparent materials, where optical methods are not suitable. 

Before continuing, some words must be said with regard to the terms rare earths and f elements used in this chapter. The term rare earths includes the elements Sc, Y and the lanthanides La through Lu. However, this chapter solely deals with divalent or trivalent rare-earth ions which are optically active, i.e., possess a partially filled f-shell. Thus, although the term rare earths is used in this chapter, it should be kept in mind that the elements Sc, Y, La, and Lu are excluded. In some exceptional cases the more general term f elements will be used, as for example when high pressure studies on actinide ions with a partially filled 5f shell are discussed. There are only few studies on 5f elements in non-metallic compounds under pressure, however, it seems interesting to compare the results found for these ions with those for the 4f-elements. 

The optical studies performed on most samples of table 1 were aimed at different aspects of the f-electron properties. A considerable amount of the work was concerned with the energy level shifts under pressure. From these shifts, variations of free-ion parameters, crystal-field parameters or crystal-field strengths with pressure have been deduced. Other studies concentrated on changes in lifetimes or intensities, the efficiency of energy transfer between rare earths or rare earths and other impurities or on electron-phonon coupling effects under pressure. The various aspects investigated under high pressure will be presented within the next sections. 

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