Energy levels under pressure

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. 

The shift of electronic energy levels under pressure predicts a redshift in all spectra eminating from regions of high space-time curvature. This includes all galaxies and quasars and their immediate environments. Consistent interpretation of observed redshifts on these grounds would drastically modify the picture currently based on cosmological Doppler shifts. 

When combustion proceeds under high pressure, separate atoms and molecules may no longer be regarded as isolated systems. Owing to their interaction, broad energy bands arise from the sharp energy levels of the atoms and molecules. The continuous spectra of flames... 

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. 

Non-metallic rare-earth compounds studied under high pressure. In almost all cases the energy level shifts as a function of pressure have been determined. The second column gives details concerning the measurements and evaluations made. In particular the following abbreviations are used L Luminescence-, A Absorption-, E Excitation-, S Site-selective spectroscopy, O Other methods, EPC Electron-Phonon Coupling, Int Intensities, LT Lifetime, CFP Crystal-Field Parameters, FIP Free-Ion Parameters, IP Intrinsic Parameters, ET Energy Transfer... 

On the other hand, the luminescence quantum efficiency can decrease. Apart from a decrease of the oscillator strengths themselves, many other mechanisms can cause such reduced quantum efficiencies, as, for example, enhanced electron-phonon coupling, generation of new paths for deexcitation or energy transfer processes. In fact, these processes will not affect the oscillator strengths of the transition but simply influence the occupation of the excited level. Therefore, the oscillator strength of a given transition may still increase under pressure, however, this increase is completely covered up by a fast depletion of the excited level. 

The main emphasis in this chapter was put on the optical investigations under high pressure. These studies reveal a considerable part of the energy level scheme and can be used also to study lifetimes and intensities. All these quantities are related to and influenced by the electronic states of the other constituents in the crystal. However, it should be noted that many other physical methods have been employed in connection with high-pressure devices. 

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