Energy bands, relation earths

The intensification of the band identified by Moncuit aig a u for the free ion (situated at 2,800 A for the platinocyanides and at 2,400 A for the palladocyanides) when working on thin polycrystalline sheets can easily be explained by the preferential orientation of the crystals on the quartz plate. It must be remembered that this band related to incident vibrations polarized perpendicular to planar groups no longer appears clearly at this energy for the complexes with alkali-metal and alkaline-earth cations. 

Yttrium aluminum borate, YAlj (603)4 (abbreviated to YAB), is a nonlinear crystal that is very attractive for laser applications when doped with rare earth ions (Jaque et al, 2003). Figure 7.9 shows the low-temperature emission spectrum of Sm + ions in this crystal. The use of the Dieke diagram (see Figure 6.1) allows to assign this spectrum to the " Gs/2 Hg/2 transitions. The polarization character of these emission bands, which can be clearly appreciated in Figure 7.9, is related to the D3 local symmetry of the Y + lattice ions, in which the Sm + ions are incorporated. The purpose of this example is to use group theory in order to determine the Stark energy-level structure responsible for this spectrum. 

Location of the localized states related to divalent and trivalent rare-earth ions relative to the valence and conduction bands of the host lattice is one of the most important factors that control the luminescence properties of rare-earth ions in solids. The location of the ground states of Ln ions with respect to the valence band can be estimated from the energies of the charge-transfer transitions (CTT), which are responsible for the broad bands in the excitation spectra of Ln ions. CTT is considered to be a transition of an electron fi om figands to the Ln ion. In the energetic diagram, it corresponds to the transition fi om the top of the valence band to the Ln " " level. The location of Ln " " can be estimated if the energy of the ionization transition (IT) is known. The IT is the opposite process to the CTT and corresponds to the transition of an electron from the Ln ion to the conduction band. 

Electron energy-loss spectroscopy (EELS), a powerful technique for material characterization at nanometer spatial resolution, has been widely used in chemical microanalysis and the studies of solid-state effects [4]. In EELS, the L ionization edges of transition metal and rare earth elements usually display sharp peaks at the near-edge region, known as white lines. For transition metals with unoccupied 3d states, the transition of an electron from 2p state to 3d levels lead to the formation of white lines. The L3 and L2 lines are the transitions from 2p / to 3d / 3d and from 2p / to 3d /, respectively, and their intensities are related to the unoccupied states in the 3d bands [5,6]. 

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