Host lattice absorption

As we have seen above, absorption of radiation does not necessarily take place in the luminescent center itself, but may also occur in the host lattice. It is obvious to make a simple subdivision into two classes of optical absorption transitions, viz. those which result in free charge carriers (electrons and holes), and those which do not. Photoconductivity measurements can distinguish between these two classes. 

An example of the former class is ZnS, the host lattice for cathode-ray phosphors. This compound is a semiconductor. Optical absorption occurs for energies laiger than Eg, the width of the forbidden gap. This absorption cremes an electron in the conduction band and a hole in the valence band. Since the lop of the valence band consists of levels with predominant sulfur character and the bottom of the conduction band of levels with a considerable amount of zinc character, the optical transition is of the charge transfer type. Its position can be shifted by replacing Zn and/or S in ZnS by other elements (see Table 2..3). 

The processes occurring upon irradiating a material with high-energy radiation like cathode rays, X rays or y rays are rather complicated. After penetration into the solid, this radiation will give ionisation depending on the type and the energy of the radiation. This ionisation creates many secondary electrons. After thermalisation we are left with electron-hole pairs, as we do after irradiation with ultraviolet radiation just over the band gap. 

We have described in this chapter the processes and transitions which are responsible for the absorption of radiation, with special attention to ultraviolet radiation and absorption by the center itself. Our systems are now in the excited state. In the following chapters we will consider how they return again to the ground state. Sometimes they follow simply the reverse of the absorption transition, but more often they prefer a different way without hesitating to make large detours. 

Henderson B, Imbusch OF (1989) Optical spectroscopy of inorganic solids. Clarendon, Oxford 

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Fig. 14. Energy level scheme of the Tm 4f ) ion in La202S. Energy E in 10 cm" Host lattice absorption starts at 35,000 cm". ...

Figure 28 shows some of their results for YVO4 and YVO4—Bi. The former has been discussed in Sect. 3.2. The latter shows a blue and a yellow emission. The blue emission can only be excited at low temperatures upon irradiation into the vanadate host lattice. At low temperatures no energy transfer occurs and the blue emission results. The yellow emission can be excited by irradiation below the host lattice absorption, i.e. directly into the -Bi complex, or by VO excitation at higher temperatures (followed by energy migration to the VO —Bi complex). 

This is nicely confirmed by a study of some Eu -activated phosphates and vanadates with zircon structure (Blasse and Bril, 1969). The observed ratio of electric to magnetic dipole emission of the Eu " luminescence in these hosts is correlated with the position of the lowest excitation (and absorption) band of these materials and the intensity ratio. This absorption band is a c.t. transition in which either europium or vanadium or both are involved. It has, therefore, been proposed that the parity-forbidden 4f-4f transitions of the Eu " ion borrow intensity from the lowest strong absorption band (either host lattice absorption or charge-transfer absorption within the centre) and not from the 4f-5d absorption band. In conclusion we find that for intense forced electric-dipole emission from Eu two conditions must be fulfilled, viz. absence of inversion symmetry at the Eu " crystallographic site and c.t. transitions at low energies. 

The divalent rare-earth ion Eu has the 4f electronic configuration at the ground states and the 4f 5d electronic configuration at the excited states. The broadband absorption and luminescence of Eu are due to 4f - 4 f 5d transitions. The emission of Eu is very strongly dependent on the host lattice. It can vary from the ultraviolet to the red region of the electromagnetic spectrum. Furthermore, the 4f-5d transition of Eu decays relatively fast, less than a few microseconds [33]. 

It is useful to view optical absorption and emission processes in such a system in terms of transitions between distinct vibrational levels of the ground and excited electronic states of a metal atom-rare gas complex or quasi-molecule. Since the vibrational motions of the complex are coupled with the bulk lattice vibrations, a complicated pattern of closely spaced vibrational levels is involved and this results in the appearance of a smooth, structureless absorption profile (25). Thus the homogeneous width of the absorption band arises from a coupling between the electronic states of the metal atom and the host lattice vibrations, which is induced by the differences between the guest-host... 

It is difficult to predict the effect of surface functionalization on the optical properties of nanoparticles in general. Surface ligands have only minor influence on the spectroscopic properties of nanoparticles, the properties of which are primarily dominated by the crystal field of the host lattice (e.g., rare-earth doped nanocrystals) or by plasmon resonance (e.g., gold nanoparticles). In the case of QDs, the fluorescence quantum yield and decay behavior respond to surface functionalization and bioconjugation, whereas the spectral position and shape of the absorption and emission are barely affected. 

In passing we mention that the c.t. transitions of Ti + in these host lattices are crystal-structure dependent (7). These absorption bands must be ascribed to the 2tiu- d transition (5). Indeed these levels depend on the nature of the bonding... 

Table 2. Absorption bands, crystal-field splitting and centre of gravity of the Sd level of Eu2+ in several host lattices all values in kK)...

Host lattice Coordination Eu2+ ion Absorption bands Crystal field splitting Centre 54 level Ref. 

As already mentioned above, it is possible to include small quantities of a particular anion in the lattice of an alkalihalide. The alkalihaUdes, because of the low frequency of their absorptions, are particularly suited to be host lattices however, salts such as sulphates, nitrates, perchlorates, to name a few, have also been used as host lattices. 

It has been possible to measure the angular dependence of scattering by e-h droplets in germanium because the host is quite transparent between 1.66 jum (the band gap) and 25 jum (the lattice absorption band). From these measurements it was ascertained that the condensate indeed existed in the form of droplets with radii between about 1 and 10 jum. 

Molecules in crystals or dispersed in host lattices are often present in a range of environments, and this results in a broadening of the electronic absorption spectrum. Such an inhomogeneously broadened absorption band (envelope of transitions) may be considered as a superposition of several distinguishable sites. A narrow line laser can saturate one of the transitions under the envelope and the corresponding molecules will no longer take part in the absorption process. This phenomenon is referred to as hole... 

Oxyhalides. The oxyhalides of yttrium, lanthanum, and gadolinium are good host lattices for activation with other rare-earth ions such as terbium, cerium, and thulium. The use of LaOCl Tb3+ as the green component in projection-television tubes has been discussed [5.419]. LaOBr Tb3+ and LaOBr Tm3+ exhibit high X-ray absorption, and they are used in X-ray intensifying screens [5.420]. 

Cr can be doped into the cubic elpasolite lattices Cs2NaInCl and Cs2NaYClg. The Cr + site symmetry is exactly octahedral, which makes these elpasolite systems particularly attractive. With Cr + doping levels of approximately 2% broad-band luminescence corresponding to the 2g— 22 transition is observed. Figure 6 shows the 6K emission spectra. They exhibit a great deal of fine structure, much more than in any spin-allowed d-d band ever measured in absorption. We also notice some differences in the intensity distribution between the two lattices. They result from a difference of approximately 0.1 8 in the M + - Cl distance in the host lattices. 

The absorption spectrum of Bk(III) in a lanthanum chloride host matrix at 77 K was first obtained in 1964 (102). A prediction of the energy level structure of Bk(III) was made by others the same year (103). Extensive, low-temperature spectroscopic studies of BkCl3 showed the absence of transitions to excited J = 0 and J = 1 states (104, 105). This provided good evidence for a fi = 0 ground level for Bk(III), consistent with that of Tb(III)-LaCl3 (106). Experimental and theoretical studies of the crystal field parameters of Bk(III) in a LaCl3 host lattice have also been reported (107). 

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