Doped alkali halides

Illustrative examples doped alkali halides and silica glasses... 

Rb NMR shieldings have been shown experimentally to depend linearly on the distance of the Rb" ion to its nearest neighbor shell of halide ions in the family of Rb-doped alkali halides, and theoretical calculations bear out the experimental linear trends. The shielding derivatives with respect to the ionic distance are positive and are very similar, largely independent of the identity of the halide ion for the distances that naturally occur in the alkali halide lattices. At much shorter distances, we would not expect this to be the case. 

The uranium-doped alkali halide systems show, that different types of luminescent uranate centres are possible. 

The emission of the Tl" -doped alkali halides is due to the P - So transition on the Tl ion (see Sect. 3.3.7). It is usually assumed that the afterglow is due to hole trapping in the host lattice (trapped exciton, see Sect. 3.3.1), whereas the electron is trapped by the activator. In Csl Na the emission is due to an exciton bound to a Na ion, in Csl to self-trapped exciton emission. 

Lead, with an electron configuration [Xe]4/ " 5d °6i 6p, has been widely perceived to be spectroscopically silent because it has a electronic configuration in all three common oxidation states (0, +2, and +4). However, nothing could be farther from the truth Although lead compounds do not exhibit d-d transitions, they do exhibit both intraatomic and charge-transfer (CT) transitions. As early as 1952, Klotz et al. (50) reported absorption spectra attributed to lead [Pb(II)] binding to proteins (37, 51-53). Even earlier, the absorption spectra of both aqueous (54—60) and solid-state (61-68) lead-doped alkali halides had been reported, as well as the absorption spectra of some Pb(II) doped alkaline earth compounds (69-73). 

However, detailed insights into the electronic transitions of lead-thiolate complexes can be gained from studies on T1(I) [which is isoelectronic with Pb(ll)] and Pb(ll)-doped alkali halides in the solid state (Fig. 5) (37, 50, 54, 113). The details of the electronic transitions in T1(I) doped alkali halides and related compounds have been studied extensively (both theoretically and experimentally) because these compounds have interesting luminescent properties and are useful in phosphors. [We will not discuss the emission spectra of these compounds, as they are not relevant to our discussion of lead-thiolate CT in coordination complexes rather, the reader is directed to several extensive reviews of luminescence in doped alkali halide systems (95, 113, 114).] The characteristic absorption spectra of alkali halides doped with a Tl(l) type ion consist of four bands, known as the A, B, C, and D bands. The A band is at lowest energy, followed by B, C, and D respectively the extinction coefficients of the bands follow the general trend D > C > A > B (Fig. 6) (115, 116). Two weaker bands labeled D and D" are also shown in Fig. 6, which are attributed to the same CT transitions as the main D band (116). In Section II.E, we will... 

The earliest theoretical treatment of thallium-doped alkali halides is due to Seitz (115), who interpreted the absorption spectra in terms of a substitutional... 

In 1962, Sugano showed that the Seitz model (115) could be interpreted as a molecular orbital model (123), an interpretation that clarifies analysis of these systems. In this interpretation, the absorption bands observed in the TI(I) doped alkali halide system come from the electronic transition aigf a g) hu), but the excited states are still calculated assuming an ionic interaction between the metal and the hgand. Since the thallium-chlorine bond is actually largely covalent, Bramanti et al. (118) modified the approach and used a semiempirical molecular orbital (MO) calculation to describe the energy levels of T1(I) doped KCl. Molecular orbitals were constructed by the linear combination of atomic orbitals (LCAO) method from the 6s and 6p metal orbitals and the 3p chlorine orbitals. Initial calculations were conducted with the one-electron approximation the method was then expanded to include Coulomb and spin-orbit interactions. The results of Bramanti et al. were consistent with experimental... 

Although most of the work on this series of doped alkali halides has dealt with T1(I), several studies have been conducted on Pb(II) doped systems (63-68). From this work, it is clear that for the most part, the same trends that have been observed and explained in the T1(I) case are relevant to Pb(ll) as well that is, the same basic series of bands is observed in the absorption spectrum, and these bands are due to a combination of intraatomic and CT transitions. The C band is a multiplet that has been interpreted in terms of the Jahn-Teller effect... 

The interpretation used to explain the absorption spectra of Tl(l) and Pb(ll) solid-state complexes can also be extended to aqueous solution spectra of Pb(ll) complexes (Fig. 5) (54—60, 62). Although aqueous lead halide absorption spectra reported by Fromherz et al. (54, 55) were not originally interpreted in the context of CT spectroscopy (56, 125), in retrospect it is clear that the bands reported are the same as those observed in doped alkali halide crystals, as are later spectra reported by Bendiab et al. (59, 60). For example, in solution a spectral shift to longer wavelengths is observed for increasing atomic number of the halogen ligand (54, 55), which is consistent with a CT process (126). 

Cusso et al. (1991) stress that Eu -doped alkali-halides suit well as personnel protection dosimeters and as a tool for studying biological effects at low UV doses. 

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