Absorption Spectra of Alkali Halides

It has to be clearly understood, however, that the electronic excitation and the formation of the metal and halogen atoms are different processes. Thus temperature, which has little effect on the ultra-violet absorption spectrum of potassium bromide in a wider range (100-400°C) has a marked influence on the quantum efficiency of the reaction forming colour centres31 32. Thus at — 100°C no metal atoms are formed whereas at 400°C almost every quantum absorbed gives rise to a metal atom. The quantum efficiency at 0°C is J. This implies that the production of metal atoms is a secondary process, dependent on the thermal oscillations of the crystal lattice, which, however, has little or no effect on the primary process of electron excitation on absorption of light quanta. The reaction may therefore be represented as follows for the alkali halide, MX. 

There is considerable evidence that in the excited state of the complex considerable charge transfer has occurred and that the primary process of photo-excitation is more akin to the reaction 

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ULTRA-VIOLET AND VISIBLE SPECTROSCOPY Absorption Spectra of Alkali Halides... 

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... 

A major problem associated with the use of 2.11.6 has been the lack of accurate electron affinities. However, recent measurements of A for the halides were made with great precision by Berry and Reimann by u.v. absorption spectra of alkali halide vapours heated by shock waves. With accurate A values, eqns. 2.11.6 probably represent the best method for the evaluation of A/ffit and AGjat since the heats and free energies of formation are known for most of the common salts. " There are several cases for which the thermodynamic data required for the calculation of AGfat are incomplete, and providing A i t is known, the former quantity can be obtained from the relation... 

Br- (g). The electron affinity of Br (g) is calculable by the method of lattice energies. Selecting the crystal RbBr, because Rb+ and Br have exactly the same nuclear structure, and taking the exponent of the repulsive term to be 10, we have computed, for the reaction, RbBr (c) = Rb+ (g)+Br g), Dz= —151.2 whence the electron affinity of Br (g) becomes 87.9. Using the lattice energies of the alkali bromides as calculated by Sherman,1 we have computed the values 89.6, 85.6, 84.6, 83.6, and 89.6, respectively. Butkow,1 from the spectra of gaseous TIBr, deduced the value 86.5. From data on the absorption spectra of the alkali halides, Lederle1 obtained the value 82. See also Lennard-Jones.2... 

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

Fig. 3. Absorption spectra of 21 (n = l)-alkali metal halide-Et3N systems in chloroform...

Ochsenfeld C, Gauss J, Ahlrichs R. An ab initio treatment of the electronic absorption spectra of excess-electron alkali halide clusters Na , , CI up to Na18Cl17. J Chem Phys 1995 103 7401-7407. 

The ultra-violet and visible absorption spectra of irradiated alkali halides were first studied by Pohl21, who observed an intense band between 400 and 800 m/x. The centres responsible for this absorption were called F-centres and the band F-band. The fact that only one band was observed in the absorption range indicated that only one electron was involved in the transition. 

Other Azides. Preliminary absorption spectra of CUN3 [86], CdNe, and HgNg [121] have been reported by Deb. Thin films were prepared by the solid-solid reaction technique described for PbNg. Mixed crystal systems of metal azide halides were probably formed in the reactions, since the results differed for each combination of alkali azide-metal halide. An earlier CUN3 spectrum was interpreted by Evans and Yoffe [82] to indicate an w = 1 exciton, but here again that appears theoretically unsound (Section C.3.b). 

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). 

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). 

STRUCTURES AND ABSORPTION SPECTRA OF NONSTOICHIOMETRIC ALKALI HALIDE AND ALKALI HYDRIDE CLUSTERS... 

Komarov, V.E. and Nekrasova, N.P. (1980) Absorption spectra of uranyl ions in molten alkali halides(in Russian). 

Wiberley noted that -C=N absorption in ferro- and ferri-cyanides as a function of grinding time was modified in potassium bromide. Likewise, in a study of ultraviolet spectra in alkali halides, Curran and his co-workers found that platinum (II) and palladium (II) complexes were sensitive to the alkali halide in that the M-Cl and M-Br covalent bonds were practically completely replaced by M-I bonds, but that the M-I bonds reacted very little. This same trend of sensitivity was reported by Stimson and Smith [ ], who found that as little as one iodide ion per 1000 chloride ions was detectable in the p-chloroanilinium ion spectrum. 

These defects were first produced by exposing alkali halide crystals to high-energy radiation such as X rays. This causes the crystals to become brightly colored with fairly simple bell-shaped absorption spectra. The peak of the absorption curve, rn lx, moves to higher wavelengths as both the alkali metal ion size and halide ion... 

Krynauw and Schutte 178-181> in a series of papers discussed the infrared spectra of solid solutions of CI04 and C103 in alkali-halide lattices, measured the absolute absorption intensities and discussed the lattice-dependence of the v3 and v4 -modes of the C104 ion in these lattices. 

Special cases of charge-transfer spectra are the so-called charge-transfer-to-solvent (CTTS) spectra [17, 68]. In this type of CT transitions, solute anions may act as electron-donors and the surrounding solvent shell plays the role of the electron-acceptor. A classical example of this kind of CTTS excitation is the UV/Vis absorption of the iodide ion in solution, which shows an extreme solvent sensitivity [68, 316]. Solvent-dependent CTTS absorptions have also been obtained for solutions of alkali metal anions in ether or amine solvents [317]. Quantum-mechanical molecular simulations of the CTTS spectra of halide ions in water are given in reference [468]. 

The optical absorption spectra were obtained earlier and interpreted for a very large number of ionic crystals. We have made use of such data in obtaining the numbers given in Tables 14-1 through 14-3. Reference was made in particular to the recent ultraviolet photoelectron-emission studies of the alkali halides by Poole et al. (1975), who reviewed other optical studies as well. We have noted the trends that are present in the electronic structure and that are reflected in these studies. 

The absorption or reflection by the solid sample of photons of various energies, from the ultraviolet to the infrared regions, are probably the most accessible and widely used techniques [8]. Absorption of infrared radiation by mulls or dispersions in alkali-halide discs are standard procedures. The sample holder may be capable of being heated so that peaks of interest can be monitored during the progress of decomposition. Hisatsune et al. [9] have successfully used infrared measmements to follow the decompositions of metal carboxylates (Chapter 16) incorporated in KBr discs. Spectra of powders may differ significantly from those of the same material in the form of larger crystals. 

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