Optical properties of crystal

The optical properties of crystals are usually quite reliable criteria for identification but occasionally crystals have sifbmicroscopic cracks and cavities, and although appearing quite normal, give refractive indices lower than those of an entirely solid crystal. This phenomenon, which is obviously very misleading, is fortunately very rare, but has been observed in anhydrite (calcium sulphate) and calcite (calcium carbonate) prepared in the laboratory. In cases erf doubt, X-ray powder photographs should be taken—see Chapter V. 

A closely related properly is exhibited by certain substances known as "liquid crystals," which appear to be intermediate between merely cybotactic liquids and true crystals. In these there appear to be large groups of molecules which, though able to move and tum about, retain their structural arrangement. Such mesomorphic substances manifest even some of the optical properties of crystals, which the former type do not. Sec also Liquid Crystals. 

It is primarily the structure, and, accordingly, the outer and inner symmetry properties of the crystal, that determines its many outstanding physical properties. The mechanical, electrical, magnetic, and optical properties of crystals are all in close conjunction with their symmetry properties [18],... 

The interaction of crystals with light depends both on the arrangement of atoms in their crystal structure and on the nature of these atoms. The optical properties of crystals indicate the directions of symmetry axes in the crystal and, in certain cases, provide useful preliminary information... 

It has been known for almost 200 years that starch gives a deep blue color when a solution of potassium iodide and iodine is added [47]. More than a century later it was suggested that the complex consisted of a helical polysaccharide, with triiodide in the center of the helix [48]. Using flow dichroism, it was demonstrated that the triiodide was stacked in a linear structure, as required for the helical model [49]. Another study of the optical properties of crystals of the amylose-triiodide complex showed it to be consistent with a helical structure [50] and X-ray diffraction showed the triiodide complex gave the dimensions of a unit-cell of a helix with six glucose residues per turn [51]. This confirmed a helical structure for the amyiose complex with triiodide that predated the helical models proposed by Pauling for polypeptides [52] and the double helical model for DNA by Watson and Crick [53] by 10 years. 

Numerous theoretical and experimental studies have been carried out in this field so that a whole branch of molecular optics - the optics of molecular crystals and molecular liquids - has been established. Even before Frenkel put forward his exciton concept, workers in this branch of optics had developed a variety of exact and approximate methods for the theoretical description of optical phenomena many of these methods were also substantiated in experimental studies. However, after the discovery of excitons the use of these methods became increasingly rare and many of the results obtained with them have not been sufficiently understood in the framework of exciton theory. Therefore, further development and generalization of these methods were impeded. On the other hand, since the results of pre-excitonic molecular optics were underestimated, the optical properties of crystals were treated in terms of only exciton theory even in those cases when this could be done much more easily by using the earlier, simpler... 

In the preceding sections of this chapter the local field method was applied mainly to the theory of optical properties of crystals and crystalline solutions. In this section we will investigate the influence of the local field corrections on the resonance interaction between the impurity molecules. Just this interaction determines the transfer of intramolecular (electronic or vibrational) excitation energy from one impurity molecule to another. 

Narasimhamurty, T. S., Photoelastic and Electro-Optic Properties of Crystals, Plenum Press, New York, 1981, pp. 405-407. 

The gap Eq is not the minimum gap Eg which determines the absorption edge and temperature dependence of carrier concentration, but the average gap corresponding to the separation of the bonding and antibonding states in the covalent bond picture. It has fundamental importance for many basic properties of the solid and appears in the theories of the relation between the chemical and optical properties of crystals (Phillips (1970)). 

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