Vibration direction

It is tlius of great interest to carry out experiments tliat excite tire intennolecular bending and stretching vibrations directly, witliout exciting tire monomers as well. These transitions he deep in tire far infrared, typically in tire 20-... 

The analyzer is removed and the color of the sample is observed in plane-polarized light. If the sample is colored, the stage is rotated. Colored, anisotropic materials may show pleochroism—a change in color or hue when the orientation with respect to the vibration direction of the polarizer is changed. Any pleochroism should be noted and recorded. 

In anisotropic crystals, the amplitudes of the atomic vibrations are essentially a function of the vibrational direction. As has been shown theoretically by Karyagin [72] and proved experimentally by Goldanskii et al. [48], this is accompanied by an anisotropic Lamb-Mossbauer factor/which in turn causes an asymmetry in quadra-pole split Mossbauer spectra, for example, in the case of 4 = 3/2, f = 1/2 nuclear transitions in polycrystalline absorbers. A detailed description of this phenomenon, called the Goldanskii-Karyagin effect, is given in [73]. The Lamb-Mossbauer factor is given by... 

On entering the crystal it faces a choice of two vibration directions and a random chance of following one or the other. 

The first point to be made is that in a crystal the refractive index depends not on the direction in which the electromagnetic waves are travelling but on the direction of the electrical disturbances transverse to the line of travel—the vibration direction . We have to consider the shape of the graph connecting refractive index with vibration direction for each crystal system, and the methods available for measuring the refractive indices of crystals in different vibration directions. 

Fig. 44. Refractive indices of monammonium phosphate, NR4H2P04. Arrows indicate vibration directions.

Thus, extinction occurs when the vibration directions of the polarizers coincide with those of the crystal. 

The vibration directions and refractive indices of crystal fragments of mon ammonium phosphate lying on a microscope slide in any orientation are given by the indicatrix in the following way. A crystal fragment, oriented with its unique axis at any angle 6 to the line of vision, is mentally replaced by the indicatrix (Fig. 48). Perpendicular to the... 

Biaxial crystals under similar optical conditions produce convergent light figures like that shown in Fig. 52 6, when the acute bisectrix of the optic axes lies along the line of vision and the vibration directions... 

The distinction between the vibration directions of higher and lower refractive indices can always be made in this way for crystals having inclined extinction no less than for those with parallel extinction. When refractive indices are measured by the methods already given, the use of the quartz wedge is hardly necessary (unless for confirmation of conclusions already reached) but in other circumstances (for instance, when crystals are being examined in their mother liquor), quartz wedge observations are useful clues to optical character. 

Dispersion. The principal refractive indices of a crystal vary in magnitude with the frequency of light and in crystals of monoclinic or triclinic symmetry, the vibration directions of the principal indices may vary with frequency. Such variation is known as dispersion. 

Optical properties of twinned crystals. Each individual in a twin exhibits its own optical characteristics. If a gypsum twin is seen along its b axis and examined between crossed polarizers, it can be seen that each individual extinguishes independently. The twin plane (100) is a plane of symmetry of the composite whole, and the vibration directions of the two individuals, like all the other properties, are related to each other by this plane of symmetry (see Rig. 57 a). 

The relations between the optical properties of the two individuals are clear in the case of gypsum because the crystals lie on the microscopf slide on their (010) faces, so that the (100) twin planes are parallel to the line of vision., In some crystals the twin planes are inclined to the line of vision when the crystals are lying on their principal faces so that one is looking through two crystals in which the vibration directions ar not parallel to each other. In these circumstances, in the overlapping regions extinction does not occur when the polarizers are rotated. Whed... 

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