Grain scattering

Attenuation in solids due to viscosity may be treated by a similar analysis. There may well be other damping mechanisms, such as heat conduction (i.e. imperfectly adiabatic conditions) which also gives an f2 law, and other phenomena associated with solid state defects that may have more complicated frequency and temperature dependence. In polycrystalline solids, especially metals and alloys and also ceramics, elastic grain scattering may cause much greater attenuation than any inelastic damping (Papadakis 1968 Stanke and Kino 1984). 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

Transmission electron microscopy (tern) is used to analyze the stmcture of crystals, such as distinguishing between amorphous siUcon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain stmctures. Tern requires samples that are very thin (10—50 nm) sections, and is a destmctive as well as time-consuming method of analysis. 

Probably the most striking and useful characteristic of common siUcate glass is its transparency to visible light. This transparency results from the absence of grain boundaries and delocalized electrons, which tend to scatter and absorb light. 

Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surface segregation. More frequent use of the element-specific CEELS version of REELM to complement SAM in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low Eq with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems. 

Another new and much used variant is a procedure called orientation imaging microscopy (Adams ci al. 199.5) patterns created by electrons back-scattered from a grain are automatically interpreted by a computer program, then the grain examined is automatically changed, and finally the orientations so determined are used to create an image of the polycrystal with the grain boundaries colour- or thickness-... 

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