The Electromagnetic Spectrum and Optical Spectroscopy

The short (UV) wavelength limit of the optical range is imposed by instrumental considerations (spectrophotometers do not usually work at wavelengths shorter than about 200 nm) and by the validity of the macroscopic Maxwell equations. These equations assume a continuous medium in other words, that there is a large number of ions within a volume of. The long (IR) wavelength limit of the optical range is basically imposed by experimental considerations (spectrophotometers work up to about 3000 nm). 

Inner electrons are usually excited by X-rays. Atoms give characteristic X-ray absorption and emission spectra, due to a variety of ionization and possible inter-shell transitions. Two relevant refined X-ray absorption techniques, that use synchrotron radiation, are the so-called Absorption Edge Fine Structure (AEFS) and Extended X-ray Absorption Fine Structure (EXAFS). These techniques are very useful in the investigation of local structures in solids. On the other hand, X-Ray Fluorescence (XRF) is an important analytical technique. 

F ure 1.2 The possible emerging beams when a sohd sample is illuminated with a beam of intensity /q. The circles represent atoms or defects in the sohd that are interacting with the incoming light. 

We shall now focus our attention on the optical spectroscopy of solids, which is the subject of this book. 

If a solid sample is illuminated by a light beam of intensity /q, we perceive that, in general, the intensity of this beam is attenuated after crossing the sample that is, the intensity f of the transmitted beam is lower than /o. The processes that contribute to this attenuation are as follows  

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