Optical Constants and Electronic Structure

It is well known that the electronic structure of molecules or solids can be directly connected to optical properties, our physical observables, through the complex dielectric constant at optical frequencies. A detailed analysis of this subject is discussed in several textbooks [18]. Here we shall only highlight the main results. From the solid state point of view, direct optical transitions, i.e., transitions in which the energy tko is conserved and the photon momentum is negligible, are related to the imaginary part of the complex dielectric constant = i + i 2 by 

0 and excited state m. From the previous relations it is clear that the electronic structure derived from the theory can be directly compared with the 

1 spectra, thus providing a deeper physical insight than indirect comparisons with rough absorbance spectra. 

In addition to the important connection with the electronic structure of the material described previously, n (n2 = e) plays a major role in the performance of optoelectronic devices. Its value is very important in characterizing 

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