Bulk Optical Absorption

The absorption of impurity centres is observed in the transparency domains of semiconductors and insulators, which are limited by their intrinsic electronic and vibrational absorptions. Further, a brief account of the relevant physical processes and an overview of the intrinsic optical properties of these materials and of their dependence on temperature, pressure and magnetic field is given in this chapter. Some semiconductors have been or are now synthesized in quasi-monoisotopic (qmi) forms because of improvements in their physical properties like thermal conductivity. A comparison of their intrinsic optical properties with those of the crystals of natural isotopic composition is also given. The absorption related to free carriers, due mostly to doping is also discussed at the end of this chapter. A detailed account of the optical properties of semiconductors can be found in the books by Yu and Cardona [107] and by Balkanski and Wallis [4]. 

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Bulk Optical Absorption of Phthalocyanine Particles. The visible and near infrared optical absorption of the phthalocyanine pigments was measured on a Cary 17 spectrophotometer using the technique described in Reference 8. A thin film of phthalocyanine dispersed in the polymer was coated onto 3 mil Mylar substrate using wirewound rods. The dry films were index matched by overcoating with a thick film of polyvinyl alcohol. The differential absorption between two films with different thicknesses were recorded to minimize reflection and light scattering losses. The solution absorption spectra of some pigments were also measured in a-chloronapthalene. 

In Yield, all electrons, independent of whether they suffered energy losses or not, contribute to the electron current as long as their energy lies above the threshold for emission into vacuum. Expressions for the Yield, Y(hv), as a function of photon energy hv have been worked out by Bandis and Pate on the basis of the three-step model [73], More sophisticated treatments of the photoemission process have shown that the three-step model is a viable approximation as long as the assumption of bulk optical absorption is good. 

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