الفهرس 
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Abstract
We developed a treatment in order to investigate the optical constants of the crystalline semiconductor and dielectric materials. The lifetime , τ ,of the electronic excited state was taken to be finite. This treatment was based on a semiclassical treatment of the interaction of radiation with matter.the derivation of the optical constants is restricted to energies cloce to the band gap edge. One-photon, as well as two- photon spectroscopies were considered with one-electron model. The
Expressions for the absorption coefficients for both electronic transitions were derived. These expressions were found to depend on the lifetime,τ ,and the photon energies. The essential feature of the absorption spectra include peaks which occur at energies close to the threshold energy . At energis far from the edge, other peaks and shoulders occur. The lifetime is strongly enhanced as we approach the fundamental onset . from the present study we deduced a maximum valua of the lifetime,τ, where large peaks for the absorption coefficient occur in the vicinity of the band gap. from a numerical estimation for a number of semiconductors we obtained for the excited state lifetime, τ, the value 2.7x10-14 sec . this value was found to be common for most semiconductor and dielectric materials. We have computed other values of the lifetime,τ, from a previous work based on a last square curve fitting computer program . the results obtained are two and three orders of magnitude smaller than our value depending on the number of critical points taken in the summation in their calculation.