Band concept light absorption

The effective masses of electrons and holes are estimated by parabolic approximation a large curvature corresponds to a small effective mass and a small curvature corresponds to a large mass. With this band concept, light absorption and luminescence are interpreted as follows Light is absorbed by the transition from valence band to conduction band. Therefore, the broadening of the absorption spectrum originates basically from the one dimensionality of the joint density of states, which is described by (E - g) . Excited electrons and holes relax to the bottom of the bands and then recombine radiatively. Therefore, the photoluminescence of the spectrum is very sharp. The energy difference between two peaks is called the Stokes shift. 

A model of the photochromic effect based on the concept of the pseudo JTE was suggested in [ 196- 197]. This vibronic model allows us to examine the microscopic physical origin of the nuclear rearrangement and the underlying mechanism of the photochromic effect at the electronic level. At the same time the pseudo JTE approach can be considered as a background for the dynamical (quantum-mechanical solution) of the problem of light absorption and emission including the crucial question of interpretation of the anomalous lifetimes for the excited states and shape-functions of the optical bands. 

Photoconductivity in zinc oxide, on the other hand, appears to be influenced by the surface through a different effect. Absorption of light effectively excites the electrons trapped in surface levels into the conduction band. This chapter will be primarily devoted to a consideration of this concept, proposed by Melnick (11), that photoconducting electrons are produced through the ionization of surface levels, specifically the adsorbed oxygen levels on zinc oxide. The decay of the photoconductivity. 

The most well-developed Generation III concept is that of tandem PV cells. These consist of layers of semiconductors having different band gaps, each layer being optimized for absorption of light with energy close to its band gap. Theoretically, with enough individual layers an efficiency of 87% can be achieved [5],... 

Luminescence spectroscopy is an analytical method derived from the emission of light by molecules which have become electronically excited subsequent to the absorption of visible or ultraviolet radiation. Due to its high analytical sensitivity (concentrations of luminescing analytes 1 X 10 9 moles/L are routinely determined), this technique is widely employed in the analysis of drugs and metabolites. These applications are derived from the relationships between analyte concentrations and luminescence intensities and are therefore similar in concept to most other physicochemical methods of analysis. Other features of luminescence spectral bands, such as position in the electromagnetic spectrum (wavelength or frequency), band form, emission lifetime, and excitation spectrum, are related to molecular structure and environment and therefore also have analytical value. 

Photocondactivity — Photoconductivity is light-sensitive conductivity, due to the generation of free charge carriers when photons are absorbed in a semiconductor. It is called intrinsic if the photon absorption involves electron transition from occupied states of the valence band to unoccupied states of the conduction band. It is called extrinsic if it involves electron excitation from defect levels to the conduction band or from the -> valence band to defect levels. Photoconductivity was discovered by W. Smith in 1873 in selenium. The phenomenon was explained in 1922 by B. Gudden and R. Pohl using quantum mechanical concepts. See also photocurrent. 

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