Transmissivity radiative properties

Radiation is emitted by every point on a plane surface in all directions into the hemisphere above the surface, ITie quantity that describes the magnitude of radiation emitted or incident in a. specified direction in space is the radiation intensity. Various radiation flu.xes such as emissive power, irradiation, and ra-diosity are expressed in terms of intensity. This is followed by a discussion of radiative propertie.s of materials such as emissivity, absoiptivity, reflectivity, and transmissivity and their dependence on wavelength, direction, and lemperatiire. The greenlioiijie effect is presented as an example- of the con.sequenccs of the wavelength dependence of radiation properties. We end tliis chapter with a dis cussion of attno.spheric and solar radiation. 

Some other materials, such as glass and water, allow visible radiation to penetrate to considerable depths before any significant absorption takes place. Radiation through such scmitranspareiu materials obviously cannot be considered to be a surface phenomenon since the entire volume of the material interacts with radiation. On the other hand, both glass and water ace practically opaque to infrared radiation. Therefore, materials can exhibit different behavior at different wavelengths, and the dependence on wavelength is an important consideration in the study of radiative properties such as emissivity, absorptivity, reflectivity, and transmissivity of materials. 

This section has been devoted to the study of the surface excitons of the (001) face of the anthracene crystal, which behave as 2D perturbed excitons. They have been analyzed in reflectivity and transmission spectra, as well as in excitation spectra bf the first surface fluorescence. The theoretical study in Section III.A of a perfect isolated layer of dipoles explains one of the most important characteristics of the 2D surface excitons their abnormally strong radiative width of about 15 cm -1, corresponding to an emission power 10s to 106 times stronger than that of the isolated molecule. Also, the dominant excitonic coherence means that the intrinsic properties of the crystal can be used readily in the analysis of the spectroscopy of high-quality crystals any nonradiative phenomena of the crystal imperfections are residual or can be treated validly as perturbations. The main phenomena are accounted for by the excitons and phonons of the perfect crystal, their mutual interactions, and their coupling to the internal and external radiation induced by the crystal symmetry. No ad hoc parameters are necessary to account for the observed structures. 

To solve Equation (38) boimdary conditions which describe the reflection and transmission of radiation at the boimdaries are required. In principle, boimdary conditions can only be established in a rigorous manner for the radiative intensity, not for G, because the optical properties of the interfaces depend on the direction of incidence of radiation. Because the PI approximation solves for an integrated quantity like G instead, approximate boundary conditions must be established (Modest, 2003). One possibility is the Marshak boundary condition (Marshak, 1947), which comes from considering the continuity of the radiative flux through the interface. If this continuity is considered together with the assumption (34) of the PI approximation and Equation (37), the following equation is obtained (Spott and Svaasand, 2000)... 

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

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