Other Absorption Phenomena

Generally the optical transmission of dielectric media is limited at high frequencies either by the bandgap energy or by the electronic polarization near cao and at the lower frequencies by the reststrahlen region or by molecular absorption. However, there are other absorptive processes that can occur in the intervening region. 

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Thermodynamically, it can be reasoned that the major contribution of heat generation or absorption must have come from source other than the lattice, i.e., such as electrons and changes in their thermodynamic state. Similar phenomena exist in stretching a piece of rubber or other cooperative phenomenon such as the transition between ordinary helium to helium-11. 

Infrared and Raman spectroscopy are often grouped together, since both techniques provide information on the vibrational modes of a compound. However, since the two spectroscopic techniques are based on different physical principles the selection rules are different. Infrared spectroscopy is an absorption phenomenon, while the Raman spectroscopy is based on a scattering phenomenon (Raman and Krishnan 1928). In general, infrared energy is absorbed by polar groups, while radiation is more effectively scattered in the Raman effect by symmetric vibrations and nonpolar groups (Colthup et al. 1990 Ferraro and Nakamoto 1994). For most molecules other... 

There are many requirements for adequate assay. The reference material should have the same type of absorption phenomenon as the experimental material. In other words, it is not usually productive of parallel and therefore comparable dosage response curves if an esterified, slowly absorbed preparation is compared with a rapidly absorbed compound. All tests must be applied with intelligence and care. 

The molecular absorption phenomenon can only be accurately measured provided that the ratio of transmitted intensity, 7, of the UV radiation to that of the incident intensity, /q, is due to the presence of the dissolved solute and not to scattering of the incident beam. If the optical windows (see below) absorb UV radiation, then the absorbance which is related to the logarithm of the ratio ///q would cause an increase in sample absorbance hence, this would lead to an erroneous result. The student will encounter two types of optical window material. One consists of glass and is said to have a UV cutoff (UV wavelengths below this would absorb) of 300 nm (near UV) and the other consists of quartz with a UV cutoff of 190 nm. The rectangular cuvette is depicted as follows ... 

In 1857, Thomson (Lord Kelvin) placed the whole field on firmer footing by using the newly developing field of thermodynamics (qv) to clarify the relationship between the Seebeck and the Peltier effects. He also discovered what is subsequently known as the Thomson effect, a much weaker thermoelectric phenomenon that causes the generation or absorption of heat, other than Joule heat, along a current-carrying conductor in a temperature gradient. 

The other primary thermoelectric phenomenon is the Peltier effect, which is the generation or absorption of heat at the junction of two different conductors when a current flows in the circuit. Whether the heat is evolved or absorbed is determined by the direction of the current flow. The amount of heat involved is determined by the magnitude of the current, I, and the Peltier coefficients, 7T, of the materials ... 

We have found for polypropynoic acid that this series of polymers reveals selective fluorescence spectra together with nonselective absorption. To account for this phenomenon, a scheme was proposed according to which PCSs are characterized by energy transfer from excited levels of some conjugation sections to the lower levels of other sections, followed by luminescence from the latter40 41,246,248,249,253. ... 

There are many other examples of changes in which a solid passes into a liquid, or a liquid into a gas, with absorption of heat at constant temperature. The constant temperature may be called the transition temperature the heat absorbed is called the latent heat of the transition. The latter name is due to Joseph Black, the discoverer of the phenomenon (1757) he appears to have regarded the heat as existing latent in the body in some sort of chemical combination, just as fixed air exists latent in chalk. In both cases the entity has lost its properties by chemical combination, but may be set free again in a suitable way. 

We wish to show that no points to the leftbb of 2 on the isotherm 62 are accessible from point 1 via any adiabatic path, reversible or irreversible. Suppose we assume that some adiabatic path does exist between 1 and 2. We represent this path as a dotted curve in Figure 2.11a. We then consider the cycle I —>2 —> 1 — 1. The net heat associated with this cycle would be that arising from the last step 1 — 1, since the other two steps are defined to be adiabatic. We have defined the direction 1 — 1 to correspond to an absorption of heat, which we will call qy. From the first law, the net work vv done in the cycle, is given by w = —q, since AU for the cycle is zero. Thus, for this process, iv is negative (and therefore performed by the system), since qy is positive, having been absorbed from the reservoir. The net effect of this cycle, then, is to completely convert heat absorbed at a high temperature reservoir into work. This is a phenomenon forbidden by the Kelvin-Planck statement of the Second Law. Hence, points to the left of 2 cannot be reached from point 1 by way of any adiabatic path. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

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