Absorption Phenomena

Applications Involving Nonlinear Absorption Phenomena. Saturable absorption (hole-burning) is a change (typically a decrease) in absorption coefficient which is proportional to pump intensity. For a simple two level system, this can be expressed as... 

Not previously considered in the above reference is a-particle self-absorption phenomena. However, attempts are at present being made to estimate radon and thoron progeny deposited on the surface of materials by y-spectrometry and gross y-count in order to eliminate self-absorption effects. 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

The Pourbaix diagram for c -[Ru (0)2(bpy)2] is shown in Figure 8. Similar electrochemical behavior is observed for c -[Ru (0)2(L)2] (L = 6,6 -Me2bpy, 2,9-Me2phen). The cyclic voltammogram in acidic medium shows all four couples, though not all of them are reversible. The electrochemical steps have not been easily observed individually, the difficulties being attributed to absorption phenomena or precipitations on the electrodes, and slow electrochemical kinetics. 

The physicochemical considerations discussed in Chapter 3 that govern the passage of drugs across biological barriers are applicable to both excretory and absorptive phenomena. 

For the present, let us turn our attention to absorption phenomena. As the flux passes through the absorbing gas, each successive lamellar slice of... 

Another important property is the structure (see Section 4.1) which characterizes the coalescence of primary particles into aggregates resembling chains or bunches of grapes. The dibutyl phthalate (DBP) absorption is commonly accepted as a measure for the carbon black structure. Due to absorption phenomena the DBP number increases with increasing specific surface area. Oil absorption (or, in general, the vehicle demand) is as well an indicator for the structure, but it also depends on the wettability of the black surface. Since linseed oil is a polar system, oil absorption declines as the concentration of surface oxides rises. 

Non resonant techniques are only of limited use to determine microwave losses with high precision, in particular when the losses are very small. Flowever, for the investigation of nonlinear absorption phenomena (i.e. rf power dependent on surface impedance or loss tangent) by intermodulation distortion measurements broad-band test devices are more common. Typically, a planar transmission line with an impedance of 50 Ohms can be employed for intermodulation... 

A variety of selection rules derived from quantum mechanics governs the intensity of the various types of absorption phenomena. 

Idson, B. In "Topics in medicinal chemistry, Vol.4, absorption phenomena." John Wiley, Chichester 1971. 

In contrast to technologies that are exclusively based on adsorption or absorption phenomena and result in pollutant transfer with the need for supplementary treatments, photocatalysis completely mineralizes the organic pollutants or, at least, enables... 

The absorption phenomena observed, depend largely on the extent of depletion of the amine in the mass transfer zone and can be classified into three regimes negligible interaction, 2 medium interaction and 3 extreme interaction between H2S and CO2 absorption. In the latter regime, desorption of one of the gaseous components is observed although, based on its overall driving force, absorption would be expected. 

In 1985 a major step in the theoretical analysis of oral drug absorption phenomena took place [156], when solubility and dose were also taken into account for the estimation of the absorption potential A P of a drug apart from the pH-partition hypothesis related parameters (lipophilicity, and degree of ionization). According to this concept, the AP is related proportionally to the octanol/water partition coefficient Pc, the fraction of the un-ionized species, at pH= 6.5, and the physiological solubility cs of the drug and inversely proportional to the dose r/o ... 

These models are dependent on the temporal variable associated with the drug transit along the small intestine. Drug absorption phenomena are assumed to take place in the time domain of the physiological mean transit time. For those dynamic models that rely on diffusion-dispersion principles both the spatial and temporal variables are important in order to simulate the spat.iotemporal profile of the drug in the intestinal lumen. 

The small-intestinal transit flow is a fundamental process for all gastrointestinal absorption phenomena. However, the structure of the gastrointestinal tract is highly complex and it is practically impossible to explicitly write and solve the equations of motion for the drug flow. Instead, numerical computer simulation techniques that incorporate the heterogeneous features of the gastrointestinal wall structure and of the drug flow are used in this section to characterize the intestinal transit process in humans. 

Subsequently, it has been recognized that other IR techniques (reflection, emission and photothermal techniques) can also be applied, and that IR spectroscopy only gives partial information on the vibrational structure of most polyatomic species. In fact, selection rules apply to IR light absorption phenomena, so that only vibrational modes that are associated with changes of the molecular dipolar moment can be directly excited. 

It remained, however, for Gustav Kirchhoff and Robert Wilhelm Bunsen in 1859 and 1860 to explain the origin of the Fraunhofer lines. Bunsen had invented his famous burner (Figure 24F-2) a few years earlier, which made possible spectral observations of emission and absorption phenomena in a nearly transparent flame. Kirchhoff con-... 

Goris L., Haenen K., Nesladek M., Wagner P., Vanderzande D., de Schepper L., d Haen J., Lutsen L. and Manca J. V. (2005), Absorption phenomena in organic thin films for solar cell applications investigated by photothermal deflection spectroscopy , /. Mat. Sci. 40, 1413-1418. 

The interactions between the sample components and the mobile and stationary phases can be classified as either adsorption or absorption (also called partition chromatography). In adsorption chromatography the sample is attracted to the surface of the phase, usually to the surface of a solid stationary phase. In absorption chromatography the sample diffuses into the interior of the stationary phase. Most chromatographic separations are a combination of both adsorption and absorption phenomena. 

As mentioned earlier, most chromatographic separations are a result of the interaction between adsorption and absorption phenomena. Both involve physical forces between the analyte and the phases of the chromatographic system. There is also a phenomenon called chemisorption which consists of a chemical reaction between the analyte and the stationary phase. The next few paragraphs describe the types of forces present in chromatography. 

Micronic particles (0.3 < a < 30). This intermediate domain corresponds to the validity limit of the last models (diffusion and diffraction), because diffusion is also influenced by reflection, refraction and absorption phenomena. These can be taken into account by the complex theory of Lorenz-Mie. 

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