Primary absorption

Application of the exact continuum analysis of dispersion forces requires significant calculations and the knowledge of the frequency spectmm of the material dielectric response over wavelengths X = 2irc/j/ around 10-10 nm. Because of these complications, it is common to assume that a primary absorption peak at one frequency in the ultraviolet, j/uv. dominates the dielectric spectrum of most materials. This leads to an expression for the dielectric response... 

A typical absorption curve for vitreous siUca containing metallic impurities after x-ray irradiation is shown in Eigure 12. As shown, the primary absorption centers are at 550, 300, and between 220 and 215 nm. The 550-nm band results from a center consisting of an interstitial alkah cation associated with a network substituent of lower valency than siUcon, eg, aluminum (205). Only alkaUes contribute to the coloration at 550 nm. Lithium is more effective than sodium, and sodium more effective than potassium. Pure siUca doped with aluminum alone shows virtually no coloration after irradiation. The intensity of the band is deterrnined by the component that is present in lower concentration. The presence of hydrogen does not appear to contribute to the 550-nm color-center production (209). 

The use of Caco-2 cell monolayers has gained in popularity as an in vivo human absorption surrogate moreover, the monolayers are generally accepted as a primary absorption screening tool by several pharmaceutical companies [10]. However, Caco-2 cell permeability measurements exhibit certain limitations due to the mechanisms involved. Both passive and active pathways exist active transport tends to increase the absorption across the cells and, since Caco-2 cells overexpress the P-glycoprotein (P-gp) efflux pump, the absorption of some compounds across these cells may be underestimated. 

In many cases, the lack of correction for reflection will not affect the shape of the optical spectrum very much, but it will just give an inflated value for absorption. However, there are frequently cases where the shape of the absorption spectrum is also appreciably changed after the reflection correction is carried out. Also, if the primary absorption is a weak one, then correction for reflection, and in particular for scattering, is crucial, since the absorption may be masked by these effects. 

From the preceding it is apparent that the nature of the first excited state has been clarified for CTTS transitions of simple anions, and that both for these and for intramolecular processes following on primary excitation the events are known with some certainty up to the stage of asymmetrization. Also, after the pair of solvated electron and parent species have been established by at least one diffusive translation (so that at least one solvent molecule separates them) all steps following have been elucidated. There remains the question of the detailed nature of the processes in the time interval between 10 u and 10 n to 10-9 sec. after primary absorption. 

To deliver drugs to their primary absorption site in their most concentrated form (e.g., drugs that are optimally absorbed in the small intestine or colon)... 

TABLE 17.3. C (Is) NEXAFS Approximate Transition Energy Ranges and Assignments of Primary Absorption Peaks... 

Fig. 8-65 Therma-radiation spectrums for the sun and earth with primary absorption bands indicated by shaded areas. Note different scales 1 erg/cm2-s = 1 mW/m2...

The main products from the mixture of C2H4/O2/CO2 irradiated by a KrF excimer laser are C2H4O and CH3CHO under pressurized conditions. The primary absorption of the laser light produces 02(A 3AU), while the ratio of the reaction products suggests the principal contribution of 0(3P) as the real oxidant. 

The photochemical equivalence law applies only to the absorption of primary photochemical process. As a result of the primary absorption only one molecule decomposes, and the products enter no further reaction, the number of molecules reacting will be equal to the number of photons absorbed. More frequently, however, a molecule-activated photochemically initiates a sequence of thermal reactions as a result of which several or many reactant molecules may undergo chemical change. Under such conditions there will be no reaction between reacting... 

Applied dose The amount of a substance in contact with the primary absorption boundaries of an organism (e.g. skin, lung or gastrointestinal tract) and available for absorption (USEPA, 1992b). 

Three main types of absorption occur, namely fundamental, overtone, and combination. Fundamental bands are the primary absorption bands for each mode of vibration, overtone bands occur at... 

The absorption spectroscopy in the UV-Vis-NIR is especially rich for the actinides, allowing for fairly simple determinations of the metal oxidation state. The primary absorption bands result from f f transitions, f d and ligand-to-metal charge transfers. The f — f transitions are typically weak since they are forbidden under the LaPorte selection rules. Distortions in symmetry allow for relaxation in these rules and bands in the visible to near-infrared range result. Complexes that contain an inversion syimnetry, for example Pu02CLt, have weaker f- -f transitions (e < 20 cm ). The direct interactions of the 5f orbitals... 

Primary X-rays are absorbed throughout the trajectory before reaching the excited atom this is primary absorption. 

Primary absorption Absorption of the excitation beam in fluo-re.scence or phosphorescence spectroscopy compare with secondary absorption. 

Secondary absorption Absorption of the emitted radiation in fluorescence or phosphorescence spectrometry compare with primary absorption. 

There are three main matrix effects in XRF primary absorption, secondary absorption and secondary fluorescence. Primary absorption refers to the radiation that is absorbed on the beam s path to reach the atoms to be excited. Secondary absorption refers to absorption of fluorescent radiation from atoms that occur along its path inside the specimen to the detector. Secondary fluorescence refers to the fluorescent radiation from the atoms which are excited by the fluorescent radiation of atoms with a higher atomic number in the same specimen. This phenomenon is possible when energy of the primary fluorescent radiation from heavier atoms is sufficient to excite secondary fluorescence from lighter atoms in the specimen. The absorption effects reduce the intensity of characteristic X-ray lines in spectrum, while secondary fluorescence increases the intensity of lighter elements. The matrix factors of EDS analysis in an electron microscope (EM) are described later in Section 6.8. 

The dose absorbed through a primary absorption route depends on ... 

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