Stimulated absorption

Stimulated absorption of photons. In this case, the electronic transition takes place from state 1 to state 2 in response to the action of an external radiation of the appropriate frequency. Atomic absorption spectrometry (AAS) is based on this process. On the other hand, atomic fluorescence spectrometry (AES) corresponds to the sequential combination of a stimulated absorption followed by spontaneous emission. 

Let neq and /ieq rad be the equilibrium spin population differences between the a and y3 levels in the absence and presence of radiation, respectively. Let pap Ppa be the probabilities per second for stimulated absorption and emission of radiation, (a) Show that... 

Advances have been made in the localization of the cellular sites of l,25-(OH)2D3 in target tissue. Receptor proteins have been extracted, and, in the case of the chick intestinal receptor, purified to homogeneity.451 The ability of l,25-(OH)2D3 to stimulate absorption of calcium is blocked reversibly by inhibitors of RNA and protein synthesis. This suggests that l,25-(OH)2D3 functions by a nuclear mechanism. 

Following this model description of the absorption process, the rate at which stimulated absorption transitions are induced is given by... 

Finally, case (iv) leads to the observation of enhanced stimulated absorption again molecular beam techniques which select the lower (a) state can be used. It is interesting to note that all four population cases are observed in different interstellar gas clouds. 

The relation between the Einstein coefficients A and By is A — Snhc0v3 By. The Einstein stimulated absorption or emission coefficient B may also be related to the transition moment between the states i and j for an electric dipole transition the relation is... 

Of these alfacalcidol has been the most widely used. It is hydroxylated in the liver to calcitriol, thus bypassing the kidney. Unlike calcitriol, which acts in the small intestine to stimulate absorption, doxercalciferol remains inactive until it reaches the liver, where it undergoes transformation to active vitamin D (2). As a result, doxercalciferol minimizes the risk of hypercalcemia by 67-80%. The manufacturers claim that doxercalciferol is as potent as calcitriol in lowering parathyroid hormone concentrations. 

Opiates and opioid derivatives (a) delay the transit of intraluminal contents or (b) increase gut capacity, prolonging contact and absorption. Enkephalins, endogenous opioid substances, regulate fluid movement across the mucosa by stimulating absorptive processes. Limitations to the use of opiates include an addiction potential (a real concern with long-term use) and worsening of diarrhea in selected infectious diarrhea. 

Pharmacology and Mechanism of Action. Active vitamin D suppresses PTH secretion by stimulating absorption of serum calcium (and phosphorus) by intestinal cells and through direct activity on the parathyroid gland to decrease PTH synthesis. As a result, the serum calcium concentration is raised and the parathyroid glands decrease the rate of secretion and formation of PTH. The set point for calcium (i.e., the calcium concentration at which PTH secretion is... 

In addition to the spontaneous emission of excited molecules, fluorescence and phosphorescence (Section 2.1.1), the interaction of electromagnetic radiation with excited molecules gives rise to stimulated emission, the microscopic counterpart of (stimulated) absorption. Albert Einstein derived the existence of a close relationship between the rates of absorption and emission in 1917, before the advent of quantum mechanics (see Special Topic 2.1). 

Bnm Einstein s coefficient of (stimulated) absorption Bmn Einstein s coefficient of stimulated emission Amn Einstein s coefficient of spontaneous emission... 

The probability of stimulated emission by an excited molecule, Bpv, is the same as that of the reverse process, absorption by the ground-state molecule. This is a consequence of the law of microscopic reversibility if the number of excited molecules Nm is equal to the number of ground-state molecules N , then the rates of stimulated absorption and emission must be equal. If by some means a population inversion can be produced, Nm > Nn, the net effect of interaction with electromagnetic radiation of frequency v will be stimulated emission (Figure 2.4). This is the operating principle of the loser. LASER is an acronym for light amplification by stimulated emission of radiation (Section 3.1). 

It should be realized that absorption and emission as discussed in Chapters 2 tuid 3 are different processes, since the former needs a radiation field and the latter not. Einstein considered the problem of transition rates in the presence of a radiation field [31]. For the transition rate from lower to upper level he wrote w = Bp, where B is the Bin.stein coefficient of (.stimulated) absorption and p is the radiation density. 

The next stage involves the synthesis of specific calcium-binding proteins, typified by the intestinal CaBP discussed in Section 62.1.3.4.5, which probably stimulates the transport of calcium. The role of the protein in vitamin D-dependent absorption of calcium is supported by the good correlation between the concentration of CaBP and the rate of calcium absorption. Under conditions of low calcium or phosphorus diets, chicks and other animals produce more intestinal CaBP to increase the efficiency of uptake of calcium. In general, adaptation to a low calcium diet involves increased synthesis of l,25-(OH)2D3 and the intestinal CaBP. Lowered requirements for calcium in old age are manifested by lower levels of both factors. Advances have been made in the localization of the cellular sites of l,25-(OH)2D3 in target tissue. Receptor proteins have been extracted, and, in the case of the chick intestinal receptor, purified to homogeneity." The ability of l,25-(OH)2D3 to stimulate absorption of calcium is blocked reversibly by inhibitors of RNA and protein synthesis. This suggests that l,25-(OH)2D3 functions by a nuclear mechanism. 

The intensity of absorbed radiation is proportional to the Einstein transition probability for stimulated absorption, Bu while that for emitted radiation is proportional to the transition probability for spontaneous emission, Aji [Eqs. (5) and (6)]. The intensities are also proportional to the frequency, v, of absorbed or emitted radiation. 

Einstein coefficient of stimulated absorption from state w to BWO backward wave oscillator... 

One function of polythionic acids is to act as cathodic depolarizers, thereby stimulating dissolution of chromium-depleted grain-boundary material. Another possible function is that their cathodic reduction products (H2S or analogous compounds) stimulate absorption to interstitial hydrogen by chromium-depleted... 

Atomic absorption (AA) is based on stimulated absorption since the number of photons absorbed is dependent on the photon flux of the incident light. However, the fraction of the light absorbed is relatively independent of source intensity until the incident intensity reaches extremely high levels and begins to populate substantially the excited state. At that point the probability of absorbing a photon... 

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