Band saturation effect

Even with the band saturation effect, it is incorrect to conclude that because there is already so much CO2 in the atmosphere, more C02 can have no additional effect on absotption of outgoing radiation. When gases are present in small concentrations, doubling the concentration of the gas will approximately double its absorption. When... 

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S2 - Sq fluorescence in condensed media has so far been found in several types of molecules. However, metalloporphyrins are contrasted with these compounds by another arresting feature such that the S2 fluorescence can be observed even upon photoexcitation to the state. Stelmakh and Tsvirko have first noticed the anomalous S2 - Sq fluorescence in metalloporphyrins (15,16). Figure 1(a) shows the fluorescence spectra of ZnTPP in EPA taken by the 540 nm excitation of a nitrogen pumped dye laser. The fluorescence band at around 430 nm observed by visible excitation is safely assigned to the S2 state fluorescence. The laser power dependence of the fluorescence intensity is quadratic at low power density of excitation (<5 x 10 photons cm"2 pulse ) but shows typical saturation effect with increasing the laser intensity. It should be emphasized here that the S2 fluorescence of ZnTPP can be observed without focusing of the laser beeim. 

Problems are associated with quantitative analysis using IR. First, deviation from Beer s law affect quantitative analyses profoundly, especially those deviations resulting from saturation effects. Variations in the path length that are not accounted for can also cause problems. Second, specific interactions between components in the sample can influence the quantitation, especially those interactions that are temperature and pressure sensitive. Third, if the quantitation is based on the peak being due to only one absorbance when in reality it is a result of overlapping bands, then there will be a bias in data that is not necessarily linear. Currently available IR spectrometers have software packages containing matrix methods that simplify the operations associated with multicomponent... 

A simple differential saturation method has been proposed (63) in which the differential saturation effects are obtained at the same overall irradiation level due to an audio-frequency modulation of the resonance frequency in the continuous-wave mode of operation. This results in the appearance of sidebands in addition to the centre band of the spectrum. By a judicious selection of the modulation index, the saturation of signals in the sidebands may be made to amount to 10 — 0-1% of those in the centre band. The lineshape of such a combination of the centre band and the two closest sidebands may be adjusted to that corresponding to the function ... 

Despite these problems of saturation of vibrational bands IR spectroscopy, described in the next subsection, has been recently shown to nevertheless remain an especially powerful method to observe H2O molecules. Special recently proposed set-ups can avoid saturation in the whole conventional IR region, thus taking full advantage of the power of IR to study H-bond networks. They are first described, before the contribution of recent time-resolved nonlinear IR spectroscopy is examined. Other methods such as NIR or Raman spectroscopy, which are intrinsically free of this saturation effects can also be used to study the HjO molecule. They are often limited to some specific problems, as they do not display the power of ordinary IR spectroscopy for the study of H-bonds or of H2O molecules and cannot consequently be considered as general methods. They are described in the last subsection of this section on vibrational spectroscopy. 

We have given a summary of Allen s excellent review article. We refer the reader to the original papers for further details. Allen clearly points out that it is important to include interband effects in the formalism. This also follows from experiments since saturation effects in the resistivity at high temperatures seem to occur in metallic compounds and alloys with unfilled d or f bands. Allen and Chakraborty s theory may therefore lead to the understanding of saturation effects if s-d(f) scattering and electron-phonon scattering effects are treated on... 

If we compare the results of the distributions evaluated by Patel s formula and the actual intensity distribution of the corresponding laser transitions, there is a discrepancy in the intensity relations of the rotational components within a vibrational band. The peak intensity occurs at a higher J-value than calculated. This discrepancy is due to saturation effects. Patel s formula gives the intensity for "small signal gain", whereas the experimental peak intensity is limited by saturation effects. 

It will be clear that this explanation of the difference between the magnetic moment in crystalline and amorphous materials is not restricted to alloys of 3d metals with rare earth. Its general validity follows for instance from the results shown in fig. 49, where the crystalline curves pertaining to the crystalline states are invariably below those of the amorphous states. Here we wish to stress again that the above explanation of the differences in saturation moment does not mean that band structure effects or charge transfer effects can be completely neglected (Malozemoff et al., 1983). The present analysis only shows that CSRO effects play a rather prominent role in the determination of the magnetic properties. 

Saturation effects, which can occur for stronger absorption bands, are the main limitation of photoacoustic spectroscopy. These distortions can be overcome by using thin samples [93] or by analyzing the phase of the photoacoustic signal... 

The absorption of a photon can result in reemisaon of another photon, dissociation or ionization of the molecule, and many experiments do not distinguish between these processes. If the photon energy is insufficient to ionize the molecule and if the absorption is continuous, it is likely that photodissociation is the dominant process. In other cases, additional information on the fluorescence and/or ionization cross sections is needed to infer the dissociation probabilities For some species, the fluorescence cross sections have been measured explicitly. Figure 46 compares the absorption and fluorescence results for NO. It appears that for some bands the fluorescence yield is large, whereas for other bands, photodissociation occurs with almost 100% probability. The absorption cross sections for continuous processes measured at low resolution are quite accurate. However, for discrete absorptions, high resolution and low pressures are essential to obtain reliable experimental results, because saturation effects can easily cause orders of magnitude errors. Another... 

As for the Tc saturation in A3C60 with large lattice parameters, it has been repeatedly pointed out that electron correlation effects cannot be ignored in systems with very narrow bands. Such effects become progressively more important as the inter-fullerene spacing increases. As a result, localization of the conduction electrons should occur at a certain lattice constant, being accompanied by metal to Mott-insulator and to molecular semiconductor/insulator transitions [85,131], and Tc will saturate. 

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