Absorption inhomogeneous

There is an early report in the literature claiming absence of the autocatalytic reaction enhancement in TS if the reaction is induced by UV-excitation of the monomer crystal. The implication would be that thermal and UV-polymerization involve different mechanisms. Later on, however. Chance and Patel found this to be an artifact caused by the neglect of spatially inhomogeneous absorption by polymer molecules which effectively competes with monomer excitation at increasing conversion and prematurely terminates the reaction. Although it is difficult to correct X(t)-curves obtained under UV-excitation for polymer absorption quantitatively, particularly if irradiation is done with unpolarized non-monochromatic light, it turns out that there is a qualitative agreement between X(t)-curves obtained under y-and UV-irradiation. Application of this correction, however, does not solve the puzzle why in case of y- or UV-polymerization of TS, the reaction rate increases less dramatically with conversion, than observed upon thermal conversion. 

If molecules are embedded in solid hosts (e.g., in glassy or crystalline states) and temperatures are sufficiently low, sharp homogeneous absorption bands begin to show up within the broad inhomogeneous absorption band of the dopant [61]. A solute molecule and its noncovalently bonded solvent shell can be classified as a supermolecule [62]. Each solute molecule will thus create a unique supermolecule with its own arrangement of the solvent shell. Each supermolecule exists in a local energy well with a potential energy barrier which caimot be overcome at low temperatures. These supermolecules can show different physical properties— specifically, a spread in their absorption. In essence every supermolecule is different and can be addressed individually. 

An exact calculation of the amount of decomposition and its distribution within the sample must take into account the inhomogeneous absorption of the light the transport, if any, of free charges or excitons the diffusion of nitrogen and the effects on the decomposition reaction of radiation-induced defects. These complications have only recently been taken into consideration, but are clearly important in interpreting rate of gas evolution vs. time curves. 

If, by absorption of monochromatic light, i.e. a laser beam, a molecule can be so modified in its structure or in its interaction with the local environment that a laser pulse of the same wavelength cannot - even at a later time - again be absorbed (bleaching), then this molecule is excluded from the ensemble of molecules which contribute to the broad, inhomogeneous absorption line. In the broad absorption band, a hole then appears at the position of the individual absorption of the bleached molecule. This forms the basis of the method of hole-burning (cf Fig. 12.17). 

One way to dissect fluorescence from a heterogeneous sample is fluorescence line narrowing, in which the temperature is lowered to freeze out fluctuations of the solvent. The underlying idea is the same as in holebuming absorption spectroscopy (Sect. 4.11). A tunable dye laser with a narrow spectral bandwidth is used to excite the sample selectively and thus to pick out a subpopulation of molecules that absorb at a particular wavelength. The emission spectrum for the subpopulation can be much narrower than the spectrum for the ensemble as a whole, and it often shifts as the excitation wavelength is tuned over the inhomogeneous absorption band [184]. 

Figure 5 shows a hole in the inhomogenous absorption band of protoporphyrin IX-substituted myoglobin. The insert shows the hole on an enlarged scale. 

Pullerits T, Visscher K J, Hess S, Sundstrom V, Freiberg A, Timpmann K and Van Grondelle R 1994 Energy-transfer in the inhomogeneously broadened core antenna of purple bacteria-a simultaneous fit of low-intensity picosecond absorption and fluorescence kinetics Biophys. J. 66 236-48... 

BeryUium aUoys ate usuaUy analyzed by optical emission or atomic absorption spectrophotometry. Low voltage spark emission spectrometry is used for the analysis of most copper-beryUium aUoys. Spectral interferences, other inter-element effects, metaUurgical effects, and sample inhomogeneity can degrade accuracy and precision and must be considered when constmcting a method (17). 

The dependence of the in-phase and quadrature lock-in detected signals on the modulation frequency is considerably more complicated than for the case of monomolecular recombination. The steady state solution to this equation is straightforward, dN/dt = 0 Nss — fG/R, but there is not a general solution N(l) to the inhomogeneous differential equation. Furthermore, the generation rate will vary throughout the sample due to the Gaussian distribution of the pump intensity and absorption by the sample... 

The unit cell (Table 1) and orientation matrix were determined from the XYZ centroids of 8192 reflections with I > 20c(7). The intensities (SAINT [8]) were corrected for beam inhomogeneity and decay, and the esd s adjusted using SADABS [9]. An absorption correction was applied (Tmin 0.949, Tmsx 0.983) and symmetry and multiply measured reflections averaged with SORTAV [10]. 

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