Optical extinction spectroscopy

Time-resolved optical absorption spectroscopy experiments have shown that arenesul-fonyl radicals decay with clean second-order kinetics14 the values of 2 k,/a h where s2 is the extinction coefficient at the monitoring wavelength, increased linearly with decreasing viscosity of the solvent, further indicating that reaction 16 is clearly a diffusion-controlled process. 

Although most polysilanes adopt main-chain helical structures, most of them do not show a preferred screw sense. Thus, if a helical polymer shows a preferred screw sense it can be either left-handed or right-handed type. These possibilities can be detected by CD (circular dichroism) spectroscopy. Thus, in regular optical absorption spectroscopy the extinction coefficient or molar absorptivity (e) is related to the absorbance (A), concentration (c) and path length of the detecting cell (1) by the famous Beer-Lambert s expression (see Eq. 7.17). 

The other notable feature of the optical spectroscopy of the lanthanide ions is that their absorption bands generally have very low extinction coefficients, because f-f transitions are... 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

We perform a series of experiments. We purchase a stock of identical plates and prepare several solutions of beetroot juice of varying concentration. One plate is soaked per solution. The chemisorptive bond between the dye and plate is strong, and equilibrium is reached after only a few seconds. The excess juice is decanted off for analysis, e.g. by means of optical spectroscopy and the Beer-Lambert law, provided we know the extinction coefficient s for the juice. 

At infrared wavelengths extinction by the MgO particles of Fig. 11.2, including those with radius 1 jam, which can be made by grinding, is dominated by absorption. This is why the KBr pellet technique is commonly used for infrared absorption spectroscopy of powders. A small amount of the sample dispersed in KBr powder is pressed into a pellet, the transmission spectrum of which is readily obtained. Because extinction is dominated by absorption, this transmission spectrum should follow the undulations of the intrinsic absorption spectrum—but not always. Comparison of Figs. 10.1 and 11.2 reveals an interesting discrepancy calculated peak extinction occurs at 0.075 eV, whereas absorption in bulk MgO peaks at the transverse optic mode frequency, which is about 0.05 eV. This is a large discrepancy in light of the precision of modern infrared spectroscopy and could cause serious error if the extinction peak were assumed to lie at the position of a bulk absorption band. This is the first instance we have encountered where the properties of small particles deviate appreciably from those of the bulk solid. It is the result of surface mode excitation, which is such a dominant effect in small particles of some solids that we have devoted Chapter 12 to its fuller discussion. 

Spectroscopy A spectrum of PtPcCP4, which is typical of these phthalocyanines is shown in Fig. 2. The most intense band is the Q band which occurs between 640 nm and 680 nm for the different metal phthalocyanines. It is the lowest allowed n - n transition of the phthalocyanine ring. In dilute solution, the Q band of the monomer typically had a molar extinction coefficient of 2 x 105 //mole-cm in agreement with previous reports. (2) Additional bands, which have been assigned to phthalocyanine aggregates (2)(3), were observed on the short wavelength side of the Q band in the relatively concentrated solutions used for the nonlinear optical studies. 

Characterisation of vesicles was achieved using a combination of methods, including photon-correlation spectroscopy, video-enhanced and cryo-electron microscopy. Measurements of the cmc of the surfactants (in the absence of salt) were made using uv-visible spectrophotometry and electrical conductivity (k). For cmc measurements, there is a convenient change in the extinction coefficient of the benzene chromophore at 262 nm. The onset of vesicle formation, and hence the determination of the esc, can be measured by 90° scattering and 180° optical turbidity measurements at 300 nm. 

The recommended unit for MCD spectroscopy, Aem, is based on the extinction coefficient for differential absorbance of a 1M solution of the solute at a field strength of 1 T. The original 9 ellipicity unit is still sometimes used. The conversion factors are 6 = 32.98 AA, where 9 is expressed in units of mdeg or [0]m = 3298A6m where, [0]m is expressed in units of deg cm -dmol . Room temperature spectral measurements are usually measured in solution, ideally in a solvent which is optically transparent in the 280-1000 nm region. 

As mentioned above, absorbance is defined as log IjE, older textbooks refer to the term as extinction, while even older manuscripts call it optical density. All three terms mean the same, but absorbance is the expression that should be used in all analytical spectroscopy... 

A is a constant, roughly analogous to the extinction coefQcient of conventional optical spectroscopy... 

PNA products released from the solid support are analyzed for purity by reverse-phase HPLC and MALDI-TOF mass spectroscopy (Fig. 8). Reconstitution of the PNA precipitate for analysis is best accomplished in a solution of 0.1% TFA in H2O as described earlier. The quantity of PNA produced is measured in absorbance units as a total optical density (OD) at 260 nm. The OD values provide a rapid early evaluation of the synthesis performance and should range between 80 and 180 for a 2-/u,mol preparation. This measurement varies with the length and purine content of the sequence. The OD is directly related to the concentration of the solution by the Beer-Lambert law A = scL where A is the absorbance at 260 nm (OD), c is the concentration, L is the path length, and e is the molar extinction coefficient. 

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