Molar extinction coefficient 592 INDEX

Solutions of isolated dioxiranes, characteristically dimethyldioxirane (DMD) in acetone, possess a pale yellow color, which serves as a convenient analytical index for monitoring the dioxirane consumption in oxidation reactions and kinetic studies. For DMD, the absorption maximum (n-jt transition ) centers at ca 325 nm, with a molar extinction coefficient (e) of 12.5 0.5 M cm in acetone. The alternative and more rigorous analytical method for dioxirane quantification utilizes iodometry (Kl/starch). 

The thickness of the interface layer where the dopant concentration is different from that of the bulk is roughly estimated as follows. Since a molar extinction coefficient of carbazolyl chromophore in PVCz at 295 nm is 1.54 x 10 cra M", the depth where the excitation intensity is 1/e of the initial value is calculated to be 0.065 um under the normal condition. On the other hand, the penetration depth of the evanescent wave is a function of the incident angle, and it is difficult to calculate it here because the complex refractive index cannot be estimated correctly from the large absorbance at the laser wavelength. At present we can say that the TIR phenomenon was really observed and that the effective thickness under the TIR... 

Molar absorptivity e = Ajbc Molar absorbancy index, molar extinction coefficient, molar absorption coefficient... 

The molar extinction coefficient is an index of the efficiency with which a compound absorbs light at a given wavelength. It should also be noted that this relationship has been used extensively in analytical chemistry because concentration is directly related to absorbance that can be measured quite easily with a spectrophotometer. 

Figure 5 shows concentration dependence of refractive index for DAAB/PMMA films(iO). Refractive indices were measured for unirradiated and irradiated samples. Unirradiated samples include the trans form alone. Photoirradiation was carried out by a Hg lamp with a filter of V-VIA (Toshiba). The conversion of trans-tocis form in the photochromic reaction was 0.70 as calculated by using the molar extinction coefficients of both isomers. Similar to the results of measurements for NBD derivatives, refractive indices are higher in the region of higher any dye content, and refractive indices of unirradiated samples are higher than those of irradiated samples at any contents. It reflects the decrease in polarizability caused by disappearance of the jt-jt transition band of the trans form. 

Absorptivity, absorbance index, absorption cor i-cient the proportionality constant e, in Beer s law for light absorption A = elc, where A is absorbance, / the length of the light path, and c the concentration. If concentration is expressed on a molar basis, e becomes the molar absorptivity, molar absorption coefficient or molar extinction coefficient, i.e. e = A/lc, where I is the length of the light path in cm, and c is the molar concentration. 

An is the contribution to the refractive index due to the solute whose concentration is C (mol/L), c is the speed of light, and e(vO is the molar extinction coefficient. An is the difference between the refractive indexes of the solution and the solvent. An is essentially the isotropic contribution to the refractive index by the solute whose absorption spectrum determines e(vO. It should not be confused with birefringence, which uses the same symbol (see under Birefringence). A common application of the Kramers-Rronig relation appears in light scattering, especially when one is interested in the effects of scattering on absorbance measurements. 

The attenuation of the intensity of a beam of light on passing through an absorbing medium can be measured by the absorption coefficient, which is the imaginary part of the refractive index. A more usual measure is the optical absorbance (A) or molar extinction coefficient e, which is defined in terms of the Beer-Lambert law as ... 

If the calibration curve (Fig. 10) of the sample in 10 mm cuvette is perfectly linear this means that the (s) molar extinction coefficient depiendence on the (n) refractive index of the solution is negligible (Skoog Leary, 1992). This is the case in our study where only up to OD = 0.2 the calibration curve is linear within experimental error. 

FIGURE 2.1. Simplified presentation of a dichroic band (Cotton effect), (a) Positive Cotton effect (b) negative Cotton effect. Molar extinction coefficients for ieft- and right-hand polarized iight components and (2) Refraction index of left- and right-hand polarized iight components and Circular dichroism Optical rotatory dispersion . 

Here pis the quantum efficiency of the sensitizer (ti = Tp/xp = l/3forpentacene)in the O4 site of p-terphenyl at 4 K, n is the index of refraction (n = 1.7 for the p-terphenyl crystal), and Na is the Avogadro s number. The integral in (H9) is calculated from the normalized fluorescence spectrum/(v) and the decadic molar extinction coefficient e(v) of pentacene at O4 site. The critical interaction distance is the sensitizer-activator separation for which the transfer rate is equal to the intrinsic decay time. Although derived for low temperatures. Equation H9 is also vaUd for arbitrary temperatures. In fact, the temperature dependence of the resonant energy transfer rate is contained in the spectral overlap integral. 

The error in (a) is stated to compare favourably with calibration from benzene, since the absolute value of R90 is hardly known to this accuracy. In (b) the concentration of DNA was measured spectrophotometrically via the molar phosphorous extinction coefficient of 6415 (with a standard deviation of 2%). The low error in (c) arises from low levels of dust achieved as well as the integration over a period of 10 secs of the readings on a digital output. The specific refractive index increment used in (d) was an experimental one from the literature. In point of fact the assess-... 

P has a very suggestive form in relation to Figure 8.26. For a large concentration of acceptors, the second term in the denominator can be made considerably smaller than 1 (i.e., Xt is proportional to acceptor concentration [A]), and P will be independent of concentration. On the other hand, for a small concentration of acceptors, the second term in the denominator can be made considerably larger than 1, and P will fall off linearly as the concentration is reduced. The scale factor in all of this is Q. With Q large, the transition from concentration independence to linear concentration dependence will be at low acceptor concentrations. P falls to 5 when the second term in the denominator of Eq. (8.27) is equal to 1, and so a critical concentration of acceptors [A], /2 can be defined to characterize the falloff. Expressing Xt in terms of molecular parameters (x, = em[A] ln(10)/, where n is the particle refractive index, em is the molar decadic extinction coefficient, [A] is the concentration of acceptors, and k is 2n/X) yields... 

The spectra Fo(v) and Ca(v) are represented on the wavenumber scale and the fluorescence spectrum (F(v)) of the donor is normalized on this scale n is the refractive index, e iv) is the molar decadic extinction coefficient of the acceptor and To is the radiative lifetime (s) and R(nm) is the D-A center to center distant. For very strong coupling the rate is given by... 

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