Wavelength dependence sample absorption

Lifetime-based sensing can also be based on the existence of two forms of the probe, the fraction of which depends on the analyte concentration. This case is presented in Figure 10.4 where it is assumed that the absorption spectra overlap and both forms can be excited at the same wavelength. The sample displays two lifetimes (TFandifi) which are characteristic of the analyte-free (F) and analyte-bound (B) forms of the probes. The change in lifetime (from rf to zb) is due to binding of the analyte... 

Fig. I. Comparison of the wavelength dependences of the enhanced absorption decay time for the (a) two examined materials (b) two Fe203 samples with different particle size distributions...

A / is the absorbance of the sample when the light is polarized parallel to a reference axis, and Aj is the absorbance of light which is polarized perpendicular to this axis. The strength of the absorption depends on the orientation of the electric field vector of the light and the transition moment of the chromophore - parallel orientation results in maximum absorption whereas perpendicular orientation leads to zero absorption. By dividing the LD value by the absorbance of the unoriented sample under isotropic conditions (Aiso), the reduced linear dichroism (LDr), i.e. the wavelength-dependent LD, is obtained (Eq. 7) [36]. 

We remeasured, therefore, the cell-paste absorption by using, in this case, a quasi-optical spectrometer (23). In order to avoid wavelength-dependent diffraction of the sample, the millimeter-wave frequency was veried between 40 and 60 GHz, and the average of the apparent transmission was measured. This averag-... 

When measuring absorption spectra, one records a signal that is related to the wavelength-dependent probability of making a spectroscopic transition. From the molecular point of view, this probability is proportional to the dot product jl p where (L is the molecular transition moment and p is the photon polarization direction. When the orientational distribution of the molecules is isotropic (not crystalline, liquid crystalline, or bound to a surface), its absorption spectrum represents the orientationally averaged probability of making a spectroscopic transition and the measured spectrum is independent of polarization direction. When the orientational distribution of the molecules is anisotropic, the probability of making a spectroscopic transition depends on the polarization direction, and that dependence can be exploited to deduce the direction of the transition moment relative to the laboratory frame. Because transition moments are often trivially related to the orientation of the molecule, structural information can be deduced from polarized absorption measurements on anisotropic samples. 

From the above it is apparent that is a function of two variables, namely time and wavelength. The time dependence leads to dynamics information as outlined above the wavelength dependence provides spectral information, useful for assignment or structural purposes. When values of A[, t) are obtained for a photo-initiated sample, a dynamic surface (T-A- ) can be constructed this contains the totality of the information available from an optical absorption experiment. 

In this method fluorescent intensities are calculated from first principles. The data needed for the calculation are (1) the spectral distribution (intensity vs. wavelength) of the primary beam from the x-ray tube, (2) mass absorption coefficients ju/p of all elements in the sample, and (3) fluorescent yields w of all elements. The calculations require that measured line intensities I from the unknown be converted into relative intensities R by comparison with pure metal standards. (Use of R rather than / values eliminates the need to know the wavelength-dependent efficiencies of reflection by the analyzing crystal and detection by the counter.)... 

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