Absorption in Normal-Transmission Geometry

With little error the exponential in the numerator of the rightmost fraction of Eq. (7.2) is expanded in a Taylor series resulting in 

We observe that Ft = V is the irradiated volume. The maximum of It is found at 

The relation is sketched in Fig. 7.3. From the shape of the curve we anticipate that good scattering signals are obtained, if the thickness of the transmitted sample is in the range 0.5/jJ. f 3/jtt. 

Moreover, the intensity is additionally increasingly dampened with increasing scattering angle. The corresponding absorption and background correction 

The Experimental Determination of the Absorption Factor is based on two flux measurements by means of ionization chambers, one placed before (7i), 

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Figure 7.2. Absorption in normal-transmission geometry. The path of the photon through a sample of thickness t before and after its scattering about the angle 20... 

Figure 7.3. Effect of absorption in normal-transmission geometry. The total transmitted scattering intensity, It, as a fnnction of the rednced sample thickness. The highest scattering signal is obtained at utopt/cos (26) = 1 with tgpt being the optimum sample thickness...

For USAXS and SAXS studies in normal-transmission geometry it is more convenient to carry out this step later - after the absorption and background correction. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Fig. 11. Azimuthal dependence of FT-RAIRS spectra for TiO2(110)-Rh(CO)2 [72], The azimuthal angle (j) is defined as 0° when the incident radiation is aligned in a plane parallel to the <110> direction. The Vsym(C-O) dynamic dipole is aligned normal to the surface and couples to Pn (transmission band), and Vasym(C-O) is aligned parallel to the surface in the <110> direction, and couples to Pt (absorption band). Two possible adsorption geometries consistent with the FT-RAIRS azimuthal dependence are shown for the gem-dicarbonyl.

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