Wavelength dependence anomalous scattering

The anomalous components of the total scattering are wavelength dependent and the use of radiation close to an absorption edge may increase or optimise the contribution due to the anomalously scattering atoms. Ramaseshan (1962) pointed out that data collected at multiple wavelengths optimising the anomalous dispersion effects would improve the quality of phase determination. 

A/" and are the real and imaginary components, respectively, of the anomalous scattering factor and they depend on both the atom type and the wavelength. 

Each reciprocal lattice point can give rise to a reflection with some wavelength within the experimental bandwidth. A variety of wavelength-dependent factors affect the measured reflection intensity. These factors include the SRS spectral profile, the effect of optical elements, sample scattering efficiencies, absorption of components in the beam and detector response. In addition, the presence of anomalous scatterers in a crystal introduces a wavelength variation in the structure amplitudes and, incidentally, in the phases (see chapter 9). 

In contrast, Karle s detailed theory leads to expressions for the structure amplitudes and phase differences defined in terms of the non-anomalous atomic scattering factors for all the atoms in the structure and whereby the wavelength-dependent parts are treated separately. The notation in common use is that of Hendrickson (1985), which is essentially the one used below. Compared with equation (2.6) (hkt) is now written as (h) and (hxj+ky + zj) as (h-r,) for brevity to allow the extra nomenclature now needed to be clearer. 

Strictly speaking, A/ depends also on the atomic number of the scattering element, which means that a different correction curve is required for every element. But the variation of A/with Z is not very large, and Fig. 13-8, which is computed for an element of medium atomic number (about 50), can be used with fairly good accuracy as a master correction curve for any element. Figure 13-8 was calculated from data in James [G.7, p. 608]. Anomalous dispersion is also discussed by Guinier [G.21]. Values of A/ as a function of Z at constant 2, for five characteristic wavelengths, can be calculated from data in [G.ll, Vol. 4]. 

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