International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by E. Arnold, D. M. Himmel and M. G. Rossmann © International Union of Crystallography 2012 |
International Tables for Crystallography (2012). Vol. F, ch. 2.1, p. 55
Section 2.1.4.4. Anomalous dispersion^{a}Laboratory of Biophysical Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands |
In classical dispersion theory, the scattering power of an atom is derived by supposing that the atom contains dipole oscillators. In units of the scattering of a free electron, the scattering of an oscillator with eigen frequency and moderate damping factor was found to be a complex quantity:where is the frequency of the incident radiation [James, 1965; see also IT C (2004), equation (4.2.6.8) ]. When in equation (2.1.4.4), approaches unity, as is the case for scattering by a free electron; when , approaches zero, demonstrating the lack of scattering from a fixed electron. Only for does the imaginary part have an appreciable value.
Fortunately, quantum mechanics arrives at the same result by adding a rational meaning to the damping factors and interpreting as absorption frequencies of the atom (Hönl, 1933). For heavy atoms, the most important transitions are to a continuum of energy states, with or etc., where and are the frequencies of the K and L absorption edges.
In practice, the complex atomic scattering factor, , is separated into three parts: . f is the contribution to the scattering if the electrons are free electrons and it is a real number (Section 2.1.4.3). f′ is the real part of the correction to be applied and f″ is the imaginary correction; f″ is always in phase ahead of f (Fig. 2.1.4.8). is the total real part of the atomic scattering factor.
The imaginary correction is connected with absorption by oscillators having . It can be calculated from the atomic absorption coefficient of the anomalously scattering element. For each of the K, L etc. absorption edges, is virtually zero for frequencies below the edge, but it rises steeply at the edge and decreases gradually at higher frequencies.
The real correction can be derived from by means of the Kramers–Kronig transform [IT C (2004), Section 4.2.6.2.2 ]. For frequencies close to an absorption edge, becomes strongly negative.
Values for f, and are always given in units equal to the scattering by one free electron. f values are tabulated in IT C (2004) as a function of , and the anomalous-scattering corrections for forward scattering as a function of the wavelength. Because the anomalous contribution to the atomic scattering factor is mainly due to the electrons close to the nucleus, the value of the corrections diminishes much more slowly than f as a function of the scattering angle.
References
International Tables for Crystallography (2004). Vol. C. Mathematical, Physical and Chemical Tables, edited by E. Prince. Dordrecht: Kluwer Academic Publishers.Hönl, H. (1933). Atomfaktor für Röntgenstrahlen als Problem der Dispersionstheorie (K-Schale). Ann. Phys. 18, 625–655.
James, R. W. (1965). The Optical Principles of the Diffraction of X-rays, p. 135. London: G. Bell and Sons Ltd.