最初描述原子在X射线照射下散射忽略了波长相关性，这种处理方式被称为“正常散射”。对正常散射进行波长相关性校正，也称为反常散射，校正必须描述振幅和相位的变化，最初分别给出符号Δf′和Δf′′。因此，原子的X射线散射因子由以下方程描述。当可调谐同步辐射源可用时，命名法发生了变化，Δ前缀被删除，因为两个波长之间的变化需要双重Δ标签，这很麻烦。因此，Δf′现在意味着f′在两个波长之间的变化。f′′的Δ前缀被删除以保持一致，即使它的使用是基于其在单个波长上的值。f′和f′′的值在所讨论的元素的吸收边缘变化最大。因此，这种共振效应有时会导致术语被称为“共振散射”。然而，由于非共振f′′效应广泛用于分子手部（其手性）的晶体结构测定，所以“反常散射”这个词语是最好的，即最广泛的术语。另一个常用术语是多波长异常色散（“MAD”，Multiple-wavelength Anomalous Dispersion），它涉及在共振条件下进行的测量，显然是在多个波长下进行的。The history of the description of the scattering of an atom when illuminated with X-rays is that initially wavelength dependencies were ignored. This was initially referred to as 'normal scattering'. The wavelength dependencies were then corrections to the normal scattering and also called anomalous. These had to describe changes in amplitude and phase, respectively initially given the symbols Δf′ and Δf′′. Thus the X-ray scattering factor of an atom is described by the equationThe nomenclature changed when tunable synchrotron sources became available and the Δ prefixes were removed because changes between two wavelengths would then have required a double Δ label, which is cumbersome. Thus the Δf′ now means the change in f′ between two wavelengths. The Δ prefix to f′′ is dropped for consistency even though its use is based on its value at a single wavelength.The values of f′ and f′′ change most at the absorption edge of the element in question. Thus this resonance effect sometimes leads to the term being referred to as 'resonant scattering'. However, since the off resonance f′′ effect is extensively used in crystal structure determination of the hand of a molecule (its chirality) 'anomalous scattering' is the best i.e. most widely embracing term. Another commonly used term is Multiple-wavelength Anomalous Dispersion ('MAD'), which involves measurements made at the resonance condition and at more than one wavelength obviously.![]()
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