Section 9.1.9. Wavelength

The wavelength of X-radiation can be tuned only at synchrotron sources. Rotating-anode generators produce radiation at a fixed wavelength which is characteristic of the metal of the anode, usually copper with λ = 1.542 Å.

The proper selection of the wavelength is most important for collecting data containing an anomalous-scattering signal. In general, the imaginary component Δf″ of the anomalous-dispersion signal is high on the short-wavelength side of the absorption edge of the anomalous scatterer present in the crystal. Near the absorption edge, both components, real Δf′ and imaginary Δf″, vary significantly. This variation is utilized in the MAD technique, the strict requirements of which are discussed in Chapter 14.2 .

If the data are collected using a single wavelength with the aim of measuring Bijvoet differences, , the requirements are not as strict as for MAD. However, it may be advisable to record the fluorescence spectrum around the region of the expected absorption edge. If the fluorescence signal from the crystalline sample is too weak, the appropriate metal or salt standard can be used. However, the chemical environment of the anomalous scatterers may cause a shift of the edge by up to 10 eV, and it is safer to use a wavelength which is 0.001–0.002 Å shorter (or use an energy 10–20 eV higher) than the edge recorded from the standard. When using anomalous scatterers displaying large white lines within their spectra, the wavelength should be accurately adjusted on the basis of the spectrum measured from the actual sample.

For collecting data without an anomalous signal, there are no strict requirements concerning the wavelength. The maximum intensity provided by the beamline depends on the energy of particles in the synchrotron storage ring and on the beamline optics. Typically, wavelengths around 1 Å or shorter are used at most synchrotrons, assuring high beam intensity and low absorption of X-rays by the sample and air, thus reducing the radiation damage of the crystal. This is of particular importance at the very bright beamlines at third-generation synchrotrons. To diminish the effect of air absorption further, it is possible to fill the space between the crystal and the detector with helium. Short wavelengths are advantageous for collecting high-resolution data, since the diffraction angles are smaller and there is no need to use a very short CTDD. The effect of profile elongation owing to the oblique incidence of diffracted X-ray beams on the detector is then smaller, and the blind region is narrower.