Wavelength-modulated diffraction: a new method for phase determination

A new diffraction method has been developed in which the intensity of Bragg reflections is measured while changing continually the wavelength of the radiation over a range in the vicinity of the absorption edge of an atom contained in the crystal. It is shown that the intensity gradient with respect to the wavelength of the hkl reflection is in a simple relation to the real and imaginary parts of the structure factor of that reflection and, if the positions of the anomalously scattering atoms are known or properly assumed, the phase of F(hkl) can be derived by solving simultaneous linear equations. The procedure is particularly simple when the crystal is centrosymmetric. The method, called the wavelength-modulated diffraction (WMD) method, is free from the problem of intensity scaling encountered in other methods of phase determination. Synchrotron radiation is most suited to WMD measurements. Suggestions on how to measure the intensity gradient are given and the possible errors involved are discussed.