On Predicting Scan Profiles: the Nature of the 'Aberration Function' BY RICCARDO DESTRO

In an earlier t reatment [Destro & Marsh (1987). Acta Cryst. A43, 711-718], an at tempt was made to predict the shapes of high-angle 0-20 scan profiles by convoluting a low-angle profile with the presumedly known spectral distr ibution function for the incident (crys ta l -monochromat ized Mo K a ) radiation but it was found necessary to introduce a third component, an 'aberrat ion funct ion ' , that varied with the Bragg angle 0. It is shown here that the pr imary purpose of the aberrat ion funct ion is to correct for defects in the spectral-distr ibution function. In particular, the effective intensity ratio between the Ka2 and Kal spectral lines can apparent ly deviate greatly (by more than 10%) from the theoretical value of 0.499, depend ing upon the a l ignment of the monochromator crystal, and an appreciable amount of white radiat ion may also be present. By a suitable modif icat ion of the spectral-distr ibution function, high-angle scan profiles can be predicted from accurate measurements of a low-angle profile; as a result, scan-truncat ion losses can, for the most part, be removed. However, model ing the spectral distribution funct ion appropria te to a part icular exper iment remains a difficult empir ical procedure.