Magic-angle theorem in powder x-ray-absorption spectroscopy

We demonstrate that texture effects can seriously distort the measurement of x-ray-absorption data from powdered specimens. We show that the isotropic average dipolar x-ray-absorption spectrum can always be measured irrespective of the degree of texture, provided axial symmetry in the particle distribution function exists normal to the specimen surface and a linearly polarized x-ray beam is employed. A simple modification to synchrotron-radiation experiments to achieve this consists of rotating the specimen by ${\mathrm{\theta }}_{\mathit{m}}$ about an axis perpendicular to the polarization vector and beam direction from normal incidence. ${\mathrm{\theta }}_{\mathit{m}}$ is given by ${\sin }^2$${\mathrm{\Theta }}_{\mathit{m}}$=1/3.