Probe coherence volume and the interpretation of scattering experiments

We discuss the importance of the probe coherence volume in the interpretation of diffraction experiments. The availability of highly monochromatic radiation, neutron or x-ray, calls into question the analysis of data by deconvolution at the level of intensities. When the probe coherence volume approaches that of microscopically ordered regions in the sample, new effects may be anticipated. For instance, under strong absorption the intensity dependence on the incident energy can be used to locate the source of the scattering below the sample surface. Two examples, based on experiments at an x-ray synchrotron source are discussed. [S0031-9007(98)07438-9].