X-ray refractive-index measurement in silicon and lithium fluoride

The refractive indices n of silicon and lithium fluoride were measured noninterferometrically with Mo K and Ag K x rays to a sub-part-per-billion accuracy. This high accuracy allows experimental determination of the real dispersion correction f to 2 millielectron accuracy. The f values obtained are in excellent agreement with the best interferometric measurements, part of which are less accurate than the present results. The predictions of both the Cromer-Liberman and the modified Hönl theories are found to deviate significantly from the measured f values, thus indicating the need for modification of the wave functions or, more likely, the exchange potential used.