Observed rocking-curve fine structure on the Aufhellung side of side of Renninger-scan peaks

Rocking curves of the Si(222) reflection have been measured with sigma-polarized incident radiation in the Aufhellung region of a Renninger scan near an n-beam interaction point. Even with an angular resolution DELTA-theta = +/-8", much larger than the intrinsic width of this weak reflection, these curves show a structure with two or more peaks, deviating markedly from that of the standard shape of a (convoluted) weak reflection. Details of the structure vary with the azimuthal angular distance from the interaction point, with the reciprocal-lattice vectors involved in the interaction, and with wavelength. Several typical examples of rocking curves obtained under different experimental conditions are presented and the probable origin and consequences of this structure are discussed.