Extended x-ray absorption fine structure (EXAFS) spectroscopy is used to study the size-dependent structural evolution of free clusters. K-edge EXAFS data are obtained for Ar clusters in beams having mean cluster size ranging from 12 to 2900 atoms per cluster. EXAFS analysis yields size-dependent features which indicate the presence of an fcc structure at ͗N͘ $ 200. It has been proposed in the literature that small rare-gas clusters have an icosahedral structure. The transition of this structure to the experimentally observed fcc lattice of rare-gas solids has been of long-standing interest and debate. The implications of our results are discussed in this light.

Small-angle options of the upgraded ultrasmall-angle x-ray scattering beamline BW4 at HASYLAB Rev. Sci. Instrum. 77, 085106 (2006); 10.1063/1.2336195 X-ray absolute intensity measurement at HASYLAB ultrasmall angle x-ray scattering beamline BW4 Rev. Sci. Instrum. 68, 4009 (1997); 10.1063/1.1148374 X1A: Secondgeneration undulator beamlines serving soft xray spectromicroscopy experiments at the NSLS Rev.

No abstract

Studies of superlattices are important in order to synthesize 3D metals in a new crystalline structure which may exhibit exotic magnetic properties. The challenge is to relate these properties to the details of the chemical gradient and the local strain. We report the use of diffraction anomalous fine structure (DAFS) spectroscopy and a crystallographic analysis of the DAFS data to separate the x-ray-absorption fine structure (XAFS)-like information about Fe atoms located at different crystallographic sites of an Fe͞Ir(100) superlattice. We obtained the first and the second (in the plane of growth) nearest-neighbor distances of Fe atoms at the Ir-Fe interface. [S0031-9007 (97)02789-0] PACS numbers: 61.10.-i, 07.85.Jy, 78.70.DmDiffraction anomalous fine structure (DAFS) spectroscopy provides a site-selective x-ray-absorption fine structure (XAFS)-like information about the local atomic environment and the chemical state of the "edge atom" through resonant x-ray scattering and diffraction. Several years after a few pioneering works [1], Stragier et al. [2] presented an elegant demonstration of this method on a copper single crystal and later, several groups applied it on different types of samples like thin films, superlattices [3,4], powders [5], and single crystals [6]. Most results were obtained with materials of which structural information could be extracted with single site-selective Bragg reflections. Site-selective information with real samples was successfully extracted in the DANES (diffraction near edge structure) region of the spectrum; however, due to insufficient statistics no information in the EDAFS (extended diffraction anomalous fine structure) region could be obtained [6]. Regarding superlattices, only the spatial selectivity of the DAFS has been used. The first DAFS experiments on several pseudomorphic Fe͞Ir(100) superlattices were carried out at the Ir edge to determine separately the Ir crystallographic parameters in the buffer and the superlattice [4] (spatial selectivity). We present here a crystallographic-based site-selective EDAFS study of an Fe͞Ir(100) superlattice to separate the XAFS-like information about Fe at the Ir-Fe interfaces.Fe͞Ir superlattices were synthesized to stabilize Fe in new crystalline structures with the aim of obtaining new magnetic properties. Different strained Fe͞Ir structures may be synthesized by varying the thicknesses of the Fe and Ir layers [7]. It has been shown that Fe relaxes into a bcc structure (as bulk Fe) in thick Fe layers (more than five atomic planes) grown at most on three Ir atomic planes, whereas in thin Fe layers (less than five atomic planes) Fe has a body centered tetragonal structure (bct) with a c͞a ratio near 1.25 [8] (pseudomorphic regime). For intermediate thicknesses with thick Fe and Ir layers(more than five atomic planes each) the Fe structure is still not accurately known. High resolution electron microscopy experiments showed that the superlattices are damaged with the disappearance of 2D growth and the presence of a large amount of...

Diffraction anomalous ®ne structure (DAFS) experiments were applied to an epitaxially grown (Ga,In)P layer on a [001] GaAs substrate as a single-crystalline model substance. The requirements for the reliable measurement of re¯ection intensities as a function of photon energy, as well as the quantitative DAFS analysis resulting in the complex-valued ®ne-structure function of the scattering factor, are described. In the case of single crystals, effort had to be put into performing the DAFS measurements in order to hold the position of the Bragg re¯ection exactly during the energy scan. Using the zinc-blende-type structure as an example, it is shown for the ®rst time that, similar to singlecrystal structure analysis, the lack of inversion symmetry has a signi®cant impact on the DAFS signal, so that DAFS may contribute to structure analysis as well.