Showing papers by "Christoph Quitmann published in 2004"

Determination of the absolute chirality of adsorbed molecules.

Abstract: Pasteur's separation of the optical antipodes of ammonium sodium tartrate tetrahydrate in 1848 and the concept of the asymmetrically substituted carbon atom, introduced by van't Hoff and le Bel, marked the birth of stereochemistry. Although the Fischer–Rosanoff convention based on d-(+)glyceraldehyde got general acceptance, it was not possible to link the optical rotary power to the absolute configuration of chiral molecules. In 1951, however, Bijvoet and co-workers determined the absolute configuration of (+)-sodium rubidium tartrate through anomalous X-ray scattering and showed that the arbitrary d-/l-convention matches reality. Increasing computational power and improved quantum mechanical codes nowadays allow to connect chiroptical spectra with absolute configuration, but this approach is still limited to relatively small or rigid molecules. Other methods for determining the absolute configuration, such as wetting or enantioselective adsorption on polar crystals and measurements of the electric polarization in ferroelectric liquid crystals, have been reported. The conformation of chiral molecules adsorbed on a surface plays an important role in biomineralization and in stereoselective heterogeneous catalysis. For example, tartaric acid serves as chiral modifier for the enantioselective hydrogenation of b-ketoesters over supported nickel catalysts. To gain deeper insight into the mechanisms of enantioselective surface chemistry and biomineralization, a detailed analysis of the local adsorbate structure is necessary. The absolute chirality of molecules adsorbed on surfaces has been inferred from scanning tunneling microscopy (STM) images. However, in most cases of molecular adsorption it is very difficult to correlate the observed electron densities with the absolute atomic positions, and modeling of STM images requires extensive theoretical and computational effort. Herein we demonstrate that the absolute configuration of chiral molecules adsorbed on single-crystal surfaces can be determined in a straightforward fashion by means of anglescanned X-ray photoelectron diffraction (XPD), a method delivering direct real-space structural information. For our study we chose the classic tartaric acid molecule adsorbed on the Cu(110) surface. This adsorbate system has been studied in great detail. At low coverages and after activation at 405 K, tartaric acid becomes doubly deprotonated, with the resulting bitartrate species forming long-range ordered chiral structures on the surface. Based on IR spectroscopy results, a local C2 symmetry for the bitartrate adsorbate complex has been proposed. Figure 1 illustrates the basic principles of the XPD experiment for the case of the adsorbed bitartrate species.

Uncompensated spins in a micro-patterned CoFeB / MnIr exchange bias system

Abstract: The element specific domain configuration of a microstructured Co86Fe10B4∕Mn77Ir23∕Ni80Fe20∕Si3N4 exchange bias film was studied by photoemission electron microscopy using x-ray magnetic circular dichroism. In the dots the magnetization shows less preferred orientation along the exchange bias direction than in unstructured areas. Uncompensated Mn spins at the CoFeB/MnIr interface with an antiferromagnetic coupling between the Mn and the Co magnetic moments could be studied by imaging and spectromicroscopy.

Fabrication of nanoscale antidot arrays and magnetic observations using x-ray photoemission electron microscopy

Abstract: Antidot arrays in thin cobalt films have been fabricated with periods ranging from 2 μm down to 100 nm and with different size to separation ratios. The resulting magnetic domains were observed with a photoemission electron microscope using x-rays. For periods larger than 1 μm, we observe magnetic domain configurations which substantiate micromagnetic calculations found in the literature. At smaller periods the domain configuration changes dramatically resulting in chains of magnetic domains running parallel to the intrinsic hard axis.