N. Haack

Shape resonances of oriented molecules

Abstract We report ab initio calculations of the X-ray absorption cross section for the near edge X-ray absorption fine structure of C 2 H 6 , C 2 H 4 and C 2 H 2 , at the carbon K-edge, based on a full multiple scattering formalism. We find that shape resonances dominate the photoabsorption carbon K-edge spectra close to the edge, leading to a spectacular enhancement in cross section of order 3–4 Mbarn. Our analysis highlights the strong influence of the molecular vibrations on the intensity, asymmetric profile and energy of this spectral feature. Calculations with and without hydrogen atoms, allow to estimate the amount of C–H character of this feature. A quantitative comparison between theory and experiment, using spectra of oriented molecules on surfaces is performed. Molecular orientation in the experiment is obtained by means of physisorption which has the advantage of obtaining molecules very close to their gas phase state.

Magnetic extended x-ray absorption fine structure at the edges of 3d elements

We report normal and magnetic extended x-ray absorption fine-structure (EXAFS) measurements made on 30 ML Fe films on Cu(001) substrates at the edges. The magnetic EXAFS at the L edges of 3d metals is particularly important as it can be used to probe the magnetism of the d states. Magnetic EXAFS oscillations were detected up to 500 eV above the edge, corresponding to in k-space. Over such a large range, we were able to see long-wavelength wiggles and separate them from Fe nearest-neighbour backscattering and the fast oscillations that had been previously seen. It is shown that without using any deconvolution procedure, a meaningful analysis can be performed despite the interference of the and edges. The experimental data were compared with the results of multiple-scattering calculations, and this enabled us to assign all of the features in both the normal and the magnetic EXAFS Fourier transforms to various single- and multiple-scattering paths. Also, there was found to be less temperature-dependent damping for magnetic EXAFS as than for the normal EXAFS between 75 K and 300 K.

Magnetic extended x-ray absorption fine structure at the L3,2 edges of Fe and Co on Cu(001)

We report magnetic extended x-ray absorption fine structure (EXAFS) measurements on 30 monolayer Fe and Co films on Cu(001) substrates at L3,2 edges. For 3d transition metals, the magnetic EXAFS at the L edges is of great interest since it probes final d states which mainly contribute to magnetism. Magnetic EXAFS oscillations were detected up to 500 eV above the edge corresponding to 11.5 A−1 in k space. Over such a large range, the long wavelength wiggles from the Fe and Co nearest neighbor backscattering and previously observed fast oscillations are clearly seen. It is shown that without using any deconvolution procedure a meaningful analysis can be performed despite an interference of L3 and L2 edges. In the case of Fe, experimental data are compared with multiple-scattering calculations.

Surface EXAFS study of the p4g(2 x 2) reconstruction of C on Ni(100)and C on Ni films

We report temperature- and angular-dependent surface extended X-ray absorption fine structure spectroscopy (SEXAFS) measurements of the p4g(2 x 2)C/Ni(100) system. Both a Ni(100) single crystal and ...

Vibrational fine structure in the N 1s→π∗ resonance of the N2 molecule physisorbed on the Cu(100) surface

Abstract High-resolution photoabsorption spectra at the N 1s→π∗ resonance of N2 physisorbed on the Cu(100) surface are reported. The vibrational fine structure of this resonance could be resolved, allowing the determination of the vibrational constants, the lifetime and intramolecular distances of the core-excited states of the physisorbed molecules. The obtained fundamental vibrational constants are compared with the corresponding vibrational parameters for the gas phase.

Influence of exchange coupling on the Ni magnetization in Co/Cu/Ni trilayers.

Abstract The X-ray magnetic circular dichroism has been used to study trilayers consisting of two ferromagnets (Ni, Co) separated by a non-magnetic spacer (Cu). The coupling of Ni and Co layers was found to be both ferromagnetic and anti-ferromagnetic depending on spacer thickness. Two magnetization curves vanishing at two different temperatures were recorded for Ni and Co in a coupled trilayer. For Ni, this temperature is larger than the Curie temperature ( T C ) of the Cu-capped Ni layer, due to the effect of interlayer exchange coupling.