A systematic study of the near-edge x-ray-absorption fine structure for single-layer coverage of C2H2, C2H4, and, for the first time, C2H6 chemisorbed on Cu(lOO) and Ag(lOO) allows the determination of the molecular orientation and bonding strength. The first surface extended x-ray-absorption fine structure at the carbon K edge yields a nearest-neighbor distance of 1.9(1) A for C2H2 and C2H4 on Cu. A comparison of the a shape-resonance position in the gas phase, chemisorbed on Ag, Cu, and Pt, is presented. PACS numbers: 68.55Jk, 68.45.Da, 78.70.Dm Acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are three important hydrocarbons which model the triple, double, and single C-C bonds. C2H6 has a fully saturated C-C bond and little dependence of the C-C bond length upon the strength of chemisorption on a metal substrate is expected. The contrary is true for C2H2 with its double n bonding. The chemical reactions of these three species on metal surfaces is of practical importance for catalysis. C2H4 on Pt(lll) rehybridizes upon adsorption below room temperature to the surface, and in the presence of atomic oxygen it combusts to CO2 and H2O at higher temperatures. * Similar reactions are known on Ag(llO).^ A first step in order to understand those processes is the knowledge of the microscopic structure of chemisorption; i.e., the orientation of the molecules on the substrate, the coordination to the substrate atoms (hollow, bridge site, etc.), the bonding length to the substrate, and an eventual change in the intramolecular bond length. Synchrotron radiation, which offers the advantage of a tunable photon energy and a linear polarization, can be used in order to try to answer some of those questions. Recently the near-edge x-ray-absorption fine structure (NEXAFS) of C2H2 and C2H4 on Pt(l 11) has been reported.^ The molecules were found to be lying flat and strongly chemisorbed to the Pt(lll) surface, strongly perturbed by it, and exhibited a longer C-C bond length than on the gas phase. ^ Whether a correlation of C-C bond length and the energy of the a resonance can be deduced in broad generality is subject to debate."^ In this Letter we present the first surface extended xray-absorption fine structure (SEXAFS) and NEXAFS study to obtain the C-Cu distance and the orientation of hydrocarbon molecules on noble-metal substrates. We investigated three similar hydrocarbons (C2H2,C2H4, C2H6) on two different (Ag,Cu) substrates. The adsorption process was characterized by thermal desorption spectroscopy (TDS) measurements, and the energy of the molecular resonances from angle-dependent NEX-AFS measurements was correlated to the strength of adsorption. In particular, we find the molecules lying flat on the Cud00) surface. On Ag(lOO) this is also the case except for C2H2, where the C-C axis makes a finite angle to the surface. C2H6 yields the same a resonance energy on the two substrates as in the gas phase. The TDS data show that the adsorption on Cu is stronger than on Ag. This can be correlated to a monotonic decreas...

No abstract

The pre-emptive stabilization of a neoclassical tearing mode, NTM, requires the calculation of the tokamak magnetic equilibrium in real-time. A launcher mirror is positioned to deposit electron cyclotron current drive on the rational surface where the NTM should appear. A real-time Grad-Shafranov solver using constraints from magnetic probe, flux loop and Motional Stark Effect measurements has been developed to locate these rational surfaces and deliver this information to the mirror controller in real-time. A novel algorithm significantly reduces the number of operations required in the first and second step of the solver. Contour integrals are carried out to calculate the q profile as a function of normalized radius and the rational surfaces are found by spline interpolation. A cycle time of 0.6 ms for calculating two tokamak equilibria in parallel using four current basis functions with magnetic constraints only and using six current basis functions with magnetic and MSE constraints has been achieved. Using these tools, pre-emptive stabilization

Real-time magnetic equilibria are needed for NTM stabilization and disruption avoidance experiments on ASDEX Upgrade. Five improvements to real-time magnetic equilibrium reconstruction on ASDEX upgrade have been investigated. The aim is to include as many features of the offline magnetic equilibrium reconstruction code in the real-time equilibrium reconstruction code. Firstly, spline current density basis functions with regularization are used in the offline equilibrium reconstruction code, CLISTE [1]. It is now possible to have the same number of spline basis functions in the real-time code. Secondly, in the presence of edge localized modes, (ELM's), it is found to be necessary to include the low pass filter effect of the vacuum vessel on the fast position control coil currents to correctly compensate the magnetic probes for current oscillations in these coils. Thirdly, the introduction of ferromagnetic tiles in ASDEX Upgrade means that a real-time algorithm for including the perturbations of the magnetic equilibrium generated by these tiles is required. A methodology based on tile surface currents is described. Fourthly, during current ramps it was seen that the difference between fitted and measured magnetic measurements in the equilibrium reconstruction were larger than in the constant current phase. External loop voltage measurements and magnetic probe pairs inside and outside the vessel wall were used to measure the vacuum vessel wall resistivity. This is the first step towards including vacuum vessel currents during the plasma current ramp in the real-time equilibrium reconstruction. Fifthly, the introduction of a constraint of the safety factor on the magnetic axis is found to be a helpful method to improve the prediction of the location of rational surfaces for NTM stabilization and disruption avoidance experiments. Soft X-ray tomography is used to assess the quality of the real-time magnetic equilibrium reconstruction using this internal constraint.

No abstract