A HREEL investigation of adsorption and dissociation of NO on a Rh(110) surface

The adsorption and dissociation of NO on a Rh(110) surface in the temperature range from 100 to 300 K has been studied by means of high-resolution electron energy loss (HREEL) spectroscopy. At 100 K only one adsorption state of NO, assigned to bridge-bonded NO species, is observed at the whole NO coverage range. The N-O stretching frequency of this species increases from 1560 to 1710 cm-1 with increasing NO coverage. NO decomposition, which occurs readily at temperatures above 170 K has been studied for NO coverages less than 0.3 of the saturated NO coverage at 100 K. The HREELS data have shown that the fraction of NO molecules which undergo dissociation increases with increasing temperature and with decreasing initial NO coverage. For the highest NO coverages considered (0.3 of saturation at 100 K) all NO molecules decompose at 240 K. A variety of loss features are observed in the HREEL spectra after decomposition of different amounts of NO. These HREEL data are explained on the basis of comparison with the HREEL spectra measured for oxygen, nitrogen and mixed oxygen and nitrogen layers on Rh(110). It has been established that the variety of loss features observed after dissociation of NO is due to different oxygen states on the surface. The observed effect of the dissociation products on the N-O stretching frequencies have heen discussed considering the factors that can account for the blue-shifts observed in the presence of electronegative surface modifiers

Stress engineering at the nanometer scale: Two-component adlayer stripes

Spontaneously formed equilibrium nanopatterns with long-range order are widely observed in a variety of systems, but their pronounced temperature dependence remains an impediment to maintain such patterns away from the temperature of formation. Here, we report on a highly ordered stress-induced stripe pattern in a two-component, Pd-O, adsorbate monolayer on W(110), produced at high temperature and identically preserved at lower temperatures. The pattern shows a tunable period (down to 16 nm) and orientation, as predicted by a continuum model theory along with the surface stress and its anisotropy found in our DFT calculations. The control over thermal fluctuations in the stripe formation process is based on the breaking/restoring of ergodicity in a high-density lattice gas with long-range interactions upon turning off/on particle exchange with a heat bath.Comment: 6 pages, 4 figure

Seeded x-ray free-electron laser generating radiation with laser statistical properties

The invention of optical lasers led to a revolution in the field of optics and even to the creation of completely new fields of research such as quantum optics. The reason was their unique statistical and coherence properties. The newly emerging, short-wavelength free-electron lasers (FELs) are sources of very bright coherent extreme-ultraviolet (XUV) and x-ray radiation with pulse durations on the order of femtoseconds, and are presently considered to be laser sources at these energies. Most existing FELs are highly spatially coherent but in spite of their name, they behave statistically as chaotic sources. Here, we demonstrate experimentally, by combining Hanbury Brown and Twiss (HBT) interferometry with spectral measurements that the seeded XUV FERMI FEL-2 source does indeed behave statistically as a laser. The first steps have been taken towards exploiting the first-order coherence of FELs, and the present work opens the way to quantum optics experiments that strongly rely on high-order statistical properties of the radiation.Comment: 24 pages, 10 figures, 37 reference

Oxidation of methanol on Ru catalyst: Effect of the reagents partial pressures on the catalyst oxidation state and selectivity

In situ core level photoelectron spectroscopy and mass spectrometry have been utilized to study the methanol oxidation on a model RuO2 catalyst at pressures ranging from 10-6 to 10-1 mbar. The experiments were carried out varying the O2/CH3OH molecular mixing ratio from 0.25 to 3.3 and the reaction temperature from 350 to 720 K. The Ru 3d5/2 and O 1s core level spectra were used to characterise the dynamic changes in the Ru oxidation state by exposing the oxide pre-catalyst to different reagents partial pressures and temperatures. Full oxidation to CO2 + H2O or partial oxidation to CO + H2O + H2 have been observed in the whole pressure range for specific reaction conditions, which preserve the oxide catalyst state or reduce the oxide to metallic Ru. The selective oxidation to formaldehyde is observed only at pressures in the 10-1 mbar range, catalyzed by a RuO_x surface oxide formed by partial reduction of the oxide pre-catalyst

Ultrafast spin-switching of a ferrimagnetic alloy at room temperature traced by resonant magneto-optical Kerr effect using a seeded free electron laser

Ultrafast magnetization reversal of a ferrimagnetic metallic alloy GdFeCo was investigated by time-resolved resonant magneto-optical Kerr effect measurements using a seeded free electron laser. The GdFeCo alloy was pumped by a linearly polarized optical laser pulse, and the following temporal evolution of the magnetization of Fe in GdFeCo was element-selectively traced by a probe free electron laser pulse with a photon energy tuned to the Fe M-edge. The results have been measured using rotating analyzer ellipsometry method and confirmed magnetization switching caused by ultrafast heating