Adsorption and coadsorption of CO and H on ruthenium surfaces

The interaction of CO with the Ru(0001) surface at several coverages (11.1, 25.0, and 33.3%) was studied, as well as the interaction of CO with a stepped Ru(0001) surface. The preference for the adsorption site (atop vs. hcp) was analyzed with d. of states diagrams. Hydrogen layers can be densely packed; 1 ML could, in fact, correspond to >100% coverage, where 100% coverage would correspond to 1 adatom for each metal atom on the surface. Calcns. were made for 1 ML of adsorbed hydrogen ?300% coverage for 2 * 2 supercells. The H coadsorption with CO (2 * 2 (CO + nH), n = 1, 3, 4) is discussed for different adsorption sites. The lateral interaction H-CO is repulsive. Hads and COads prefer to form islands rather than mixed structures. CO is little influenced by coadsorption, except when 1 ML of at. hydrogen is preadsorbed. H is strongly affected by coadsorption. The H adsorption sites become highly asym. if H and CO share 1 metal atom

Dynamical studies of zeolitic protons and interactions with adsorbates by picosecond infrared spectroscopy

We have investigated zeolite-adsorbate interactions with time- resolved (picosecond) non-linear infrared spectroscopy, In these experiments, the zeolite hydroxyl (the catalytically active, acid site) is vibrationally excited after which the energy relaxation can be monitored real-time. The effect of the presence of adsorbates on energy relaxation rates and pathways provides information on catalyst-adsorbate interaction dynamics. Furthermore, the effect of the presence of adsorbates on the homogeneous linewidth, investigated with infrared hole- burning spectroscopy, provides information on the nature of the interaction potential. (C) 1997 Elsevier Science B.

Orientation in molecule-surface interactions

Heinzmann U, Holloway S, Kleyn AW, Palmer RE, Snowdon KJ. Orientation in molecule-surface interactions. JOURNAL OF PHYSICS-CONDENSED MATTER. 1996;8(19):3245-3269.The role of the orientation of a molecule in its interaction with a surface is examined for a number of cases. At first the determination of the static orientation of a molecule at the surface is discussed. A dependence of the orientation on state and coverage is found for O-2 molecules adsorbed on Ag(110). The orientation dependence of NO in chemisorption, displacement reactions, and chemical reactions is discussed. Rotational excitation of NO in collisions with surfaces is shown to exhibit a strong orientation dependence. Dissociative chemisorption of hydrogen molecules is found to depend on the initial orientation of the molecule. Finally, in dissociative collisions of fast molecules with surfaces the role of the initial orientation is examined and the final orientation of the molecular axis is determined. Several mechanisms for molecular dissociation are discussed

CO adsorption on hydrogen saturated Ru(0001)

The interaction of CO with the Ru(0001)(1 x 1)H surface has been studied by density functional theory (DFT) periodic calculations and molecular beam techniques. The hydrogen (1 x 1) phase induces an activation barrier for CO adsorption with a minimum barrier height of 25 kJ mol(-1). The barrier originates from the initial repulsive interaction between the CO-4 sigma and the Ru-d(3z2-r2) orbitals. Coadsorbed H also reduces the CO adsorption energy considerably and enhances the site preference of CO. On a Ru(0001)(1 x 1)H surface, CO adsorbs exclusively on the atop position. (C) 2001 American Institute of Physic

A differentially pumped argon plasma in the linear plasma generator Magnum-PSI: gas flow and dynamics of the ionized fraction

Magnum-PSI is a linear plasma generator designed to reach the plasma–surface interaction (PSI) regime of ITER and nuclear fusion reactors beyond ITER. To reach this regime, the influx of cold neutrals from the source must be significantly lower than the plasma flux reaching the target. This is achieved by a differential pumping scheme, where the vacuum vessel is divided by skimmers into separate chambers which are individually pumped. The non-magnetized expansion of 5 Pa m3 s-1 (3 slm) argon in a low background pressure was studied in the differentially pumped vacuum vessel fitted with non-cooled flat skimmers. The behavior of the neutral component was studied with direct simulation Monte Carlo simulations and Rayleigh scattering measurements. Thomson scattering and double Langmuir probe measurements were performed on the ionized fraction. It was found that the electrons and neutral particles are not completely coupled in the shock front. The neutral fraction shows clear signs of invasion from hotter background gas, causing the average temperature and density to increase before the shock. This is also shown in the ionization ratio, which has been determined in front of and behind the first skimmer. This study helps us to understand the behavior of the gas flow in the machine and validates our modeling