Dissociation of non-screw dislocations in B.C.C. lattice

A non-planar dissociation of non-screw 1/2[¯111] (110) dislocations is proposed. The dissociation is possible for dislocations of [1¯11] and [1¯10] directions and the dislocations of these directions become sessile. The [1¯11] and [1¯10] directions are in agreement with experimentally observed shape of dislocations in the (110) glide plane.     

Polar- and azimuth-angle-dependent rotational and vibrational excitation of desorbing product CO2 in CO oxidation on palladium surfaces

Clear polar and azimuth angle dependencies were found in rotational and vibrational energies of product CO2 in CO oxidation on Pd surfaces. On Pd(110)-(1x1), with increases in polar angle, both energies decreased in the [001] direction but remained constant in [110]. On the Pd(110) with missing rows, both energies increased in [001] but decreased in [110], indicating that the transition state changes with the geometry of the substrate. On Pd(111), the rotational energy greatly increased, but the vibrational energy decreased. Such angular dependence of internal energy provides new dimensions in surface reaction dynamics.     

Stark effect of adsorbate vibrations

The theory of the vibrational Stark effect of adsorbates is discussed. In particular, the reported linear variation of CO vibrational frequency with potential difference across the Pt-electrolyte interface is shown to be consistent with a recent measurement of the Stark tuning rate of CO on Ni [110] in UHV and the differential capacitance of the Pt-electrolyte interface. We also demonstrate that the Stark tuning rate of CO on Ni [110] is itself in agreement with theoretical prediction. Our vibrational Hamiltonian applies to atomic adsorbates and molecules. It relates frequency shifts of the adsorbate to the local electric field at an adsorbate free surface.     

Angular distribution patterns of photoelectrons from orbitals of CO adsorbed on Ni(100), Pt(111) and Pt(110)

The synchrotron radiation from BESSY has been used to measure the photoemission from CO orbitals adsorbed as ordered overlayers on Ni(100) c(2 × 2), Pt(111) c(4 × 2) and Pt(110) (2 × 1)p2mg. Angular distribution patterns of photoelectrons from CO orbitals were recorded with a display-type analyzer. The data were compared with differential photoionization cross sections calculated for free and oriented molecules. The results demonstrate the upright orientation of CO on Ni(100) and Pt(111), while CO on Pt(110) shows a marked difference which can be explained by assuming that the CO molecules are tilted in the [001] directions of Pt(110), yielding a (2 × 1)p2mg superstructure observed in LEED. The tilt angle is estimated to about 20°. The structure model is supported by the shape resonances of the 4σ (5σ) orbitals of CO/Pt(110) as compared to CO/Pt(111).     

Adlayer geometry and structural effects in the CO/Ni(110) system

The formation of ordered adlayers of CO on Ni(110) and the correlation between structure and adsorption energy, sticking coefficient and adsorbate induced change of work function was investigated. LEED, TDS and work function measurements served to monitor adsorption and desorption. Models are presented for the structures formed at intermediate coverages (0.5 < θ < 0.85) - identified as a c(8×2) and a c(4×2) structure - and the (2×1) formed close to saturation: The CO molecules are adsorbed on the Ni rows in the [110] direction, their separation is dominated by short range COCO repulsions rather than by the NiCO interaction. The repulsions in the [001] direction lead only to the formation of structures with staggered configurations. In the first two structures formed only below room temperature the CO stands upright and the repulsion is weak, leading to considerable disorder (antiphase domains) and a streaky LEED pattern. In the (2×1) structure which does not thermally disorder in the experimental temperature range, the high density of the adlayer results in a lateral tilt of the CO, and subsequently also to good correlation in the [001] direction. The repulsions become evident in TDS as a low temperature shoulder at the main peak (c(8×2) and c(4×2) structure) or as a distinct extra peak at 330 K ((2×1) structure). The adsorption kinetics can be modelled by a first order precursor model (K = 0.95). The work function almost linearly increases with coverage to 1500 mV at saturation. Both quantities are not noticeably affected by the degree of order in the adlayer.     

The steady state of heterogeneous catalysis, studied by first-principles statistical mechanics

The turnover frequency of the catalytic oxidation of CO at RuO2(110) was calculated as a function of temperature and partial pressures using ab initio statistical mechanics. The underlying energetics of the gas-phase molecules, dissociation, adsorption, surface diffusion, surface chemical reactions, and desorption were obtained by all-electron density-functional theory. The resulting CO2 formation rate [in the full (T,p(CO),p(O2)) space], the movies displaying the atomic motion and reactions over times scales from picoseconds to seconds, and the statistical analyses provide insight into the concerted actions ruling heterogeneous catalysis and open thermodynamic systems in general.     

Chemisorption and dissociation of carbon monoxide on rhodium surfaces

The adsorption, desorption and decomposition of CO on Rh surfaces have been investigated using field emission microscopy and thermal desorption spectroscopy. Thermal dissociation of CO cannot be detected on clean Rh surfaces at pressures up to 10^?1 Torr and temperatures below 1000 K. This holds also for atomically rough surfaces like (210). CO dissociation can be promoted under the influence of an electron beam directed to the surface, a high electric field in the presence of CO in the gas phase and by means of discharge techniques. The growth of crystallites formed by CO dissociation and the diffusion of carbon into the bulk has been followed as a function of temperature and surface structure. The tip regions around (110) are very active in these processes. Carbon crystallites on these surfaces disappear around 1000 K by diffusion into the lattice whereas crystallites present around (311) surfaces persist up to 1150 K. The results are discussed in relation to the activity of Rh in CO/H₂ reactions.     

STM investigations of CO coadsorption with O and with S on Ni(110)

The interaction between CO coadsorbed with oxygen and sulfur on Ni (110) has been studied with room temperature STM and LEED. In the case of CO/O/Ni(110)−(θ_o0.3 ML), it is found that due to a large local repulsion between the differing species, the coadsorbed species phase separate into large domains of O−(3 × 1) and CO−p2mg(2 × 1) structure. Similarly in the case of CO/S/Ni(110)−(θ_s0.4 ML), at low local coverages of coadsorbed CO, island segregation of CO and S-c(2 × 2) is observed. At locally saturated CO coverage, the S-c(2 × 2) structure transforms into long -S-S- chains running predominantly along the [ ] direction and separated by a local p2mg(2 × 1)-CO structure; this transformation is attributed to the large CO-CO repulsion in the condensed overlayer structure.     

Pulsed laser atom-probe study of the dissociation of CO on molybdenum

The thermal decomposition of molecular CO on molybdenum surfaces has been studied using the pulsed laser atom-probe (PLAP). Field emitter tips, cleaned by vacuum field evaporation, were dosed with CO at low temperatures (<60 K) and heated to 180–350 K under fieldfree conditions. The fraction of undissociated CO⁺ ions compared to all carbon- and oxygen-containing species appearing in subsequently recorded PLAP mass spectra was taken as a measure of the extent of surface CO dissociation. Molecular (virgin) CO was found to convert to dissociated (β) CO at temperatures between 200—300 K. The analyzed surface area included both flat single crystal planes and stepped regions. Assuming first-order kinetics, the dissociation occurred with an activation energy of 0.7 ± 0.1 eV. The dissociation on the flat (110) plane, measured using the same technique but with the rest of the surface masked, occurred with a 0.12 eV higher activation energy.     

Structure of adsorbed organometallic rhodium: model single atom catalysts

We have determined the structure of a complex rhodium carbonyl chloride [Rh(CO)2Cl] molecule adsorbed on the TiO2(110) surface by the normal incidence x-ray standing wave technique. The data show that the technique is applicable to reducible oxide systems and that the dominant adsorbed species is undissociated with Rh binding atop bridging oxygen and to the Cl found close to the fivefold coordinated Ti ions in the surface. A minority geminal dicarbonyl species, where Rh-Cl bond scission has occurred, is found bridging the bridging oxygen ions forming a high-symmetry site.     

Anisotropic transport properties of charge-ordered La_(5/8-y)Pr_yCa_(3/8)MnO_3(y= 0.43) film

The anisotropic resistances along [001] and [1-10] axes are investigated for an La5/8-yPryCa3/8MnO3(y = 0.43)(LPCMO) film grown on(110)-oriented La AlO3 substrate. It is found that the charge order(CO) transition is much stronger and the resistance is larger along the [001] direction than that along the [1-10] direction. Special attention has been paid to the different effects of a magnetic field on the resistances of the two axes. The resistance is more susceptible to the magnetic field along the [001] direction compared with that along the [1-10] direction. Our results demonstrate that the anisotropic transport properties can be ascribed to the intrinsic anisotropic strain field in the film, which changes the shape of metallic domains for the phase separation manganite film. We also provide a feasible method to rule out the Joule heat effect from the electric current effect. This could be useful for future construction and application of materials and devices.     

Chemisorption,surface structural chemistry,and electron impact properties of carbon monoxide on rhodium (110)

The adsorption, desorption, surface structural chemistry, and electron impact properties of CO on Rh(110) have been studied by LEED, Auger spectroscopy, thermal desorption, and surface potential measurements. At 300 K, CO adsorbs into a single chemisorbed state whose desorption energy (E_d) is ~130kJmol^-1. The initial sticking probability is unity, and at saturation coverage a (2 × 1)plgl ordered phase reaches its maximum degree of perfection, thus demonstrating that this CO structure is common to the (110) faces of all the cubic platinum group metals. The saturated adlayer corresponds to θ = 1 and shows a surface potential of Δ? = +0.97 V. Under electron impact, desorption and dissociation of CO occur with about equal probability, the relevant cross sections being ~10^-22 m² in each case. Slow thermal dissociation of CO occurs at high temperature and pressure, leaving a deposit of C and O atoms on the surface. The thermal, electron impact, and Δ? properties of Rh(110)CO resemble those of Ni(110)CO rather closely, and are very different from those of Pt(110)CO. Surface carbon is shown to inhibit CO chemisorption, whereas surface oxygen appears to lead to the formation of a new more tightly bound form of CO with a considerably enhanced desorption energy (E_d ~ 183 kJmol^-1). Similar oxygen-induced high temperature CO states have been reported recently on Co(0001) and Ru(101&#x0304;1).     

A REMPI study of the correlation of internal excitations and substrate-adsorbate coupling for CO molecules desorbed by electron impact

We compare vibrational, translational and rotational excitations of CO molecules desorbed by electron impact, and the yield of oxygen and carbon atoms from electron-induced fragmentation of CO molecules for: (1) CO monolayers on bare transition metals [Ru(001) and Pt(111)]; (2) CO monolayers coadsorbed with well-ordered oxygen atoms; (3) weakly bound CO monolayers on epitaxially grown silver films; and (4) CO monolayers decoupled from the metallic substrate by mono-atomic xenon spacer layers. For all but the last system, we find CO molecules which are vibrationally extremely hot. This is explained by the excitation of strongly antibonding multi-electron states which are quenched in the vicinity of the metal surface before enough translational energy is acquired by the nuclei to complete dissociation. For CO/Xe/Ag(111), vibrationally hot CO molecules are missing among the desorbing particles, whereas strong fragment signals persist. Because of the isolating Xe layer, the substrate-adsorbate coupling is too weak to terminate the dissociation reaction which is induced by the electron impact before the rupture of the molecular bond.     

Decomposition of carbon monoxide on a (110) nickel surface

New investigations of the (110) nickel/carbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1×10^?7 torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.     

AES and LEED studies of xenon adsorption on (100)NaCl

The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N₂ and O₂ in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N₂ production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.     

Magnetic anisotropy of the antiferroquadrupole phase in Ce0.50La0.50B6

Magnetic phase diagrams for antiferroquadrupole (AFQ) phase II and antiferromagnetic (AFM) phase III in Ce0.50La0.50B6 with a Gamma(8) ground state have been investigated by ultrasonic measurements. The hybrid magnet (Gama) in the National Institute for Materials Science was employed for high-field measurements up to 30 T and a 3He-4He dilution refrigerator was used for low-temperature experiments down to 20 mK. The phase boundary from paramagnetic phase I to AFQ phase II under [001] magnetic fields closes at H(I-II) approximately 29 T, while the boundary is still open under fields along the [110] and [111] directions even up to 30 T. This anisotropic character of phase II in fields is consistent with the theoretical calculation based on the O(xy)-type AFQ ordering. We also found that AFM phase III reduces considerably in fields turning from the [001] to [110] and [111] directions.     

STM study of the Ni(110)-(2 × 1)-2CO system: structure and bonding-site determination

The structure of the Ni(110)-(2 × 1)-2CO system has been studied with scanning tunneling microscopy (STM) at room temperature. Contrary to many previous studies, it is found that atomically-resolved STM imaging of CO molecules is attainable. By comparing the apparent CO adlayer registry with coadsorbed low concentrations of O and S, it is found that the CO molecules bond in short-bridge sites along the Ni [11̄0] rows and alternately tilt away from the normal direction within the (001) plane.     

Orbital selection of the double [CuO2] layer compound Ca3Cu2O4Cl2

We present a first-principles investigation on the dynamics and mechanism of the oxidation reaction between water molecules and the reduced PuO_2(110) surface using ab initio molecular dynamics(AIMD) simulations in combination with density functional theory(DFT) + U calculations. We find a dominating dissociation preference of water molecules for the vacancy defect sites on the PuO_2(110) surface, irrespective of the water or vacancy coverage. Due to hybridizations between the frontier orbitals of water molecule and the electronic states of the vacancy vicinity, partial water dissociation at the vacancy sites is exothermic and barrierless. The dissociation product, an OH group, further hydrogenates the PuO_2(110) surface by occupying the vacancy site.We also observe surface vacancy diffusion induced by the interactions between the water molecules and the surface oxygen atom in the proximity of the defect sites.     

Density functional theory study of formaldehyde adsorption and decomposition on Co-doped defective CeO_2(110) surface

Formaldehyde as an air pollutant to adverse health effects for humanity has been getting attention. The adsorption and dissociation of formaldehyde(HCHO) on the Co_xCe_(1-x_O_(2-δ)(110) surface were investigated by the density functional theory(DFT) calculations. We calculated the oxygen vacancy formation energy as the function of its site around dopant Co in detail. The results showed that Co doping was accompanied by compensating oxygen hole spontaneous formation.The adsorption configurations and bindings of HCHO at different locations on the Co_xCe_(1-x)O_2(110) were presented.Four possible pathways of oxidation of formaldehyde on the catalytic surface were explored. The results suggested that formaldehyde dissociation at different adsorption sites on the doped CeO_2(110) — first forming dioxymethylene(CH_2O_2)intermediate, and then decomposing into H_2O, H_2, CO_2, and CO molecules. It was found that the presence of cobalt and oxygen vacancy significantly prompted the surface activity of CeO_2.     

Surface and interface Bleustein-Gulyaev waves along symmetry directions of cubic crystals

We have studied the dispersion relation of surface and interface Bleustein-Gulyaev waves along symmetry directions of the (001) and (110) surfaces and interfaces of cubic crystals by using an extension of the surface Green function matching method recently developed. We have shown, for the first time to our knowledge, in a general way that along the [001] direction on (001) metallized and non-metallized surfaces the Bleustein-Gulyaev mode does not exist. On the other hand for the [110] direction on the (110) surfaces, metallized or not, the Bleustein-Gulyaev mode always exists. For metallized (110) surfaces the dispersion relation is obtained in closed form, whereas for non-metallized (110) surfaces we obtain the dispersion relation as a simple equation which must be solved numerically. We have obtained the existence condition for the Bleustein-Gulyaev modes in these cases. The velocities of the Bleustein-Gulyaev modes for ZnS, ZnSe, ZnTe and GaAs are given, when they exist. The same analysis has been applied to the study of the dispersion relation of interface Bleustein-Gulyaev mode, and a brief discussion is given.