Photoemission of molecular adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N₂, and CO₂ adsorbates to discuss the various aspects of photoemission of molecular adsorbates.     

Infrared absorption of H2 adsorbed on porous glass,silica gel,and MgO

Infrared absorption of the vibrational band of H₂ in the bulk is known to arise from overlap and quadrupolar interactions. For adsorbed H₂, in addition to intermolecular interactions, an interfacial mechanism of inducing absorption has been identified in porous Vycor glass. A large enhancement of the fundamental vibrational line (by a factor of 60) from the bulk H₂ (liquid) has been observed. The vibrational overtone absorption is similarly enhanced by 150. We discuss these results as well as infrared absorption measurements for H₂ on MgO and silica gels.     

Atomic scale characterization of oxygen adsorbates on Al(111) by scanning tunneling microscopy

We present atomic scale STM pictures of clean and oxygen containing Al(111) surfaces. Little influence of the surface oxygen on the topography of the surfaces is found. Three different oxygen species can be distinguished. One of them is associated with adsorbed oxygen and found to grow in small islands upon adsorption at 300 K. Characteristic hexagonal nuclei, created upon annealing of a dilute oxygen adlayer, represent the second one. By comparison with existing spectroscopic data these are assigned to nuclei of a surface oxide.     

Precursors of long-range order and local disorder in colloids

We show the existence of precursors of crystalline and amorphous phases preceding the appearance of freezing and glass transitions in colloidal systems. The formation of crystalline clusters or amorphous local arrangements can be controlled by tuning the parameters of a proposed continuous potential and by changing the packing conditions.     

Absence of long-range order with long-range potentials

A particular form of Mermin's inequality is analyzed for repulsive inverse power potentials [V(r)=e ² r ^–m/m] in ad-dimensional space. For long-range potentials (m d) the system is put into a stabilizing background. Long-range order is shown to be excluded ford (m + 2)/2 whenm d, while for short-range potentials (m > d) we recover Mermin's result (d 2). For Coulomb systems (m=d – 2) and the experimentally studied electron surface layer (d = 2,m=1), long-range order cannot be excluded by the present argument.     

Efficient use of the surface plasmon polariton resonance in light scattering from adsorbates

Efficient collection of the scattered light from adsorbates on silver in the attenuated total reflection configuration is demonstrated with a so-called Weierstrass prism. The Raman signal from carbon contamination on silver films is enhanced by two orders of magnitude with respect to the simple external backscattering configuration.     

Preparation, characterization and application of a nanostructured composite: Octakis(cyanopropyldimethylsiloxy)octasilsesquioxane

Octakis(cyanopropyldimethylsiloxy)octasilsesquioxane was prepared and characterized by ¹³C, ²⁹Si NMR (MAS), SEM, FT-IR, XRD and thermogravimetric techniques. The four groups α, β, γ, κ (to the terminal silicon atom), associated with an acrylonitrile, were clearly seen in the ¹³C NMR (α-CH₂ at 17.9; β-CH₂ at 31.3; γ-CH₂ at 50.4; κ-CN at 59.0 ppm). The ²⁹Si NMR spectrum of the final product, exhibits only Q type silicon signal, ascribed to Q⁴ (−118.0 ppm). The presence of acrylonitrile substituted for octameric cube confers a relative change phase and thermal stability to the material. With regard to the applications for this new material, it was intended, in this case, to react with Na₂[Fe(CN)₅NH₃] by chemical substitution. This composite was incorporated into a carbon paste electrode and the electrochemical studies were performed by cyclic voltammetry. The cyclic voltammogram of the modified graphite paste electrode, showed one redox couples with formal potential () = 0.24 V versus SCE.     

Lase action on resonant molecular flows through capillaries

New experimental results are reported pointing to the possibility of a laser control of the resonant gases transit through capillaries in the Knudsen and intermediate regimes.     

Ultraviolet laser ablation and etching of polymethyl methacrylate sensitized with an organic dopant

Although polymethyl methacrylate (PMMA) is essentially transparent to light of 308 or 351 nm, it can be made sensitive to photoablation and etching by excimer laser pulses (20 ns half-width) of those wavelengths by the introduction of an organic dopant. The dopant (trade name=Tinuvin^*) is actually a quencher of the first electronic excited state of PMMA and is therefore used commercially to stabilize the polymer against photodegradation. Laser etching of Tinuvin-doped PMMA can be shown to be a photochemical process in which the Tinuvin decomposes by the absorption of two or more photons and causes the ablation of the surrounding polymer.^*[2-(2-hydroxy-3,5-diisopentyl-phenyl) benzotriazole].     

Deactivation of TiO2 photocatalytic films loaded on aluminium: XPS and AFM analyses

TiO₂ films were loaded on aluminium substrates by dip-coating method. Based on cyclic photocatalytic degradation experiments using benzamide as model molecule, XPS and AFM tests, the deactivating behaviour of the samples was studied. Experiment results show that the samples with less coating times (one to four times) deactivated very quickly, while the samples coated more than five times did not lose activity. Al element was proved to segregate from substrate and diffuse into TiO₂ films during calcination and annealing treatment, existing as mixture of Al₂O₃ and Al(OH)₃ at the boundaries among TiO₂ particles. During photocatalytic reactions in aqueous phase, the transformation of Al from Al₂O₃ to Al(OH)₃ and the leaching of the latter brought out serious alternation of surface morphology to the samples coated one to three times, on whose surface Ti³⁺ and Ti²⁺ centers were also detected after six cycles of photocatalytic reactions, while fresh films and the tested films which did not deactivate possess unique +4 valence Ti. The alteration of surface morphology, together with the change of valence of surface Ti element, resulted in the deactivation encountered in this research.     

Oxygen adsorption on the surface of In2O3 and β-(Ga0.8In0.2)2O3 mixed oxides porous structure

The paper is devoted to the oxygen sensing properties of In_1.2Ga_0.8Se₃ own oxide. The results are compared with theoretical adsorption kinetics. Temperature and time dependences of adsorption mechanisms are discussed.     

Plasma-electrolytic formation, composition and catalytic activity of manganese oxide containing structures on titanium

In the present work, we report the data about formation of TiO₂-rutile or TiO₂ and Mn₂O₃, Mn₃O₄ containing oxide structures on titanium in aqueous electrolytes by means of plasma-electrolytic deposition. The layers formed are characterized by X-ray diffraction, electron probe microanalysis and scanning electron microscopy methods. The PEO coatings on titanium formed in sodium tetraborate solution contain the TiO₂ stabile rutile modification that is important when utilizing such a structure as a catalyst carrier. Manganese acetate adding into the electrolyte leads to formation of layers that contain Mn₂O₃, Mn₃O₄ and TiO₂-rutile in outer region. The manganese content in the surface layer depends on the formation conditions as well as on manganese acetate concentration in the electrolyte. Catalytic activity of the layers in CO → CO₂ reaction is studied in the static and flow conditions. The manganese-containing layers obtained possess the catalytic activity in CO → CO₂ oxidation reaction at the temperature range of 250-350 °C. The catalytic activity depends on the concentration and surface distribution of manganese as well as on the layers morphology.     

Off-diagonal long-range order in a supersymmetric integrable model of correlated electrons

A new model for correlated electrons is presented which is integrable in one-dimension. The symmetry algebra of the model is the Lie superalgebra gl(2|1) which depends on a continuous free parameter. This symmetry algebra contains the pairing algebra as a subalgebra which is used to show that the model exhibits Off-Diagonal Long-Range Order in any number of dimensions. Received: 9 December 1997 / Revised: 12 February 1998 / Accepted: 17 March 1998     

Dynamics of photoinduced reactions at oxide surfaces

This report summarizes our work on UV-laser induced desorption of small molecules and atoms from transition metal oxides. The systems presented serve as examples for a simple photochemical reaction, the fission of the molecule surface bond. State resolved detection methods were used to record the final state distributions of the desorbing neutral molecules. Detailed results on the systems NO/NiO(1 1 1) and CO/Cr₂O₃ (0 0 0 1) are presented. The experiments include investigations on stereodynamic aspects like the angular distributions of the desorbing molecules and, in the case of CO desorption, the rotational alignment with respect to the surface normal. Large desorption cross sections of (6 ± 1) · 10^–17 cm² for NO and (3.5 ± 1) · 10^–17 cm² for CO have been found for the desorption at 6.4 eV. The wavelength dependence indicates that the primary excitation step is substrate induced. The final state distributions show a high degree of translational, rotational and vibrational excitation and are clearly nonthermal of origin. The results are consistent with the formation of a negative ion intermediate state of the adsorbate. This observation is supported from a comparison to former results on NO/NiO(1 0 0) for which extensive ab initio calculations including electronically excited states exist. A spin state dependence of the vibrational excitation of NO could only be observed for NO/NiO(1 1 1) and is absent for NO/NiO(1 0 0). We attribute this observation to a spin state dependent coupling of the desorbing molecule to the surface in case the spin lattice orientation of the surface shows a preferential orientation. In the (111) plane the spin orientation is parallel within neighbour nickel ions while it is alternating in the (1 0 0) plane. For both systems studied the velocity component parallel to the surface is constant leading to a strong peaking along the surface normal for the fast molecules. The change from a preferred helicopter rotation (angular momentum vector aligned parallel to the surface normal) to a cartwheel motion (angular momentum vector aligned perpendicular to the surface normal) with increasing rotational excitation for desorption of the flat lying CO is consistent with a change of bonding geometry during the desorption process.     

High-temperature desorption of benzene from zirconium surfaces

We report on the interaction of benzene with zirconium (0001) surfaces. Following adsorption at 150 or 170 K benzene desorbs near 715 K at exposures above one Langmuir. The high desorption temperature of benzene is indicative of the complicated kinetics that zirconium surfaces exhibit. For lower exposures benzene dissociates during heating and an increase of the oxygen content at the surface is detected. We propose that hydrogen from the dissociated layer(s) attracts subsurface oxygen and that an exchange of adsorbed carbon with this oxygen takes place.     

Adsorption of cyclic hydrocarbons on Pt and the interaction of the adsorbed species with hydrogen

The adsorption of six-membered hydrocarbon cycles and cyclopentane and the interaction of hydrogen with the adsorbed layer on polycrystalline Pt-foil have been studied. The work function change (Δφ) was followed by a Kelvin probe and the C/Pt peak ratio was determined by Auger electron spectroscopy. Combining these two techniques made it possible to distinguish between chemisorption via σ-bonds and π-complex formation. Benzene and toluene adsorbed first as π-complex while cyclohexane showed initially a partial aromatization and a π-complex-like bonding to the surface. Excess hydrocarbon or addition of hydrogen transformed the π-complex into σ-bonded species. Cyclopentane adsorbed via σ-bonds and showed no significant hydrogen effect.     

Ground-state long-range order in quasi-one-dimensional Heisenberg quantum antiferromagnets: high-order coupled-cluster calculations

We investigate the ground-state magnetic long-range order of quasi-one-dimensional quantum Heisenberg antiferromagnets for spin quantum numbers s = 1/2 and s = 1. We use the coupled cluster method to calculate the sublattice magnetization and its dependence on the inter-chain coupling J_⊥. We find that for the unfrustrated spin-1/2 system, an infinitesimal inter-chain coupling is sufficient to stabilize magnetic long-range order, in agreement with results obtained by other methods. For s = 1, we find that a finite inter-chain coupling is necessary to stabilize magnetic long-range order. Furthermore, we consider a quasi one-dimensional spin-1/2 system, where a frustrating next-nearest neighbor in-chain coupling is included. We find that for stronger frustration as well, a finite inter-chain coupling is necessary to have magnetic long-range order in the ground state, and that the strength of the inter-chain coupling necessary to establish magnetic long-range order is related to the size of the spin gap of the isolated chain.     

Off-diagonal long-range order and the Meissner effect

We provide a general, model-independent, derivation of the Meissner effect, on the basis of assumptions of off-diagonal long-range order (ODLRO) and gauge covariance of the second kind. This is an exact result that is independent of the microscopic mechanism responsible for the ordering, and so is applicable both to high- and low-T _c superconductors.     

Third order nonlinear optical properties of some organic nitroxyls

The degenerate four-wave mixing experiments have been performed in the dichloromethane solutions of 2,2,6,6-tetramethyl-pipridine-4-ol-N-oxyl p-hydroxybenzaldehyde oxime benzoic ester (TPOHBOBE) and bis-(TPOHBOBE) cobalt (II), respectively. The result is in agreement with the theoretical prediction of depending on the molecular chain length l as l ⁵. A large off-resonant third order hyperpolarizability _xxxx of 1.3 × 10^–30 esu for bis-(TPOHBOBE) cobalt (II) was obtained. The corresponding value of the third order nonlinear optical susceptibility ⁽³⁾ _xxxx for its solid state is estimated to be 7.8 × 10^–9 esu.