Production of a spin-polarized,metastable He(23 S) beam for studies in atomic and surface physics

This beam was developed as a target for a crossed-beam electron-atom scattering experiment on the interaction of a polarized spin-1/2 electron with a polarized spin-1 atom. In the future this beam will be used in “Spin-Polarized Metastable Atom Deexcitation Spectroscopy” (SPMDS) for studying ferromagnetic surfaces without and with adsorbate layers. We use a discharge source for producing a beam of metastable helium atoms, a permanent sextupole magnet with a central stop at its exit for selecting He(2³ S) atoms in the Zeeman substatem _s =+1, a zero-field spin flipper for reversing the atomic beam polarization with respect to a magnetic guiding field, and a Stern-Gerlach magnet for analyzing the atomic polarization. At a distance of 90 cm beyond the exit of the sextupole, in the “interaction region” of an experiment, the polarized beam has a circular cross section of about 6 mm FWHM and a particle density of 1 · 10⁷ atoms/cm³. The reversible spin polarization was determined asP=0.90±0.02. A possible contamination of the beam with metastable singlet atoms is included within this value; the ground-state He atoms are not considered to be part of the polarized beam. An observed contamination with long-lived Rydberg atoms can easily be destroyed by applying a high electric field.     

Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211)

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.     

Initial oxidation of a Rh(110) surface using atomic or molecular oxygen and reduction of the surface oxide by hydrogen

The formation conditions, morphology, and reactivity of thin oxide films, grown on a Rh(110) surface in the ambient of atomic or molecular oxygen, have been studied by means of laterally resolved core level spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Exposures of Rh(110) to atomic oxygen lead to subsurface incorporation of oxygen even at room temperature and facile formation of an ordered, laterally uniform surface oxide at approximately 520 K, with a quasi-hexagonal structure and stoichiometry close to that of RhO(2). In the intermediate oxidation stages, the surface oxide coexists with areas of high coverage adsorption phases. After a long induction period, the reduction of the Rh oxide film with H(2) is very rapid and independent of the coexisting adsorption phases. The growth of the oxide film by exposure of a Rh(110) surface to molecular oxygen requires higher pressures and temperatures. The important role of the O(2) dissociation step in the oxidation process is reflected by the complex morphology of the oxide films grown in O(2) ambient, consisting of microscopic patches of different Rh and oxygen atomic density.     

Surface structure and energetics of hydrogen adsorption on the Fe(111) surface

Spin-polarized density functional theory calculations have been performed to characterize the hydrogen adsorption and diffusion on the Fe(111) surface at 2/3-, 1-, and 2-monolayer (ML) coverages. It is found that the most favored adsorption site for atomic hydrogen (H) is the top-shallow bridge site (tsb), followed by the quasi 4-fold site (qff) with the energy difference of about 0.1 eV, while the top site (t) is not competitive. Furthermore, the adsorbed atomic hydrogen (H) has a high mobility, as indicated by the small diffusion barriers. The local density of state (LDOS) analysis reveals that the Fe-H (tsb or qff) bond involves mainly the Fe 4s and 4p and H 1s orbitals with less contribution of the Fe 3d orbital, while the Fe 4s, 4p, and 3d orbitals all participate in the Fe-H (top) bond. In addition, the coverage effects on the adsorption configurations and adsorption energies are addressed.     

A study of the XeF molecule by atomic beam scattering with magnetic analysis

Absolute total cross sections for scattering of fluorine atoms by xenon have been measured in the 0.9–2.0 km s^? velocity range. Information is obtained on the long-range interaction and, using a technique for magnetic analysis of substates of F atoms, a characterization is given for the bonding in the two lowest excited states of XeF.     

Hangman effect on hydrogen peroxide dismutation by Fe(III) corroles

Hangman Fe(III) corroles catalyse H(2)O(2) disproportionation at a faster rate and display a more pronounced hangman effect than their one electron oxidized analogues owing to their ability to bypass high energy intermediates by redox-leveling derived from the use of the corrole as a non-innocent ligand.     

Mixed pseudo-potential approach to the on-top chemisorption of atomic hydrogen on the (100) silver surface

A mixed pseudo-potential approach in which the Ag atoms interacting directly with the adsorbate are treated by dealing explicitly with their 11 valence electrons whereas the remaining atoms are treated as one-electron atoms proves to be satisfactory with respect to the case in which all atoms are treated as 11-electron atoms. This approach permits, on the one hand, an easier interpretation of the results obtained and, on the other hand, the possibility of extending the model without being forced to carry out expensive calculations.The results for the Ag₅H and Ag₉H systems show that the same interactions are present in the two models, leading to nearly identical values for the adsorbate-surface distance, the vibrational frequency of the adsorbate perpendicular to the surface, and the net charge on the adatom.     

Determination of atomic hydrogen in non-thermal hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry

Atomic hydrogen plays important roles in chemical vapor deposition of functional materials, plasma etching and new approaches to chemical synthesis of hydrogen-containing compounds. The present work reports experimental determinations of atomic hydrogen near the grounded electrode in medium-pressure dielectric barrier discharge hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry (MB-TIMS). At certain discharge conditions (a.c. frequency of 24 kHz, 28 kV of peak-to-peak voltage), the measured hydrogen dissociation fraction is decreased from approximately 0.83% to approximately 0.14% as the hydrogen pressure increases from 2.0 to 14.0 Torr. A simulation method for extraction of the approximate electron beam energy distribution function in the mass spectrometer ionizer and a semi-quantitative approach to calibrate the mass discrimination effect caused by the supersonic beam formation and the mass spectrometer measurement are reported.     

Coverage-dependent adsorption of atomic sulfur on Fe(110): a DFT study

Adsorption of atomic sulfur at different coverages on the Fe(110) surface is examined using density functional theory (DFT) in order to investigate the effect that adsorbate-adsorbate interactions may have on the surface properties. S is adsorbed in the high-symmetry adsorption sites: 4-fold hollow, bridge, and atop sites in the following surface arrangements: c(2 x 2) and p(1 x 1) which correspond to coverages of 1/2 and 1 monolayer, respectively. The binding energy, work function change, adsorption geometry, charge density distribution, magnetic properties, and density of states are examined and compared to our previous study of S adsorbed at 1/4 monolayer coverage and p(2 x 2) arrangement [Spencer et al. Surf. Sci. 2003, 540, 420]. It was found that S forms polar covalent bonds to the surface. The bonding goes from being S-Fe dominated at the low coverages to being S-S dominated at the higher coverages where the S atoms are located closer together on the surface and interact with each other.     

Model study of the relaxation accompanying adsorption of atomic hydrogen on the Li(100) cluster surface

Relaxation effects in the (4, 1, 4) and (5, 4) Li₉ clusters induced by interaction with H are studied using the diatomics-in-molecules method. Total electronic energies for the clusters are determined as functions of the Li-Li bond lengths, both in the absence and in the presence of hydrogen adsorbed in a position of C_4v symmetry. Two models of cluster relaxation are considered, differing in which part of the cluster is allowed to relax. The calculations reveal that hydrogen adsorbed on the (100) cluster surface causes quite a significant contraction of the metal atoms. The effect of the cluster relaxation on the nonadiabatic coupling between the lowest two Born-Oppenheimer states of the hydrogen-cluster system is discussed.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

Role of the phase transition at GaN QDs formation on (0001)AlN surface by ammonia molecular beam epitaxy

Journal of Thermal Analysis and Calorimetry - We report an original method of GaN/AlN quantum dots (QDs) formation with low density by ammonia MBE on the (0001)AlN...     

High-pressure Raman scattering on Fe2(MoO4)3 microcrystals obtained by a hydrothermal method

This study reports on the effect of high hydrostatic pressure on the vibrational spectra of Fe₂(MoO₄)₃ microcrystals using Raman scattering in combination with a membrane diamond-anvil cell. The ferric molybdate was obtained by the conventional hydrothermal method, and the structural and morphological properties of the sample were characterized using additionally X-ray diffraction, scanning electron microscopy, as well as energy dispersive spectroscopy. The X-ray diffraction measurements have shown that the crystals have a monoclinic structure. At pressures higher than 4.8 GPa, the disappearance of external modes was observed, suggesting that pressure has induced a breakdown in the translational symmetry of the crystal system. The high-pressure amorphous phase is reversible and attributed to an incomplete crystal-amorphous transformation.     

Interaction of atomic hydrogen with the beta-SiC(100) 3 x 2 surface and subsurface

We investigate clean and atomic hydrogen exposed beta-SiC(100) 3 x 2 surfaces by synchrotron radiation-based Si 2p core-level photoemission spectroscopy. The clean 3 x 2 surface reconstruction exhibits three surface and subsurface components. Upon hydrogen exposures, those surface and subsurface components are shifted to lower binding energies by large values, indicating significant charge transfer to the surface and subsurface regions, in excellent agreement with the recently discovered H-induced beta-SiC(100) 3 x 2 surface metallization. In addition, the interaction of hydrogen results in a large reactive component at Si 2p supporting an asymmetric charge transfer in the third plane below the surface, in agreement with previous experimental investigations. However, the results are inconsistent with recent ab initio theoretical "frozen" calculations predicting H atom to be in a bridge-bond position.     

Dehydrosulfurization of Isothiocyanates by Gas-Phase Fe(I) Cations

Bare Fe(I) cations act in the gas phase as dehydrosulfurization catalysts for butyl isothiocyanate by specifically coupling the S-atom with remote H-atoms from the ω and (ω ? l) position of the alkyl chain. Other losses induced by Fe(I) are due to ‘remote functionalization’, the formation of ion/dipole intermediates, and insertion/β-hydrogen shift processes.     

Interaction of gas phase atomic hydrogen with Pt(111):Direct evidence for the formation of bulk hydrogen species

Employing hot tungsten filament to thermal dissociate molecular hydrogen,we generated gas phase atomic hydrogen under ultra-high vacuum(UHV)conditions and investigated its interaction with Pt(111) surface.Thermal desorption spectroscopy(TDS)results demonstrate that adsorption of molecular hy- drogen on Pt(111)forms surface Had species whereas adsorption of atomic hydrogen forms not only surface Had species but also bulk Had species.Bulk Had species is more thermal-unstable than surface Had species on Pt(111),suggesting that bulk Had species is more energetic.This kind of weakly- adsorbed bulk Had species might be the active hydrogen species in the Pt-catalyzed hydrogenation reactions.     

Spectroscopic and catalytic characterization of a functional Fe(III)Fe(II) biomimetic for the active site of uteroferrin and protein cleavage

A mixed-valence complex, [Fe(III)Fe(II)L1(μ-OAc)(2)]BF(4)·H(2)O, where the ligand H(2)L1 = 2-{[[3-[((bis(pyridin-2-ylmethyl)amino)methyl)-2-hydroxy-5-methylbenzyl](pyridin-2-ylmethyl)amino]methyl]phenol}, has been studied with a range of techniques, and, where possible, its properties have been compared to those of the corresponding enzyme system purple acid phosphatase. The Fe(III)Fe(II) and Fe(III)(2) oxidized species were studied spectroelectrochemically. The temperature-dependent population of the S = 3/2 spin states of the heterovalent system, observed using magnetic circular dichroism, confirmed that the dinuclear center is weakly antiferromagnetically coupled (H = -2JS(1)·S(2), where J = -5.6 cm(-1)) in a frozen solution. The ligand-to-metal charge-transfer transitions are correlated with density functional theory calculations. The Fe(III)Fe(II) complex is electron paramagnetic resonance (EPR)-silent, except at very low temperatures (<2 K), because of the broadening caused by the exchange coupling and zero-field-splitting parameters being of comparable magnitude and rapid spin-lattice relaxation. However, a phosphate-bound Fe(III)(2) complex showed an EPR spectrum due to population of the S(tot) = 3 state (J= -3.5 cm(-1)). The phosphatase activity of the Fe(III)Fe(II) complex in hydrolysis of bis(2,4-dinitrophenyl)phosphate (k(cat.) = 1.88 × 10(-3) s(-1); K(m) = 4.63 × 10(-3) mol L(-1)) is similar to that of other bimetallic heterovalent complexes with the same ligand. Analysis of the kinetic data supports a mechanism where the initiating nucleophile in the phosphatase reaction is a hydroxide, terminally bound to Fe(III). It is interesting to note that aqueous solutions of [Fe(III)Fe(II)L1(μ-OAc)(2)](+) are also capable of protein cleavage, at mild temperature and pH conditions, thus further expanding the scope of this complex's catalytic promiscuity.     

P-H and P-P bond activation by Ni(I) and Fe(I) beta-diketiminato-complexes

Reactions of ((NacNac)Ni)2(mu-eta3-eta3-C6H5Me) (NacNac = HC(CMeNC6H3(i-Pr)2)2) with PH2Ph or PHPh2 proceeds in a facile manner to afford the Ni(I)-phosphine adducts (NacNac)Ni(PH2Ph) and (NacNac)Ni(PHPh2) , respectively. The related reactions of the Fe(I) complex (NacNac)Fe(eta2-CH2CPh2) with PH2Ph resulted in the formation of [(NacNac)Fe(mu2-PPh)]2. Alternatively was also obtained from the reaction of [(NacNac)Fe(mu-Cl)]2 with PHPh2 and Na/K. The Ni(II) di-phosphosphine bridged compound [((NacNac)Ni)2(mu4-(PPh)2)] was prepared via reaction of ((NacNac)Ni)2(mu-eta3-eta3-C6H5Me) with P5Ph5. Crystallographic data are reported for compounds .     

Adsorption of atomic hydrogen on the surface of the boron-carbon nanotubes

Results of theoretic studies of geometrical, electronic, and energy composition of monolayer boron-carbon BC₃ nanotubes of the zig-zag type (n, 0) and the cylindrical symmetry are presented. The mechanism of adsorption of atomic hydrogen on the outer surface of the boron-containing BC₃-nanotubes of the (6, 0) type is studied. The calculations are carried out on the basis of models of the ion-incorporated covalent-cyclic cluster using the semiempiric MNDO scheme and density functional (DFT) methods.