Quasiquadrupolar echoes in solids containing spins-12 triads undergoing hindered reorientation

The spin echo responses to resonant 90⁰-τ-β_φ⁰ rf pulse sequences in solids containing spin-$\text{1}\text{2}$ triads undergoing hindered rotations, are shown to be predicted by a model of dipolar coupled quasiparticles of spin-$\text{3}\text{2}$.     

Field desorption of common gases from iridium and tungsten

The technique of field desorption has been used to study adsorption and desorption of gases from field ion tips. The formal procedure of the experiments is quite similar to thermal flash desorption. Values for the desorption field and their change with coverage have been obtained. For H₂/W a field adsorbed state could be detected. Time-of-flight analysis was applied to determine the desorbing species. The gas supply for field ion tips could be measured; it increases exponentially with the field.     

Dielectronic recombination of positive ions—III. Rate coefficients for Mo32+

Dielectronic recombination rate coefficients for Mo³²⁺ recombining to Mo³²⁺ are given for electron temperatures of 1.4, 2.8, and 5.6 keV. Nonrelativistic results in the resonance approximation were obtained and tables of selected Auger (A_a) and radiative (A_r) rates, fluorescence yields, and dielectronic recombination rates are given. Excitation of 2p electrons to higher shells is found to be dominant over other transitions including direct capture at the temperatures considered. The effect of the cascade transitions is estimated to be of the order of 30%. Different angular momentum averaging procedures change the results very significantly.     

Reaction rate oscillations during CO oxidation over Pt/γ-Al2O3; experimental observations and mechanistic causes

In this paper we present experimental observations of reaction rate oscillations during CO oxidation over Pt/γ-Al₂O₃ at atmospheric pressures. Based on our experimental observations and prior experimental literature on this reaction we propose a mechanistic scheme, which we believe explains in qualitative terms the oscillatory behavior exhibited by this catalytic reaction system. This mechanistic scheme involves a Langmuir-Hinshelwood reaction between adsorbed CO and oxygen and a slow formation and reduction step of an inactive surface oxide species. Our experimental observations and mechanistic ideas, furthermore, support the existence of multiple time scale phenomena for this catalytic reaction system, an idea originally suggested by Chang and Aluko.     

A ΔSCF MO investigation of core-ionized states of some first-row transition metal ions

Non-empirical LCAO MO SCF computations have been carried out on a series of first-row transition metal ions in a variety of model lattice sites. The data allow a discussion of the shifts in binding energy and changes in relaxation energy as functions of the oxidation state and electronic environment. It is shown that the dominant contribution to the relaxation energy originates in the 3d valence-orbital component.     

Infrared ellipsometry of adsorbed molecules: CO on evaporated Cu films

The theoretical and experimental basis for the use of ir ellipsometry for obtaining the vibrational spectrum of molecules adsorbed onto specularly reflecting surfaces is examined, and a number of the points are illustrated using new data for CO adsorbed onto Cu films formed by evaporation under ultra-high vacuum conditions.     

Characterization of oxide impurities on Pt and their effect on the adsorption of CO and H2

The interactions between oxide support materials and Pt have been studied by incorporating silica, alumina, titania, and niobia into the surface of a clean Pt foil. Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of CO and H₂ were used for surface characterization. For all of these oxides, TPD indicated no change in the adsorption properties of CO and H₂. Peak temperatures were unaffected by the presence of oxide impurities. For silica and alumina, AES results indicated that suboxides could be formed after oxidation at 400 and 800 K respectively. Al₂O₃ and SiO₂ were formed at higher temperatures. Relatively large quantities of these oxides were required to substantially decrease the saturation coverages of CO and H₂, indicating that these oxides probably form clusters on the metal surface. For titania and niobia, AES indicated that these oxides dissolved into the Pt above 1300 K, but segregated back to the surface below 500 K. These segregated layers cover the Pt evenly and both oxides completely suppress H₂ and CO adsorption at an oxygen coverage of 1 × 10¹⁵/cm². These results are used to discuss the possible reasons for differences in the catalytic properties of Pt on these four oxide supports.     

The adsorption of CO on evaporated iridium films,studied with infrared reflection-absorption spectroscopy

The adsorption of ^12CO on Ir films evaporated under ultrahigh vacuum (UHV) conditions was studied using infrared reflection-absorption spectroscopy (IRAS). Only a single absorption band was observed at 300 K, shifting continuously from the “singleton” value ~2010 cm^?1 at very low coverages to 2093 cm^?1 at saturation coverage. This band is attributed to CO adsorbed on top of the surface atoms. Synchronously with this shift the bandwidth at half maximum intensity $\text{Δv}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ decreases from ~30 to 8 cm^?1. The integrated peak area increases linearly with coverage up to a relative coverage (θ_r) of approximately 0.4, then the increase levels off and a maximum is observed. Upon continuing adsorption the intensity decreases slightly. In addition results are presented on adsorption at 300 K of ¹²CO?¹³CO isotopic mixtures. The coverage induced frequency shift is discussed in terms of a dipole-dipole coupling mechanism and it is concluded that intermolecular coupling can explain the shift (~83 cm^?1) observed. The decrease in intensity at coverages > 0.4 is attributed to the formation of a compressed overlayer with part of the CO molecules adsorbed in a multicentre position with different spectral properties. No infrared bands of nitrogen adsorbed at 78 K could be detected at pressures up to 6.7 kPa (1 Pa = 0.0075 Torr, 1 Torr = 133.32 Pa).     

Specific adsorption of halide and pseudohalide ions at electrochemically roughened versus smooth silver-aqueous interfaces

The differential capacitance of electrochemically roughened silver surfaces in mixed perchlorate electrolytes containing chloride, bromide, iodide, thiocyanate, or azide anions has been measured as a function of electrode potential and anion concentration. These results are compared with corresponding data for electropolished silver in order to ascertain the influence of surface roughening on the double-layer structure and composition of polycrystalline silver-aqueous interfaces. The surface concentrations of specifically adsorbed anions were obtained from these capacitance-potential data using a “Hurwitz-Parsons” type of analysis. Although electrochemical roughening by means of a conventional oxidation-reduction cycle in chloride media is a prerequisite to the appearance of Surface-Enhanced Raman Scattering (SERS) for these adsorbates, it yields only moderate (ca. 1.5- to 2-fold) increases in the actual surface area and has a relatively minor effect on their average surface concentration. However, roughening does induce noticeable changes in the morphology of the capacitance-potential curves which are traced to alterations in the surface-crystallite structure. Comparisons between the potential dependence of SERS with corresponding capacitance-potential data indicate that anion coverages close to a monolayer are necessary for stable SERS. This is attributed to the stabilization of Raman-active silver clusters by surrounding adsorbed anions.     

X-Ray photoelectron diffraction study of the atomic geometry of the Ge(111)(√3 × √3)R30°-Sn surface

The azimuthal angle dependence of Sn 3d X-ray photoelectron diffraction from the Ge(111)(√3 × √3)R30°-Sn surface has been measured and analyzed kinematically. It has been found that triplets of Sn atoms, the side of the triplet being 3.1 ± 0.1 Å, are formed on the substrate surface although the bonding sites of the triplets to the substrate are yet to be determined.     

A theoretical study of the substituent effect on the infrared intensities of the CN stretching mode of benzonitrile derivatives

The substituent effect on the integrated intensities of the characteristic CN infrared band of benzonitrile derivatives was studied using the CNDO/2 method. The charge flux term plays an important role in the substituent effect on the intensities and the fixed charge term does not have a predominant effect on the intensities. The contributions of the π, , and σ electron systems to the charge flux term are discussed in detail.     

Thermal evolution of C2H2 and C4H2 on Pd(111) studied by high-resolution electron energy loss spectroscopy

The thermal evolution of acetylene and ethylene on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150K–500K. Formation of ethylidyne ( CCH₃) near room temperature is important for both molecules, whereas CH is the major surfaces hydrocarbon species formed at high temperatures.     

Mössbauer study of a ferromagnetic metallic glass: Fe74Co10B16

Amorphous Fe₇₄Co₁₀B₁₆ (METGLAS 2605CO) has been studied in the temperature range of 77 K – 700 K by Mössbauer spectroscopy. Its crystallization temperature is found to be 665 ± 5 K and Curie temperature is estimated to be 760±10 K. The observed rapid decrease in reduced hyperfine fields can be explained well by Handrich's model for amorphous ferromagnets if one assumes a temperature dependent δ, a measure of fluctuations in the exchange interactions in such solids.     

Surface characterization and catalytic CO + H2 reaction on Fe82.2B17.8 amorphous alloy

Fe_82.2B_17.8 amorphous ribbon has been used as a catalyst for the Fischer-Tropsch-type reaction of CO+H₂. Specific activity has been found to be at least an order of magnitude higher than that of either the crystallized ribbon of identical composition or the supported iron catalyst. Before and after the catalytic tests the ribbons were characterized by XRD, XPS, UPS and Mössbauer spectroscopy in transmission and in conversion electron modes. Conversion electron Mössbauer spectroscopy and UPS proved that the surface of the amorphous ribbons is being partially crystallized during 8000 min reaction time at a maximum reaction temperature of 560 K. The superior catalytic activity has been explained by stabilization of the small iron particles and Fe₂O₃ by boron atoms at the surface and by suppressed carbide formation.     

Interaction of CO,CO2 and O2 with nonpolar,stepped and polar Zn surfaces of ZnO

The nonpolar (10$\text{1}$0), stepped (40$\text{4}$1) and (50$\text{5}$1), and the polar (0001) surfaces of ZnO were prepared. Stable unreconstructed nonpolar and stepped surfaces were obtained. LEED analyses showed that the step height and the step width of the stepped surfaces were similar to the theoretical values. The polar surface showed a 1 × 1 LEED pattern of six-fold symmetry after annealing at 500°C, and evidence of a more complicated pattern at 300–400°C. Temperature programmed desorption of CO resulted in the desorption of CO from the stepped and the polar surfaces. However, desorption of CO₂ was observed from the stoichiometric nonpolar surface, and no desorption from the reduced nonpolar surface. CO₂ was also observed by interacting CO with all surfaces at elevated temperatures. A total of four temperature programmed desorption peaks of CO₂, α, β, γ, and δ were observed. The α and β peaks were observed on the nonpolar and the stepped surfaces, and the γ peak was observed on the polar surface. The α peak was assigned to adsorption on a surface ZnO pair, and the β peak was assigned to adsorption on an anion vacancy or a step. While adsorbed water enhanced the β, preadsorbed methanol reduced it. O₂ adsorption was similar on the nonpolar and the stepped surfaces, but was weak on the polar surface.     

Magnetic circular and linear dichroism of iron garnets thin films

Low temperature absorption, MCD and MLD spectra are reported for pure and substituted Y₃Fe₅O₁₂ thin films in the 13000–33000 cm^?1 spectral range. The materials were obtained by liquid phase epitaxial growth on a Gd₃Ga₅O₁₂ substrate. Above 19000 cm^?1 we demonstrate that our transmission experiments on garnet films provide the same information as do reflexion measurements on bulk crystals. Below 19000 cm^?1 however, the former are much more reliable and accurate. All our data indicate that the films are of excellent optical quality. MCD and MLD spectra show a considerably increased resolution as compared to the absorption spectrum and help in sorting out most of its numerous overlapping components. Finally we prove unambiguously that Clogston's theoretical interpretation is qualitatively correct in the 13000–23000 cm^?1 region and even probably at higher wavenumbers.     

Temperature programmed desorption study of acetylene on a clean,H-covered,and O-covered Pt(111) surface

The reactions of acetylene on a clean, a H-covered and an O-covered Pt(111) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH₄, C₂H₆, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO₂, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.     

Electron energy loss and Auger study of epitaxially grown Cu on Ni(100)

Auger and electron energy loss spectra have been measured on films of Cu epitaxially grown on Ni(100). The films were prepared under UHV conditions using a quartz crystal for monitoring the deposition rate. LEED measurements were taken to determine the orientation of the films. The presence of a monolayer of Cu on Ni(100) is enough to suppress the 3p-3d transition on the surface of the sample. The electron energy loss spectra were studied as a function of the primary electron energy (50 to 300 eV). The experimental results were qualitatively analyzed using recent theoretical calculations of Cooper and co-workers. The effect of a small Cu coverage on Ni(100) on the chemisorption of CO and O₂ was also studied. A strong suppression of CO chemisorption at room temperature was observed. In the case of O₂, large exposures are necessary in order to observe a significant amount of oxygen on the surface. The absence of any appreciable chemisorption on the surface of the metal is attributed to the lack of empty d-surface states.