Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

Piezospectroscopy of the triple acceptor neutral copper in germanium

Experimental and theoretical results are presented for the behaviour of the optical absorption lines of neutral copper, a triple acceptor, in germanium under applied uniaxial compression. The ground state for the three equivalent holes is found to be the four-fold degenerate Λ₈ state of $\text{T}{}_{\mathrm{d}}$. The interpretation of the results requires that, for F6〈111〉, there be no splitting of either the ground state or the excited state of the D-line. However, this interpretation produces a violation of the predicted selection rules for the G components with F6〈100〉.     

Surface chemistry of the metal-halogen interface: Bromine chemisorption and dibromide formation on palladium (111)

At 300 K and in the coverage regime ($\text{0<θ<}\text{1}\text{3}$) bromine chemisorbs rapidly on Pd(111); the sticking probability and dipole moment per adatom remain constant at 0.8 ± 0.2 and 1.2 D, respectively. This stage is marked by the appearance of a √3 structure: desorption occurs exclusively as atomic Br. At higher coverages, desorption of molecular Br₂ begins (desorption energy ～130 kJ mol^?1) as does the nucleation and growth of PdBr₂ on the surface. This latter stage is signalled by the appearance of a √2 LEED pattern and the observation of PdBr₂ as a desorption product (desorption energy ～37 kJ mol^?1). Some PdBr₂ is also lost by surface decomposition and subsequent evaporation of atomic Br. The data indicate that the transition state to Br adatom desorption is localised and that PdBr₂(a) ? Br(a) interconversion occurs; these surface species do not appear to be in thermodynamic equilibrium during the desorption process.     

A recombination model for large-PT reactions

A difficult theoretical problem in large P_T reactions is to reproduce simultaneously the observed single-particle distributions and the distribution of 〈P_out〉² versus X²_E for different values of $\text{s}$ and P^trig_T. The purpose of this letter is to report successful fits to these distribution in a “recombination” model and to present the predictions of the model at $\text{s}\text{=540}\text{GeV}$, corresponding to the CERN PP? collider.     

Single crystal magnetization studies of DySb

High resolution magnetization measurements in single crystal-spherical DySb samples have been performed versus T in fields to 23.6 kOe. Measurements with H along principal crystallographic and 〈11$\text{2}$〉 directions are interpreted in terms of the intermediate metamagnetic phase having the HoP structure. Magnetization components parallel and perpendicular to the applied field for field rotations in (110) and (111) crystallographic planes have been measured and are interpreted in terms of a simple model with a single critical field. For H ∥ 〈110〉, M and H are collinear. The (M - H - T) phase diagram has been determined for H ∥ 〈110〉. The metamagnetic transition is 1st order below a critical point P_c = (8.5 K, 14.7 kOe) becoming 2nd order above. Field induced order for T >T_N is observed in agreement with the results of Brun et al. The absence of hysteresis in M_∥(θ) for H in (110) and (111) planes is interpreted as evidence for the tunneling model in DySb.     

An interpretation of published data on high-angle neutron elastic scattering in Tb metal

Published data of Steinsvoll et al. on high-angle elastic scattering of polarized neutrons by Tb metal are interpreted. States of the 4f-5d system at any one atomic site are assumed to be expressible as sums of products of 4f states ¦6M〉 with 5d states ¦d_M〉, where ¦6M〉 (M = 6, 5 or 4) denote 4f states with components M of their angular momenta along the b axis, and ¦d_M〉 denote 5d states which satisfy 〈d₆¦d₅〉 = β, 〈d₆¦d₄〉 = β². A small angle ? is introduced which represents an amount of tipping of the 4f moment away from the b axis which would occur if β = 1. The radii of 4f wave functions are allowed to increase by a small fraction s above values given by LS-averaged Dirac-Fock calculations. The quantities ?, βε, s and the effective polarization P of the neutron beam used are treated as adjustable parameters subject to β ? 1, and a fifth partially adjustable parameter subject to a constraint due to input information is introduced. A good fit to the results for eighteen scattering wave vectors q for which $\text{(}\text{q}\text{4π}\text{)>5nm}{}^{\mathrm{?1}}$ is obtained with ε=(10⁺⁴_?2)°, βε=(9±1)° and s=(0.018 ± 0.003).     

A monte-carlo/molecular dynamics study of the diffusional recombination kinetics of C(a) + O(a) → CO(g) on Pt(111)

The diffusion constants for C and O adsorbates on Pt(111) surfaces have been calculated with Monte-Carlo/Molecular Dynamics techniques. The diffusion constants are determined to be D_C(T)=(3.4 × 10^?3e^{$\text{?13156}\text{T}$})cm²s^?1 for carbon and D_O(T) = (1.5×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1 for oxygen. Using a recently developed diffusion model for surface recombination kinetics an approximate upper bound to the recombination rate constant of C and O on Pt(111) to produce CO_(g) is found to be (9.4×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1.     

Is KNO scaling proved or postulated?

Assuming Feynman's scaling law, Koba, Nielson and Olesen had shown that as s → ∞, (i) 〈n^q〉/〈n〉^q → d_q where d_q is independent of s and $\text{(ii)}\text{σ}{}_{\mathrm{n}}\text{(s)/σ}{}^{\mathrm{(s)}}{}_{\mathrm{<mtext>incl</mtext>}}\text{→ (}\text{1}\text{〈n〉)}\text{Ψ(n/〈n〉)}$. The derivation of the latter result is, however, not rigorous and it does not follow, as a necessary consequence, from the scaling law.     

The chemisorption of sulphur on W(100)

The interaction of sulphur vapour with a W(100) surface is studied in detail with Auger Electron Spectroscopy (AES), LEED, work function difference (Δ?) measurements and thermal desorption spectroscopy (TDS). The dissociative adsorption of S occurs on the W surface without reconstruction. Several LEED structures are observed which indicate repulsive adatom interactions. TDS shows that the desorption energy of atomic S decreases from about 8 eV at θ = 0.1 ML to about 3 eV near saturation in close vicinity of 1 ML. Above $\text{θ =}\text{3}\text{4}$ ML, S₂ desorbs in addition to S in a high temperature peak which saturates at about 1 ML. Sulphur in excess of about 1 ML is desorbed in two low temperature peaks of which the lower one consists not only of S and S₂ but also of S₃ and S₄.     

Correlation between band structure and hydride formation in dilute palladium alloys

Data on the free energy change ΔG, following solution of hydrogen in dilute Pd-alloys Pd_1?xM_x have been reviewed for different concentrations of M (M = Au, Ag, Pt, Ir, Rh, V, Cu, Ni, Pb, Sn and Ti) in both the α and β phases. The dependence of ΔG values upon the nature of the substituents (transition metals) is consistently explained within the framework of a metal-hydrogen bonding mechanism in the hydrides. For the β-hydride the ΔG values can be calculated on the basis of the equation ΔG = ΔG_pd + a(T)(〈?^M_LB〉 ? 〈?^Pd_LB〉)x, where $\text{ΔG}{}_{\mathrm{<mtext>Pd</mtext>}}\text{= ? 0.0489}\text{eV}\text{H}$ atom and is the free energy change of solution of hydrogen in pure Pd, a(T) = 0.194 at T = 298 K, 〈?^m_LB〉 and 〈?^pd_LB〉 are the average energies of the lowest band of the pure constituents ($\text{〈?}{}^{\mathrm{<mtext>Pd</mtext>}}{}_{\mathrm{<mtext>LB</mtext>}}\text{〉 = ?9.15}\text{eV}\text{atom}$). The stability of the palladium-hydrogen bond in dilute Pd-alloys depends on the value of 〈?^M_LB〉; for substituents having lower 〈?^M_LB〉 values than Pd the bond will strengthen, while for those having higher 〈?^M_LB〉 values it will weaken. This behaviour agrees well with the general trend of the stability of the stoichiometric hydrides predicted by Gelatt, Ehrenreich and Weiss using band structure results.     

Electrical properties of narrow gap semiconductor PtSb2

Results of measurements of conductivity, Hall and Seebeck coefficients of tellurium doped n-type crystals of platinum antimonide are presented. The Hall coefficient and the Seebeck coefficient undergo sign inversion twice, below and above room temperature. The detailed analysis of the experimental results revealed that below 200 K PtSb₂ can be described by a simple conduction and valence band model. The energy gap E_g = (110?0.15 × T) (meV), the electron conductivity mass m_n^c/m₀ = 0.35, acoustic phonon limited electron mobility 〈μ_a〉_n = 3 × 10⁶ T^{?$\text{3}\text{2}$} ($\text{cm}{}^2\text{V · s}$) and mobility ratio 〈μ_a〉_n/〈μ_a〉_p = 0.4 are determined. However, at higher temperatures a more complicated valence band model is needed to account for the experimental results. The arguments for the existence of subsidiary valleys in the valence band are presented.     

A single crystal study of the initial stages of silver sulphidation: The chemisorption and reactivity of molecular sulphur (S2) on Ag(111)

Molecular sulphur undergoes rapid dissociative chemisorption on Ag(111) with an essentially constant sticking probability of unity up to the completion of the first layer of S atoms. At this stage a $\text{(√39 R 16.1° × √39}\text{R}\text{?}\text{16.1°)}$ structure is formed in which the S atom arrangement and spacing is similar to that in the (100) plane of γ-Ag₂S (the high temperature form of silver sulphide). Further dosing with S₂ leads to continued rapid uptake of sulphur and the appearance of a (√7 × √7) R 10.9° structure, the Auger, Δφ and thermal desorption data all indicate that fast formation of Ag₂S now occurs. Very well-ordered growth of γ-Ag₂S(111) is now observed, and low-temperature S₂ desorption spectra appear which show that the activation energy for S₂ desorption is ~175 kJ mol^?1 ; this value is in excellent agreement with that observed for the enthalpy of decomposition of bulk Ag₂S (2 Ag₂S(s) → 4 Ag(s) + S₂(g), ΔH = +179 kJmol^?1). All the properties of the Ag(111)-S system imply that the material characterised by the √39 structure (i.e. the first adsorbed layer of S) is very different from bulk Ag₂S. This is discussed and compared with the results of other studies on metal-sulphur systems.     

Surface self-diffusion on Ni(110): Temperature dependence and directional anisotropy

The surface self-diffusion coefficients, D_s, on a Ni(110) crystal are measured by a mass transfer technique in [1$\text{1}$0] and [001] directions in the temperature range 773–1573 K. The surface cleanliness was checked by Auger electron spectroscopy. LEED investigations showed that the sinusoidal surface profile consisted of (110) terraces and monatomic steps. The temperature dependence of D_s can be expressed by $\text{D}{}_{\mathrm{s}}\text{[1}\text{1}\text{0] = 0.009}\text{exp}\text{(}\text{?17.5}\text{kcal}\text{mole}\text{· RT)}$ and $\text{D}{}_{\mathrm{<mtext>s</mtext>}}\text{[001] = 470}\text{exp}\text{(}\text{?45}\text{kcal}\text{mole}\text{· RT)}$ at temperatures below 1150 K. Theoretical values for the activation energies of surface migration were calculated in the framework of the pairwise interaction model. Together with an estimate for the formation energy of adatoms of $\text{16.3}\text{kcal}\text{mole}$, one obtains for the activation energy of surface self-diffusion 17 and $\text{51 kcal}\text{mole}$ for [1$\text{1}$0] and [001] direction, respectively. At T > 1150 K the anisotropy in D_s begins to vanish. Surface diffusion in [1$\text{1}$0] direction at T < 1150 K is most likely taking place by a simple adatom hopping process. Circumstantial evidence indicates that diffusion in [001] direction does not occur by a simple hopping process but by a more complex mechanism involving higher energy surface diffusion states. This isotropic process is suggested to take place for both directions at T < 1150 K.     

A theory of the surface reconstruction in gold

A phenomenological theory of the unidirectional contraction of the (111) surface layer of gold at low temperatures and the isotropic contraction of it at high temperatures is presented. It is shown that the state contracted unidirectionally in one of the 〈1$\text{1}$0〉 directions is of lowest energy in an intermediate range of the magnitude of the misfit energy, being stabilized mainly by the elastic energy λe_xxe_yy of the two dimensional continuum model. The reconstruction of the (100) surface is also discussed.     

X-ray photoelectron spectroscopic study of the physical adsorption of xenon and the chemisorption of oxygen on tungsten (111)

X-ray photoelectron spectroscopy (ESCA) has been used to study the physical adsorption of Xe and the chemisorption of oxygen by W (111). An ultrahigh vacuum ESCA spectrometer has been modified such that thermal desorption behavior from the W (111) crystal can be directly compared with ESCA spectra of the adsorbed species. In addition, since the work function of a W (111) crystal covered with one monolayer of Xe is accurately known from previous work, the binding energy of the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ adsorbate level can be accurately compared to the gaseous $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ level.When Xe is physisorbed to 1 monolayer the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ level exhibits a binding energy (relative to the vacuum level) which is 2.1 eV below that found for Xe (g). At lower Xe coverages the shift becomes monotonically greater, approaching 2.6 eV at a Xe coverage of 0.05. This 0.5 eV shift downward is accompanied by an increase of only 0.05 eV in adsorption energy as coverage decreases, and may be partially caused by the presence of ~ 10–20 % of extraneous adsorption sites other than W (111) which adsorb Xe with higher adsorption energy. The adsorption energy of Xe may also be increased by coadsorption of oxygen and the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ binding energy exhibits a corresponding shift downward as adsorbed oxygen coverage is increased to θ_o = 0.5. Electronic relaxation processes affecting the final state are dominant factors in determining the magnitude of the chemical shift upon adsorption, in agreement with the predictions of Shirley. The magnitude of the relaxation effect seems to be very sensitive to small changes in Xe adsorption energy. Similar effects have been seen for chemisorption of CO.The adsorption of O₂ at 120 K by W (111) yields a single broad O(1s) peak whose line-width decreases with increasing coverage. The final spectra at θ_o = 1 monolayer are very similar to those obtained at temperatures of 300 K or above on polycrystalline tungsten.     

Backward polarization as a direct experimental measure of peripherality

Striking regularities are observed in πN backward elastic scattering for p_lab ? 1 GeV. The rich structure of the polarization and differential cross section vary rapidly in both angle and energy. For a fixed energy however, the observables approximately satisfy the derivative relation $\text{P(θ)}\text{d}\text{σ}\text{d}\text{θ}\text{∝}\text{?}\text{t6θ}\text{(}\text{d}\text{σ}\text{d}\text{θ}\text{)}$. This empirical relation is shown to be the consequence of the peripheral nature of the interaction at high energies and the dominance of successive partial waves with increasing energy W = √s at lower energies: l = 2,3,4 … for W = 1.68, 1.95, 2.18 … GeV, respectively. $\text{“l-}\text{dominance}\text{”}$ suggests a pattern where N,Δ and Δ,Δ states appear in nearly degenerate pairs with the same l and parity but with $\text{J = l +}\text{1}\text{2}\text{and}\text{J = l ?}\text{1}\text{2}$. The observed regularities are consistent with measurements in the inelastic channel π^?p → ΛK⁰ and could provide a useful tool for an extension of phase-shift analyses to higher energies where no prominent peaks in the total cross section constrain the solution. At the highest energy (p_lab = 6 GeV/c) a possible deviation from the derivative rule implies the onset of a non-peripheral contribution not present at lower energies.     

Analyzing powers for the (p, α) reactions on 58, 60, 62Ni at Ep = 22 MeV

Measurements of both the analyzing power and cross section were made for the (p, α) reactions on ^58,60,62Ni at an incident energy of 22 MeV. Data were taken for the strongly populated proton-hole states (0f_{$\text{7}\text{2}$}, 1s_{$\text{1}\text{2}$} and 0d_{$\text{3}\text{2}$}) in the residual cobalt isotopes and for 8 weakly populated low-lying states in ⁵⁵Co and ⁵⁹Co. Angular distributions were taken between θ_lab = 10° and 140° for the ground state and θ_lab = 10° and 80° for the excited states. Both the cross sections and analyzing powers exhibit a similar angular distribution for states having the same J^π values except in the transition to the $\text{3}\text{2}$^? state in ⁵⁹Co at 1.099 MeV. Using the observed J-dependence of the analyzing power, the unknown J^π values for the states at 2.982 MeV in ⁵⁵Co and 3.090 MeV in ⁵⁹Co are assigned to be $\text{9}\text{2}$^?. The shapes of the differential cross sections were well reproduced by the zero-range DWBA calculations using a triton-cluster form factor. However, all the measured analyzing powers could not be reproduced within the framework of such a simple DWBA calculation.     

Elastic and inelastic φ photoproduction

The differential cross section of the reaction (γp → pφ) has been measured in the t range 0 ? t ? 0.4 GeV² and for photon energies from 3.0 to 6.7 GeV. In particular for the small t region the measurement accuracy was better than 10%. We obtained for the slope parameter B in an exponential parametrization of the differential cross section dσ/dt = Ae^?Bt values of $\text{B ? 6 ± 0.5}\text{GeV}{}^{\mathrm{?2}}$ which are significantly larger than the slopes obtained by most other experiments at higher t values. This indicates a t dependence of B particularly in the small t region.An energy dependence of the optical point (dσ/dt)_t=0, observed in our measurements, has been explained as a kinematic effect due to the VDM relation. A fit of our measurements is in excellent agreement with all other published values of (dσ/dt)_t=0(γp → φp), this implies that σ_tot(φp) must be essentially energy independent in this energy range.Spin density matrix elements of the φ have been evaluated and an analysis of the helicity amplitudes has been carried out. This analysis confirmed s-channel helicity conservation. Moments of spherical harmonics of the $\text{K}\text{K}$ angular decay distribution have been computed for 10 MeV $\text{K}\text{K}$ mass-bins from threshold to 1.3 GeV. The mass dependence of the normalized moments is generally smooth. Contributing amplitudes have essentially only even moments. The moment 〈Y₂⁰〉/〈Y₀⁰〉 changes sign above the φ mass.Differential cross sections for the inelastic φ production γp → φX have been evaluated for the first time both with respect to t?t_min and M_K. The integrated inelastic cross sections are comparable in size with the elastic ones. The slopes of the differential cross sections dσ/dt appear to become flatter with increasing M_X.     

The interaction of oxygen with the Rh(111) surface a

The adsorption of oxygen on Rh(111) at 100 K has been studied by TDS, AES, and LEED. Oxygen adsorbs in a disordered state at 100 K and orders irreversibly into an apparent (2 × 2) surface structure upon heating to T? 150 K. The kinetics of this ordering process have been measured by monitoring the intensity of the oxygen $\text{(1,}\text{1}\text{2}\text{) LEED}$ beam as a function of time with a Faraday cup collector. The kinetic data fit a model in which the rate of ordering of oxygen atoms is proportional to the square of the concentration of disordered species due to the nature of adparticle interactions in building up an island structure. The activation energy for ordering is $\text{13.5 ± 0.5}\text{kcal}\text{mole}$. At higher temperatures, the oxygen undergoes a two-step irreversible disordering (T? 280 K) and dissolution (T?400K) process. Formation of the high temperature disordered state is impeded at high oxygen coverages. Analysis of the oxygen thermal desorption data, assuming second order desorption kinetics, yields values of 56 ± 2 kcal/ mole and 2.5 ± 10^?3 cm² s^?1 for the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient, respectively, in the limit of zero coverage. At non-zero coverages the desorption data are complicated by contributions from multiple states. A value for the initial sticking probability of 0.2 was determined from Auger data at 100 K applying a mobile precursor model of adsorption.