Co desorption and adsorption on Pt(111)

The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is $\text{k = 2.7 × 10}{}^{13}\text{exp(?31.1}\text{kcal}\text{mole}\text{· RT) s}{}^{\mathrm{?1}}$. We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to $\text{31.1 + 1.2}\text{kcal}\text{mole}$ over the coverage range 0 < θ ? 0.5. The average harmonic oscillator frequency of the adsorbed CO molecules is 191 ± 76 cm^?1.     

Chemisorption of indium on (111) silicon substrates: I. Adsorption and nucleation by mass-spectrometric technique

The adsorption and nucleation of indium on clean (111) silicon surfaces are studied by a UHV molecular beam mass-spectrometric technique. The thermal accommodation of the adatoms on the surface is complete. At very low surface coverages θ, an adsorption energy of $\text{57}\text{kcal}\text{mole}$ and a preexponential term τ₀ of the Frenkel relation equal to 8 × 10^?13 s are found from transient response measurements. The isosteric heat of adsorption E_a varies very slowly with θ, E_a is equal to $\text{59}\text{kcal}\text{mole}$ for θ ～ 10^?3 and $\text{57}\text{kcal}\text{mole}$ for θ = 0.9. The nucleation occurs without supersaturation in an adsorbed layer near a monolayer.     

The application of XPS to the determination of the kinetics of the co oxidation reaction over the Ir(111) surface

The coverages of adsorbed oxygen and CO on an Ir(111) surface have been determined using X-ray photoelectron spectroscopy (XPS) during the steady-state catalytic production of CO₂. Correlating the coverages of the reacting adsorbates with the rate of CO₂ production allows the kinetics of the CO oxidation reaction to be determined. The reaction is found to obey a Langmuir-Hinshelwood rate expression of the form , where R_CO2 is the rate of CO₂ production, k⁰ is the pre-exponential factor of the reaction rate coefficient, [CO] and [O] are the surface coverages of CO and oxygen, respectively, and E_a is the activation energy for the oxidation reaction. The activation energy for this catalytic oxidation reaction is found to be approximately 9 $\text{kcal}\text{mole}$.     

NMR study of hydrogen diffusion in uranium hydride

The diffusion of hydrogen in uranium hydride is studied employing the NMR technique. From measurements of spin-spin relaxation time T₂, the activation energy for hydrogen diffusion in β-UH₃ is determined to be $\text{E}{}_{\mathrm{a}}\text{= (19.25 ± 0.4)}\text{kcal}\text{mole}$ and the preexponential factor to be A₀ ≈ 5 × 10¹⁴ Hz. It is shown that these results are in fair agreement with spin-lattice relaxation time T₁ data. Assuming that hydrogen diffusion proceeds via vacancies whose concentration is temperature dependent, it is concluded that E_a is the sum of the energies of vacancy formation and barrier height, and that A₀ contains an entropy change factor. Using vacancy concentration data calculated by Libowitz, we estimate the barrier height energy to be E_b ≈7 kcal/mole. Using a value for the frequency of hydrogen vibration v₀ determined from inelastic neutron scattering by Rush et al., we estimate the entropy change due to vacancy formation and the hydrogen atom jump to be about $\text{S}\text{k}{}_{\mathrm{B}}\text{≈3}$. Similar measurements on samples containing less hydrogen than is needed to compose stoichiometric UH₃, show that the rate of diffusion is enhanced by the presence of excess metal in the sample. The jump frequency at 500°K in UH₃ is found to be approximately 10⁶ Hz while for the two-phase samples of H/U = 2.8 and 2.5, it is larger by a factor of about 3 and 3.5, respectively.     

Bulk-to-surface precipitation and surface diffusion of carbon on polycrystalline nickel

The time evolution of the KLL Auger spectrum of carbon as a function of temperature is used to derive the kinetics of the surface diffusion and bulk-to-surface precipitation of carbon on polycrystalline nickel. The results show that the activation energy for the surface diffusion of carbon atoms on polycrystalline nickel is 6.9 ± 0.6 $\text{kcal}\text{mole}$, and the activation energy for bulk-to-surface precipitation is 9.4 ± 0.6 $\text{kcal}\text{mole}$. The dependence on the surface diffusion coefficient D_s (cm²s^?1), on the absolute temperature T can be represented, over the experimental temperature range, 350–425° C, by: $\text{ln}\text{D}{}_{\mathrm{s}}\text{= 10.27 ?}\text{3568}\text{T}$.     

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.     

The effect of sulfur on co adsorption/desorption on Pt(S)-[9(111) × (100)]

CO adsorption/desorption on clean and sulfur covered Pt(S)-[9(111) × (100)] surfaces was studied using AES, TPD, and modulated beam experiments. CO desorption occurred from two states on the clean surface — a low temperature state associated with the (111) terraces and a high temperature state associated with the steps/defects. Thermal desorption results indicated that above small CO coverages conversion from the low temperature state into the high temperature state was activated and that back conversion was slow. Sulfur preferentially adsorbed at step/defect sites and decreased the population of the high temperature desorption state. Modulated beam experiments were performed in order to determine CO adsorption/desorption parameters as a function of sulfur coverage on the Pt crystal. The sticking coefficient and binding energy of CO decreased as the sulfur concentration increased. Sulfur adsorption at step/defect sites decreased the CO sticking coefficient only slightly but increased the effective rate constant for CO desorption significantly. Sulfur adsorption on the terraces affected CO adosrption more than sulfur at step sites. On the clean surface the effective rate constant for CO desorption was

$\text{1 × 10}{}^{15}\text{s}{}^{\mathrm{?1}}\text{exp (}\text{?36.2 kcal/mole}\text{RT}\text{)}$

Desorption occurred from both terrace and step/defect sites, but the kinetics were characteristic of the step/defect sites. For the surface on which step/defect sites were blocked by sulfur the effective desorption rate constant was

$\text{k}{}_{\mathrm{eff}}\text{= 1 × 10}{}^{13}\text{s}{}^{\mathrm{?1}}\text{exp (?}\text{27.5 kcal/mole}\text{RT}\text{)}$

indicating an appreciable decrease in CO binding on the terraces, though sulfur-CO repulsive interactions had probably made k_eff larger than the true rate constant for desorption from clean (111) planes. The results showed clearly a compensation effect in activation energy and preexponential factor.     

Chemisorption of indium on (111) silicon substrates: II. Adsorption by flash desorption technique — Adsorption energy

The adsorption of indium on clean (111) silicon surfaces is studied by flash desorption. Two adsorption phases are found: a $\text{59.5}\text{kcal}\text{mole}$ constant-energy phase which saturates at $\text{7 × 10}{}^{13}\text{atoms}\text{cm}{}^2$ and a second phase which increases with the adatom population n. The temperature of the desorption peak of this phase decreases when n increases. The adsorption energy of In on (111) Si is calculated by the Gyftopoulos theory, and the calculated and experimental results are compared. A very small charge transfer between the adatoms and the surface, equal to or smaller than 0.02 electron per adatom, is found.     

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.     

Thermal desorption of cyanogen from Pt(100)

Thermal desorption of cyanogen adsorbed on Pt(100) was studied by flash desorption mass spectrometry. By investigating the parent ion and all possible fragmentation products in the mass spectrometer during desorption it was concluded, that desorption takes place exclusively as molecular C₂N₂. Three desorption peaks were observed at 140, 410 and 480°C denoted as α, β₁ and β₂. The respective surface coverages at saturation were determined by quantitative evaluation of the flash desorption curves to be 2.0 ± 0.2 × 10¹⁴ and 5.5 ± 1.0 × 10¹⁴$\text{molecules}\text{cm}{}^2$ for the α and the β states, respectively. First order desorption kinetics was suggested by the coverage dependencé of the desorption spectra for both α and β states with desorption energies of 12 and 38–42 $\text{kcal}\text{mole}$, respectively. A large difference in the sticking probabilities of α and β states was observed with initial values of 0.06 (α) and 0.9 (β). Adsorption experiments at elevated temperatures led to the assumption, that α and β states coexist on the surface with no or very little interactions between them. The results are discussed in terms of different models for the adsorption states.     

Interaction of inert gases with a nickel (100) surface: I. Adsorption of xenon

The adsorption of Xe on a Ni(100) surface has been studied in UHV between 30 and 100 K using LEED, thermal desorption spectroscopy (TDS), work function (Δφ) measurements, and UV photoemission (UPS). At and below 80 K, Xe adsorbs readily with high initial sticking probability and via precursor state adsorption kinetics to form a partially ordered phase. This phase has a binding energy of ~5.2 kcal/mole as determined by isosteric heat measurements. The heat of adsorption is fairly constant up to medium coverages and then drops continuously as the coverage increases, indicating repulsive mutual interactions. The thermal desorption is first order with a preexponential factor of about 10¹² s^?1, indicative of completely mobile adsorption. Adsorbed Xe lowers the work function of the Ni surface by 376 mV at monolayer coverage. (This coverage is determined from LEED to be 5.65 × 10¹⁴ Xe molecules/cm^-2.) For not too high coverages, θ, Δφ(θ) can be described by the Topping model, with the initial dipole moment μ₀ = 0.29 D and the polarizability α being 3.5 × 10^?24 cm³. In photoemission, the $\text{Xe 5p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{5p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ orbitals show up as intense peaks at 5.56 and 6.83 eV below E_f which do not shift their position as the coverage varies. Multilayer adsorption (i.e. the filling of the second and third layers) can be seen by TDS. The binding energies of these α states can be estimated to range between 4.5 and 3.5 kcal/mole. The results are compared and contrasted with previous findings of Xe adsorption on other transition metal surfaces and are discussed with respect to the nature of the inert-gas-metal adsorptive bond.     

Adsorption of carbon monoxide on polycrystalline nickel films

The adsorption states of carbon monoxide on polycrystalline nickel films have been investigated by measuring the thermal desorption, the heat of adsorption, the change in resistivity, and the change in work function in dependence on coverage and temperature. It can be shown that there are two chemisorbed (β₂, β₂) and one weakly bound (γ) species. Desorption peaks appear at 170K, 310–360 K, and 460–490 K. The differential heat of adsorption is $\text{30}\text{kcal}\text{mole}$ at low coverages and approximately $\text{25}\text{kcal}\text{mole}$ between 0.3 and 0.6 monolayers. The resistivity of the nickel film is characteristically changed with increasing coverage, and there is a maximum of resistivity at half a monolayer. At low coverages the increase in the work function is proportional to the amount adsorbed; at a monolayer the total increase is 1.26 eV at 77 K and 1.46 eV at 273 K. The two chemisorbed species differ only in the structures they form in the adsorption phase, β₂ being the species that is stable at low coverages, β₁ being the species that is stable at high coverages. These results are in good agreement with those recently found for CO adsorption on single crystal surfaces.     

Phenomenological predictions of the SO(10) supersymmetric grand unified model

The phenomenological predictions of the SO(10) supersymmetric grand unified model (SO(10) SGUM) for the mass scales M₁, M₂, weak angle ifsin²θ_w, quark-leptons mass ratios $\text{m}{}_{\mathrm{<mtext>b</mtext>}}\text{m}{}_{\mathrm{\tau }}$, $\text{m}{}_{\mathrm{t}}\text{m}{}_{\mathrm{<mtext>b</mtext>}}$, $\text{m}{}_{\mathrm{\tau }}\text{m}{}_{\mathrm{\nu }}{}_{\mathrm{\tau }}$ and proton lifetime τ_p are estimated by using renormalization group analysis at one-loop level. In contrast with SU(5) SGUM, we find that the SO(10) SGUM still has problems with τ_p but not with sin²θ_w and $\text{m}{}_{\mathrm{<mtext>b</mtext>}}\text{m}{}_{\mathrm{\tau }}$, which may suggest that supersymmetry would be bro at a mass scale $\text{?10}{}^7\text{GeV}$.     

Polarization and polarization transfer in the 2H(d,n)3He reaction at 10 MeV

Angular distributions of six polarization transfer coefficients $\text{K}{}_{\mathrm{x}}{}^{\mathrm{x\prime }}\text{(θ), k}{}_{\mathrm{x}}{}^{\mathrm{z\prime }}\text{(θ), K}{}_{\mathrm{z}}{}^{\mathrm{<mtext>x</mtext><mtext>?</mtext>}}\text{(θ), K}{}_{\mathrm{z}}{}^{\mathrm{<mtext>z</mtext><mtext>?</mtext>}}\text{(θ),}\text{and}\text{K}{}_{\mathrm{yy}}{}^{\mathrm{<mtext>y</mtext><mtext>?</mtext>}}\text{(θ)}$; of the four analyzing powers A_y(θ), A_xx(θ), A_yy(θ), and A_zz(θ); and of the polarization function P^ý(θ), have been measured atE_d = 10.00 MeV for the reaction ²H(d, n)³He. Measurements were made for neutron lab angles between 0° and 80° in 10° steps. Additionally the y-axis associated quantities were measured at θ_1ab = 99°. Most of the measured coefficients are large at some angles and all show considerable variation with angle.     

Quark masses

In quark gluon theory with very small bare masses, -$\text{ψ}$$\text{M}$ψ, spontaneous breakdown of chiral symmetry generates sizable masses M_u, M_d, M_s, … We find ($\text{M}{}_{\mathrm{<mtext>u</mtext>}}\text{+ M}{}_{\mathrm{<mtext>d</mtext>}}\text{) /2 ≈ m}\text{p}\text{/ √6 ≈ 312}\text{MeV}\text{,}\text{and}\text{M}{}_{\mathrm{<mtext>s</mtext>}}\text{≈ 432}\text{MeV}$. Scalar densities have well determined non-zero vaccum expectations $\text{〈0|u}{}^{\mathrm{a}}\text{|0〉 ≡ 〈0|ψ(x) (λ}{}^{\mathrm{a}}\text{/2)ψ(x)/0〉 ≈ ?}{}_{\mathrm{\pi }}{}^2\text{M}{}^{\mathrm{a}}\text{,}\text{i.e}\text{〈0? u}{}^{\mathrm{o}}\text{/vb0〉 ≈ 8 × 10}{}^{\mathrm{?3}}\text{(}\text{GeV}\text{)}{}^3$ at an SU(3) breaking of the vacuum c′ ≡ 〈0|u⁸|〉/〈0|u^o|0〉 ≈ ? 16%     

Inclusive pion distributions in e+e− annihilation from sum rules and duality

The threshold behaviour of pion production presented in our earlier work is successfully compared with the new SPEAR data. By using duality and sum rules we derive F_T^(π+)(x) ≈ F_L^(π+)(x) ≈ F_T^(π0)(x) ? F_L^(π0)(x) for x near 1. An accompanying results is $\text{σ}{}_{\mathrm{<mtext>\pi </mtext><mtext>A</mtext>}}{}_2\text{(s) ≈ 2σ}{}_{\mathrm{\pi \omega }}\text{(s) ≈ 4σ}{}_{\mathrm{\pi \pi }}\text{(s) ≈ 9(m}{}_{\mathrm{?}}{}^2\text{/s)}{}^3\text{σ}{}_{\mathrm{<mtext>\mu </mtext><mtext>\mu </mtext>}}$ for large s.     

Two-body hadronic decays of the 3.1 GeV resonance

Hadronic two body decays of the 3.1 GeV resonance were measured and the following branching ratios were obtained: Γ_π⁺_π^?/Γ_μμ <0.0046, Γ_K⁺_K^?/Γ_μμ <0.0084, and $\text{Γ}{}_{\mathrm{p}}\text{p}\text{/ Γ}{}_{\mathrm{\mu }}{}_{\mathrm{\mu \mu }}\text{= 0.036 ± 0.010}$ assuming the p$\text{p}$ decay distribution is ≈ + cos²θ.     

CLL = (L,L; 0, 0) measurements and a structure due to a P partial wave in the pp system

Measurements of C_LL of pp elastic scattering near θ_c.m. = 90° at thirteen energies between 300 and 800 MeV are reported. These, together with previous values of C_NN, are used to extract values of two quantities, $\text{?}{}_{\mathrm{<mtext>s</mtext>}}$ and $\text{?}{}_{\mathrm{<mtext>t</mtext>}}$, which contain only spin-singlet and only coupled spin-triplet partial waves, respectively. The $\text{?}{}_{\mathrm{<mtext>s</mtext>}}$ curve, which is not dependent on C_LL, exhibits the behavior expected for the previously conjectured ¹D₂ resonance. The $\text{?}{}_{\mathrm{<mtext>t</mtext>}}$ curve also exhibits a resonance-like behavior, which could be due either to the ³P₀ or the ³P₂ partial wave.     

Site-selective adsorption of xenon on a stepped Ru(0001) surface

The adsorption of xenon has been studied with UV photoemission (UPS), flash desorption (TDS) and work function measurements on differently conditioned Ru(0001) surfaces at 100 K and at pressures up to 3 × 10^?5 Torr. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) served to ascertain the surface perfectness. On a perfect Ru(0001) surface only one Xe adsorption state is observed, which is characterized by$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 5.40 and 6.65 eV, an adsorption energy of E_ad≈ 5 kcal/mole and dipole moment of μ'_T ≈ 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the step-height and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference in adsorption energy ofΔE_ad ≈ 3 kcal/mole and finally by their strongly deviating dipole moments of μ_S = 1.0 D and μ_T = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kink-sites are distinguishable via E_ad. The E_ad heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. —The observed effects suggest selective xenon adsorption as a tool for local surface structure determination.