STM investigations of CO coadsorption with O and with S on Ni(110)

The interaction between CO coadsorbed with oxygen and sulfur on Ni (110) has been studied with room temperature STM and LEED. In the case of CO/O/Ni(110)−(θ_o0.3 ML), it is found that due to a large local repulsion between the differing species, the coadsorbed species phase separate into large domains of O−(3 × 1) and CO−p2mg(2 × 1) structure. Similarly in the case of CO/S/Ni(110)−(θ_s0.4 ML), at low local coverages of coadsorbed CO, island segregation of CO and S-c(2 × 2) is observed. At locally saturated CO coverage, the S-c(2 × 2) structure transforms into long -S-S- chains running predominantly along the [ ] direction and separated by a local p2mg(2 × 1)-CO structure; this transformation is attributed to the large CO-CO repulsion in the condensed overlayer structure.     

Interaction of NO with CO on Pd(100): ordered coadsorption structures and explosive reaction

An ordered mixed structure of c(3 × 2) is formed for a (NO + CO) coadsorption layer. The c(3 × 2) islands are considered to consist of equimolar NO and CO. The local fractional coverage (θ_NO + θ_CO) in the domain is estimated to be 0.33. Explosive production of CO₂ takes place in the c(3 × 2) islands. The vacancy requirement model is considered to be valid for the autocatalytic reaction. Since the reaction is not accompanied with any substrate reconstruction, the autocatalytic behaviour is attributed only to the formation of mixed islands. The desorption of N₂ follows, however, the second-order kinetics on Pd(100). As a result of the competition between NO and CO for the surface electrons, the CO-metal bond is weakened by the coadsorbed NO, which influences the explosive reaction. On the other hand, strengthening of the NO-metal bond is observed. When NO is in excess of CO, a p(3 × 2) structure coexists with the c(3 × 2) structure. The local coverage in the p(3 × 2) islands is estimated to be 0.33. In this coverage region, another path for the CO₂ production is available.     

Structure of the Si(011)-(16 × 2) surface and hydrogen desorption kinetics investigated using temperature-programmed desorption

D₂ temperature-programmed desorption (TPD) was used to probe the structure of the Si(011)-(16 × 2) surface. Deuterium was adsorbed at 200°C to coverages θ_D ranging up to complete saturation (approximately 1.1 ML) and the sample heated at 5°C s⁻¹. TPD spectra exhibited three second-order desorption peaks labelled β₂, β*₁ and β₁ centered at 430, 520 and 550°C. Of the proposed models for the Si(011)-(16 × 2) reconstruction, the present TPD results as a function of θ_D provide support for the adatom/dimer model with the β₂ peak assigned to D₂ desorption from the dihydride phase, while the β*₁ and β₁ peaks arise from adatom and surface-atom monohydride phases.     

Coverage dependent promoter action: K coadsorption and reactions with CO and CO2 on Pd{1 1 0}

The adsorption of CO and CO₂ on K-predosed Pd{1 1 0} at room temperature has been examined via reflection–absorption infrared spectroscopy (RAIRS). CO₂ adsorbs on 0.37 ML K-predosed Pd{1 1 0} with high sticking probability and a reactive chemisorbed intermediate, CO₂⁻, is detected in RAIRS at room temperature. Reaction of this species ultimately yields carbonate. The same high K precoverage induces dissociation of CO at low CO exposure. Carbonate is detected at higher CO exposure and is probably produced via stepwise oxidation of molecularly adsorbed CO. In contrast at low K precoverage (0.11 ML), CO remains intact but the C–O bond is considerably weakened with respect to CO chemisorbed on clean Pd{1 1 0}. These findings illustrate a dual promoter mechanism of K in the adsorption and reaction of CO or CO₂ at high K coverage. The alkali metal induces dissociation of these molecules and directly participates in the formation of a surface compound, K₂CO₃.     

Interactions of NH3, coadsorbed with PF3, on Ru(001)

NH₃, coadsorbed with PF₃, on Ru(001) has been studied by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). HREELS shows that the P---F stretching frequency decreases by 60–90 cm⁻¹ in the presence of NH₃. This is explained by the enhancement, in the presence of NH₃, of backdonation of electrons from the metal to the antibonding orbital of the PF₃. The PF₃ bending frequency does not change in the presence of NH₃. This indicates that the σ donation of electrons from PF₃ to the metal substrate is not influenced by coadsorbed NH₃. The intensity of the PF₃ vibrational bands, particularly the PF₃ bending mode, decreases drastically in the presence of NH₃. We interpret this in terms of electric field shielding created by the coadsorbed NH₃ and/or the result of direct interactions between NH₃ and PF₃.     

Electron stimulated desorption of CO from Pd1/W(110)

The electron impact behavior of CO adsorbed on Pd₁/W(110) was investigated. The desorption products observed were neutral CO, CO⁺, and O⁺. After massive electron impact residual carbon, C/W = 0.15, but not oxygen was also found, suggesting that energetic neutral O, not detected in a mass analyzer must also have been formed. Formation of β-CO, i.e., dissociated CO with C and O on the surface was not seen. The total disappearance cross section varies only slightly with coverage, ranging from 9 × 10 ⁻¹⁸ cm² at low to 5 × 10⁻¹⁸ cm² at saturation (CO/W = 0.75). The cross section for CO⁺ formation varies from 4 × 10⁻²² cm² at satura to 2 × 10⁻²¹ cm² at low coverage. That for O⁺ formation is 1.4 × 10⁻²² cm² at saturation and 2 × 10⁻²¹ cm² Threshold energies are similar to those found previously [J.C. Lin and R. Gomer, Surf. Sci. 218 (1989) 406] for CO/W(110) and CO/Cu₁/W(110) which suggests similar mechanisms for product formation, with the exception of β-CO on clean W(110). It is argued that the absence or presence of β-CO in ESD hinges on its formation or absence in thermal desorption, since electron impact is likely to present the surface with vibrationally and rotationally activated CO in all cases; β-CO formation only occurs on surfaces which can dissociate such CO. It was also found that ESD of CO led to a work function increase of the remaining Pd₁/W(110) surface of 500 meV, which could be annealed out only at 900 K. This is attributed to surface roughness, caused by recoil momentum of energetic desorbing entities.     

A LEED, TPD and HREELS investigation of NO adsorption on Sn/Pt(111) surface alloys

The adsorption of NO on Pt(111), and the (2 × 2)Sn/Pt(111) and (√3 × √3)R30°Sn/Pt(111) surface alloys has been studied using LEED, TPD and HREELS. NO adsorption produces a (2 × 2) LEED pattern on Pt(111) and a (2√3 × 2√3)R30° LEED pattern on the (2 × 2)Sn/Pt(111) surface. The initial sticking coefficient of NO on the (2 × 2)Sn/Pt(111) surface alloy at 100 K is the same as that on Pt(111), S₀ = 0.9, while the initial sticking coefficient of NO on the (√3 × √3)R30°Sn/Pt(111) surface decreases to 0.6. The presence of Sn in the surface layer of Pt(111) strongly reduces the binding energy of NO in contrast to the minor effect it has on CO. The binding energy of β-state NO is reduced by 8–10 kcal/mol on the Sn/Pt(111) surface alloys compared to Pt(111). HREELS data for saturation NO coverage on both surface alloys show two vibrational frequencies at 285 and 478 cm⁻¹ in the low frequency range and only one N-O stretching frequency at 1698 cm⁻¹. We assign this NO species as atop, bent-bonded NO. At small NO coverage, a species with a loss at 1455 cm⁻¹ was also observed on the (2 × 2)Sn/ Pt(111) surface alloy, similar to that observed on the Pt(111) surface. However, the atop, bent-bonded NO is the only species observed on the (√3 × √3)R30°Sn/Pt(111) surface alloy at any NO coverage studied.     

The CO + NH3 coadsorption over Re(0001) studied by optical second harmonic generation

The coadsorption of CO and NH₃ on Re(0001) was investigated utilizing the optical second harmonic generation (SHG) technique. The results were compared to temperature programmed desorption (TPD) and work function change measurements (Δ). It was found that the enhancement of the SHG due to ammonia is quenched very efficiently in the presence of small CO coverages. A quenching cross section by CO of the second harmonic response of the metal substrate due to ammonia adsorbates, is defined to be 100 ± 10 Ų. This extreme sensitivity of the SHG signal to the presence of CO adsorbed with NH₃ is in sharp contrast to both Δ and TPD measurements. These results present only a minor effect of CO on the Δ and TPD of ammonia on Re(0001). It is concluded that unlike the local contribution to the SHG signal generally assumed for separate adsorbates, the quenching by CO is a nonlocal effect. Moreover, the electronic interactions which govern the SHG from the NH₃-rhenium system, are very different from the interactions which lead to the TPD and Δ data. At ammonia preadsorption on the surface, the effect of the same amount of coadsorbed CO is largely reduced. This observation is discussed in terms of possible islanding mechanisms driven by the postadsorbed CO.     

Supersymmetric left-right model and its tests in linear colliders

We investigate phenomenological implications of a supersymmetric left-right model based on SU(2)_L × SU(2)_R × U(1)_B–L gauge symmetry testable in the next generation linear colliders. We concentrate in particular on the doubly charged SU(2)_R triplet higgsino , which we find very suitable for experimental search. We estimate its production rate in e⁺e⁻, e⁻e⁻, e⁻γ and γγ collisions and consider its subsequent decays. These processes have a clear discovery signature with a very low background from other processes.     

Stimulated desorption from CO chemisorbed on Cr(110)

Electron stimulated desorption (ESD) experiments using a time-of-flight pulse counting method are reported for molecular CO chemisorbed on the Cr(110) surface at 90 K. Consistent with previous qualitative observations, negligible CO⁺ and O⁺ desorption signals were measured from the ₁CO overlayer which saturates at 1/4 monolayer. For θ_CO > 0.25, a terminally-bonded (₂CO) binding mode is populated in addition to the existing ∝₁CO binding mode and the ion yield increases sharply. For ₂CO, both O⁺ and CO⁺ ions are observed; the CO⁺ ions desorb with characteristically lower kinetic energies than O⁺ ions. Near saturation coverages of CO(ads), an observed decrease in the O⁺ yield is attributed to adsorbate-adsorbate interactions which reduce the ion desorption probability, as seen in ESD studies of terminally-bonded CO on other metals. These results are considered in the context of two possible models proposed for the ₁CO binding state and related ESD observations for CO chemisorbed on Fe(001) and potassium-promoted Ru(001).     

Z-scan determination of the third-order optical nonlinearity of a triphenylmethane dye using 633 nm He–Ne laser

The third-order nonlinear optical response of a triphenylmethane dye (Acid blue 7) was studied using the Z-scan technique with a continuous-wave He–Ne laser radiation at 633 nm. The magnitude and sign of the third-order nonlinear refractive index n₂ of aqueous solution of Acid blue 7 dye were determined; the negative sign indicates a self-defocusing optical nonlinearity in the sample studied. The negative nonlinear refractive index n₂ and nonlinear absorption coefficient β were estimated to be −1.88 × 10⁻⁷ cm²/W and −3.08 × 10⁻³ cm/W, respectively, corresponding to Re(χ⁽³⁾) = −8.35 × 10⁻⁶ esu, and Im(χ⁽³⁾) = −6.88 × 10⁻⁷ esu. The experimental results show that Acid blue 7 dye have potential applications in nonlinear optics.     

A new measurement of the weak magnetism term in the betaspectra of B and N

The betaspectra of ¹²B and ¹²N have been measured with a NaI crystal as spectrometer. Assuming a shape correction factor 1 + aW + bW² and b₋ = 1.106 × 10⁻⁴ MeV⁻², b₊ = −1.397 × 10⁻⁴ MeV⁻², the spectra yield a₋ = (+0.91 ± 0.11) × 10⁻² MeV⁻¹ and a₊ = (−0.07 ± 0.09) × 10⁻² MeV⁻. the a₋ − a₊ = (+0.98 ± 0.09) × 10⁻² MeV⁻¹ is in agreement with the weak magnetism prediction.     

Co dissociation and recombination reactions on Ni(100)

The kinetics of atomic carbon and oxygen buildup on a Ni(100) surface exposed to carbon monoxide at high temperatures have been investigated by Auger electron spectroscopy. The experimental data, taken at different sample temperatures (453 , T 573 K) and at different CO partial pressures (3 ×10⁻⁷ , P_co , 3 ×10⁻¹ mbar) allowed the identification of the CO dissociation mechanism. By fitting the experimental data with a set of rate equations describing CO dissociation, CO reduction of surface oxygen, and C and O recombination, we have been able to determine the pre-exponential factors and the activation energies of these processes.     

Domain wall pinning in polycrystalline perovskite La0.7Sr0.3MnO3

Reversible and irreversible domain wall (DW) motions have been investigated in La_0.7Sr_0.3MnO₃ ceramic samples using frequency-response complex permeability with various amplitudes of AC field. We also examine the effects of temperature in the range from 293 to 368 K and transverse DC magnetic field with a maximum of 4.40×10⁵ A/m on the real part of permeability (μ′). Two relaxations corresponding to reversible wall motions and domain rotations occur in low and high frequency regions, respectively. The irreversible DW displacements can be activated as the amplitude larger than the pinning field of 3 A/m, leading to an increase in μ′. The μ′ obeys a Rayleigh law at the temperature below 343 K or under DC field of less than 4.22×10⁴ A/m. The Rayleigh constant η increases from 5.45×10⁻² to 1.54×10⁻¹ (A/m)⁻¹ as the temperature rises from 293 to 343 K, and η decreases from 5.58×10⁻² to 3.67×10⁻² (A/m)⁻¹ with increasing DC field from 1.99×10³ to 4.22×10⁴ A/m.     

Neutrino oscillations induced by two-loop radiative mechanism

Two-loop radiative mechanism, when combined with an U(1)_L′ symmetry generated by L_e − L_μ − L_τ (=L′), is shown to provide an estimate of Δm²/Δm²_atm εm_e/m_τ, where ε measures the U(1)_L′-breaking. Since Δm²_atm 3.5×10⁻³ eV², we find that Δm² ε10⁻⁶ eV², which will fall into the allowed region of the LOW solution to the solar neutrino problem for ε 0.1.     

Limits on neutrino electromagnetic properties — an update

Limits on neutrino electromagnetic properties from laboratory experiments and astrophysical arguments are reviewed with an emphasis on the currently favored range of small neutrino masses. We derive a helioseismological limit on the charge and dipole moment for all flavors of e_ν6×10⁻¹⁴e and μ_ν4×10⁻¹⁰μ_B (Bohr magneton). The most restrictive limits remain those from the plasmon decay in globular-cluster stars of e_ν2×10⁻¹⁴e and μ_ν3×10⁻¹²μ_B.     

Effect of surface orientation and sulfur coverage on the oxidation of carbon on Ni surfaces

The reaction between adsorbed oxygen and segregated carbon on a cylindrical nickel single-crystal has been examined with Auger electron spectroscopy (AES) and temperature programmed desorption (TPD), for a range of surface orientation, oxygen exposure, and sulfur coverage. It was found that for small oxygen exposures, surface carbon and surface oxygen react during TPD to form a CO desorption peak, labeled β₁. The β₁ CO peak temperature and peak shape vary with orientation. At higher oxygen coverages, the CO desorption peak split into low-temperature and high-temperature peaks. The behavior of the β₁ CO desorption peak for large oxygen exposures is consistent with a model of the carbon-oxygen recombination reaction in which the morphologies of the initial carbon and oxygen phases change during oxygen exposure as a result of repulsive lateral interactions. High oxygen exposures result in the formation of large regions of contact between the two phases; this is believed to produce the low-temperature β₁ CO desorption peak. Small segregated-sulfur coverages, and low oxygen exposures, caused the β₁ CO peak to shift to lower temperatures for all orientations. Sulfur is believed to cause more frequent contact between carbon and oxygen for small oxygen exposures because it compresses the adsorbed oxygen and segregated carbon into the sulfur-free areas of the surface. Large coverages of segregated sulfur inhibited carbon segregation on some, and oxygen adsorption on most, orientations. The absence of reactant species explains the disappearance of the β₁ CO peak during TPD from orientations which had a high sulfur coverage.     

Fabrication and growth rate estimation of PrBa2Cu3O7−δ single crystal by the modified top seeded crystal pulling method

Recently, we succeeded in fabricating single crystals of PrBa₂Cu₃O_7−δ by a modified top seeded crystal pulling method called the SRL-CP (Solute Rich Liquid-Crystal Pulling) method. Y₂O₃ and MgO polycrystalline crucibles and a MgO single crystal crucible were used to grow the single crystals. The crystal growth temperature was set in the range of 968°C to 972°C. The grown crystals were identified as PrBa₂Cu₃O_7−δ by X-ray diffraction. In the case of using Y₂O₃ crucibles the composition of the grown crystals was Y_xPr_1−xBa₂Cu₃O_7−δ (0.48 < x < 0.57) and in the case of using MgO crucibles a relatively small amount of Mg contamination to the grown crystals occurred at a typical concentration of approximately 1 at.% of the sum of cations. According to the crystal growth model of the SRL-CP method [1–5], a maximum growth rate of 1.7 × 10⁻⁵ cm/s was calculated with the aid of the phase diagram studies we reported earlier [6]. This value is reasonably in agreement with the experimental results.     

Adsorption and dissociation of Si2H6 on Ge(001)2 × 1

Adsorption and thermally-induced dissociation of disilane (Si₂H₆) on clean Ge(001)2 × 1 surfaces have been investigated using a combination of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), reflection high-energy electron diffraction (RHEED), and scanning tunneling microscopy (STM). With initial Si₂H₆ exposure at room temperature, the Si surface coverage increased monotonically, the EELS surface dangling bond peak intensities continuously decreased, and the intensity of half-order RHEED diffraction rods decreased. The low-coverage Si₂H₆ sticking probability at 300 K on Ge(001) was found to be 0.5 while the saturation coverage was 0.5 ML. A new EELS feature, GSH, involving Si-H and Ge-H bond states was observed at Si₂H₆ exposures φ 3.4 × 10¹³ cm⁻². In contrast to Si₂H₆ -saturated Si(001), the saturated Ge(001) surface significant fraction of dimerized bonds. Adsorbed overlayers were highly disordered with the primary species on saturated surfaces being SiH₂, GeH, and undissociated SiH₃· Si₂H₆-saturated Ge(001)2 × 1 substrates were annealed for l min at temperatures T_a between 425 and 825 K. Admolecules were mobile at T_a = 545 K giving rise to significant ordering in one-dimensional chains. By T_a = 605 K, essentially all of the admolecules were captured into coarsened islands. Dangling-bond EELS peaks reappeared by 625 K and the intensities of the half-order RHEED diffraction rods increased. Ge segregation to the surface, which began at T_a 625 K, occurred rapidly at T_a 675 K. All H was desorbed by 725 K.     

Surface tension and density measurements for indium and uranium using a sessile-drop apparatus with glow discharge cleaning

The sessile-drop method is used to measure the surface tension and density of liquid indium and uranium under high vacuum. Measurements are made over the temperature range 156–500°C for In and at the melting point for U. Surface oxides are efficiently removed with a glow discharge system. Drop profiles are captured by photograph and processed using nonlinear regression to yield the surface tension and density. In this regression procedure, normal distances from calculated profiles to data points are minimized. For indium, the density and surface tension measurements yield _mp = 7.05 × 10³kg/m³, d/dT = −0.776 kg/m³·°C, and γ_mp = 0.568 N/m, dγ/dT = −9.45 × 10⁻⁵ N/m·°C. The results for uranium at the melting point are _mp = 17.47 × 10³ kg/m³ and γ_mp = 1.653 N/m.