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1.
Experimental data are presented for the angular dependence of the relative flux, the mean energy and the speed ratio of deuterium molecules desorbing from a Ni(111) crystal surface at a surface temperature of Ts = 1143 K and at sulphur coverages ranging between 30% and less than 2% of a monolayer.The angular flux distribution is sharply peaked in the forward direction (cosdθwith 3 ? d ? 5) and the mean energy 〈E〉 of the desorbate depends strongly on the desorption angle θ. For normal desorption (θ = 0°) 〈E〉2k is about 700 K higher than Ts and for glancing angles (θ = 80°) it decreases to about 400 K below Ts The results obtained on sulphur free and sulphur covered Ni(111) surfaces are compared with our former data on polycrystalline nickel. The main differences in the kinetic features can be ascribed to the surface roughness. Accordingly, the angular distributions of flux, mean energy, and speed ratio, which deviate strongly from the Knudson and Maxwellian law, do not seem to depend considerably on sulphur coverage and surface structure. A qualitative explanation for these deviations is presented using the principle of detailed balancing.  相似文献   

2.
N. Pauly  S. Tougaard 《Surface science》2009,603(13):2158-2162
The thickness, xs, of the effective surface region, defined as the region in which an electron travelling inside a material experiences surface excitations, as well as Begrenzungs effect, i.e. the variation of the bulk inelastic cross section in the surface region are theoretically determined for aluminium and silicon using the QUEELS-ε(k,ω)-REELS software. This software allows to determine the energy-differential inelastic electron scattering cross sections for reflection-electron-energy-loss spectroscopy (REELS) within the dielectric response theory. This study has been carried out for electron energies between 300 eV and 5000 eV. We find that the bulk inelastic cross section decreases exponentially with the distance to the surface, that xs is practically independent of electron incidence and exit angles and that xs=[2(vB)0.4/ωs]v0.6, where ωs is the surface plasmon frequency, v the electron velocity and vB the Bohr velocity ().  相似文献   

3.
《Current Applied Physics》2015,15(10):1200-1204
A systematic study of the conventional and inverse magnetocaloric effects, and critical behaviors in an alloy ingot of Ni43Mn46Sn8In3 has been performed. Our results reveal the sample exhibiting structural and magnetic phase transitions at temperatures TCM = 166 K (TC of the martensitic phase), TM–A = 260 K (the martensitic-to-austenitic phase transformation) and TCA = 296 K (TC of the austenitic phase). The large values of refrigerant capacity (RC) around TM–A and TCA are found to be RCM–A = 172.6 and RCA = 155.9 J kg−1, respectively, under an applied field change of 30 kOe. Our critical analyses near the TCM and TCA reveal that a coexistence of the long- and short-range ferromagnetic order in the martensitic phase, while the long-range ferromagnetic order exists in the austenitic phase. Interestingly, at around TCA, the maximum magnetic entropy change (|ΔSmax|) versus magnetic field H obeys a power law, |ΔSmax| = a·Hn, where the exponent n is found to be about 0.66.  相似文献   

4.
Interactions between oxygen under low pressure and a niobium-oxygen solid solution had been studied, in the regime where adsorption is the rate-determining step, from 1000 to 1700 K. It is shown that at saturation of solid solution, there exists a constant limiting value Θl of superficial coverage, comparable to a limiting bulk concentration cl. The ratios θ = Θ/Θl and ? = c/cl are called “relative ratio of occupation” (superficial and bulk). KSV is the equilibrium constant of segregation between adsorbed and dissolved oxygen atoms: (Odiss?v) + σ ? (Ochim?σ) + v (σ and v being respectively surface and bulk sites), KSV = [(1 ? θ)/θ] [?/(1 ? ?)]. The experimentally determined expression: KSV = 5.7 exp[?(22.1 ? 12.1 θ)/ RT] shows that lateral superficial interactions have a large influence on the enthalpy of transfer between the bulk and the surface of the sample. Adsorption is direct and non activated. At the solubility limit, only a fraction of the superficial sites is occupied. We estimate it to be one half. The sticking probability b of oxygen on a niobium oxygen solid solution is given by b = (1 ? θ/2)2, its value at zero coverage being estimated as unity.  相似文献   

5.
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 Ts and mean kinetic energy normal to the surface 〈 E 〉. The desorbing molecules exhibit a cos θe 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 Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ~0.4 forE⊥ 〉 = 0.12 eV. The cos5θe 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.  相似文献   

6.
NO adsorbs on Pt(111) with a (temperature independent) initial sticking coefficient S0=0.88. The fraction of molecules not being chemisorbed is directly inelastically scattered back due to failure of translational energy accommodation. The nonlinear variation of s with coverage can well be described by a precursor-state model, the precursor state being formed by NO molecules translationally and rotationally accommodated in a physisorbed second layer. Dissociation is essentially restricted to defect sites and is negligible on perfect (111) planes. These defect sites (present in small concentration) are first populated and are also sampled by the modulated beam technique yielding an activation energy for desorption Ed = 33.1 kcal/mole and preexponential factor vd = 1015.5s?1. Isothermal desorption measurements yielded Ed and vd as a function of coverage: Ed rapidly drops from its initial value (at defect sites) to about 27 kcal/mole — which value is considered as representing the adsorption energy on a perfect (111) plane — and then decreases continuously due to effective repulsive interactions. Simultaneously vd is decreasing to about 1012 s?1 at θ = 0.25 which marks the equilibrium coverage to be reached at 300 K. If the surface is precovered with oxygen atoms the NO sticking coefficient is reduced to 0.6, and the desorption parameters are lowered to Ed = 17.1 kcal/mole and vd= 1012.6s?1 (at zero NO coverage).  相似文献   

7.
The energy distributions of low energy (E0 = 0.4–3.2 keV) Na+ ions scattered from a clean polycrystalline Ag surface were measured. The angle between the incident beam and the surface was fixed at ψ = 45° while the scattering angle (θ) ranged from 50 to 130°. The cleanliness of the surface during the measurement was maintained by simultaneous deposition of Ag atoms from an effusion source. The obtained distributions considerably differ from the corresponding distributions of noble ions. Firstly, for all measured values of E0 and θ, an intensive hump is observed in the high energy part of the distribution. In certain cases this hump is transformed into a peak. Secondly, the low energy part of the distribution is very pronounced, especially for higher values of E0 and θ.  相似文献   

8.
An analytical dependence of the cross section for the small-angle scattering of polarized neutrons at spin waves in helimagnets formed because of Dzyaloshinskii—Moriya interaction in cubic crystals without an inversion center (the space group is P213) is obtained. It is assumed that the dispersion of spin waves in helimagnets with the wave vector k s polarized by a magnetic field is larger than the critical field HC2 of the transition to the ferromagnetic phase and has the form E q = A(q ? k s ) + gμB(H ? HС2). It is shown that the cross section for neutron scattering at the two-dimensional map of angles (θ x , θ y ) is two circles of the radii θC with the centers ±θ S , corresponding to the Bragg angle of diffraction by a helix oriented along the applied magnetic field H. The radii of these two circles θC are directly related to the stiffness of spin waves A of the magnetic system and depends on the applied magnetic field: \(\theta _C^2 = \theta _0^2 - \frac{{g{\mu _B}H}}{{{E_n}}}{\theta _0}\), where \({\theta _0} = \frac{{{h^2}}}{{2A{m_n}}}\) and E n and m n are the neutron energy and mass. It is shown that the scattering cross section depends on the neutron polarization, which is evidence of the chiral character of spin waves in the Dzyaloshinskii—Moriya helimagnets even in the completely polarized phase. The cases of neutron scattering at magnons where θ0 ≤ θ S and θ S ≥ θ0 are considered. The case of neutron scattering at spin waves in helimagnets is compared with analogous scattering at ferromagnets where θ S → 0.  相似文献   

9.
The role of adsorption of dodecylethyldimethylammonium bromide (C12(EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB) at water-air and polytetrafluoroethylene (PTFE)-water and poly(methyl methacrylate) (PMMA)-water interface, in wetting of PTFE and PMMA surface, was established from the measured values of the contact angle (θ) of aqueous C12(EDMAB) and BDDAB solutions in PTFE (PMMA)-solution drop-air system, and from the measured values of the surface tension of aqueous C12(EDMAB) and BDDAB solutions. Adsorption of C12(EDMAB) and BDDAB at water-air interface was determined earlier from the Gibbs equation. Adsorption at solid-water interface was deduced from the Lucassen-Reynders equation based on the relationship between adhesion tension (γLV cos θ) and surface tension (γLV). The slope of the γLV cos θ-γLV curve was found to be constant and equal to −1, and about −0.3 for PTFE and PMMA surface, respectively (in the case of both surfactant studied: C12(EDMAB) and BDDAB, and in the whole range of surfactants concentration in solution). It means that the amount of the surfactant adsorbed at the PTFE-water interface, ΓSL, was essentially equal to its amount adsorbed at water-air interface, ΓLV. However, ΓSL at the PMMA-water interface was about three times smaller as compared to that at water-air interface. By extrapolating the linear dependence between γLV cos θ-γLV and dependence between cos θ-γLV and cos θ = 1 we determined the value of the critical surface tension of PTFE and PMMA surface wetting, γc. The obtained values of γc for PTFE surface were equal 23.4 and 23.8 mN/m, 23.1 and 23.2 mN/m for C12(EDMAB) and BDDAB, respectively and they were higher than the surface tension of PTFE (20.24 mN/m). On the other hand, the obtained values of γc for PMMA surface were equal 31.4 and 30.9 mN/m, 31.7 and 31.3 mN/m for C12(EDMAB) and BDDAB, respectively and they were smaller than the surface tension of PMMA (39.21 mN/m). Using the values of PTFE and PMMA surface tension and the measured values of the surface tension of aqueous C12(EDMAB) and BDDAB solutions in the Young equation, the PTFE (PMMA)-solution interfacial tension, γSL, was also determined. Next, the work of adhesion (WA) was deduced, and it occurred that the dependence between the WA and the surface tension (γLV) for both studied solids was linear. However, the values of the WA for PMMA change as a function of log C (C—surfactant concentration) changed from 91.7 to 68.5 mJ/m2 and from 91.8 to 65.1 mJ/m2 for C12(EDMAB) and BDDAB, respectively. On the other hand, the work of adhesion of both studied surfactants solutions to the PTFE surface was practically constant (an average value was equal 45.8 and 45.4 mJ/m2, respectively). These values were close to the value of the work of water adhesion to PTFE surface (45.5 mJ/m2).  相似文献   

10.
Numerical calculations based on the full potential muffin-tin orbitals method (FP-LMTO) within the local density approximation (LDA) and the local spin-density approximation (LSDA) to investigate the structural, electronic and thermodynamic properties of filled skutterudite EuFe4Sb12 are presented. The electronic band structure and density of states profiles prove that this material is a conductor. The present investigation is also extended to the elastic constants, such as the bulk modulus B, anisotropy factor A, shear modulus G, young's modulus E, Poisson's ratio ν, and the B/G ratio with pressure in the range of 0–40 GPa. The sound velocities and Debye temperatures are also predicted from the above constants. The variations of the primitive cell volume, expansion coefficient α, bulk modulus B, heat capacity (Cp and Cv), Debye temperature θD, Helmholtz free energy A, Gibbs free energy G, entropy S, and internal energy U with pressure and temperature in the range 0–3000 K are calculated successfully.  相似文献   

11.
We discuss the systematics of 0+, v = 0, T = |TZ| + 1 levels in odd-odd nuclei and conclude about the ground state isospin and seniority of these nuclei. For N = Z nuclei starting from the 1f72 shell vg.s = 0 |Tg.s = |Tz + 1. Otherwise vg.s = 2, Tg.s = |Tz|.  相似文献   

12.
The co-adsorption of Cu on O2 and a W{100}surface is studied by Auger electron spectroscopy (AES), thermal desorption (TD), low energy electron diffraction (LEED) and by work function change (δø) measurements. It is shown that the presence of Cu on the surface initially decreases sO, the sticking coefficient of O2. For longer oxygen exposures and for higher adsorption temperatures, θO reaches values larger than those on the clean surface for the same O2 exposure. Except at the highest θO values and temperatures, the sticcking coefficient for copper, sCu, is unity and is independent of the oxygen coverage θO in the range studied (0 ? θO ? 2). Co-adsorption at room temperatures does not produce any long range order while co-adsorption at elevated temperature leads to the ordered structures (1 × 1), p(2 × 1), p(2 × 2) and c(2 × 2). The saturation coverage of the two dimensional co-adsorbate at 800 K is given by the relation θCu + 85 θO = 2. The work function is a complicated function of θO and θCu and is determined predominantly by the temperature at which oxygen is adsorbed. At high temperatures the sequence of adsorption has no influence, in contrast to the room temperature behavior.  相似文献   

13.
Experimental data on the optical reflectance of free-standing smectic C films were analyzed within the framework of a phenomenological Landau approach. At a certain temperature T 0N (determined from experimental data), which exceeds the known temperature T c of the volume phase transition from smectic A to smectic C state, a surface phase transition takes place whereby molecules in the surface layer become sloped relative to the normal of the smectic layers. The transition temperatures T 0N s,a for N-layer films possessing synclinic (symmetric) and anticlinic (antisymmetric) textures of the order parameter (tilt angle θ) were determined. A comparison of the theoretical and experimental data allowed all parameters of the model to be determined (including critical indices of the correlation length and the surface order parameter). Three possible models of the transition from the state with transverse polarization (perpendicular to the molecular tilt plane) to the state with longitudinal polarization (parallel to this plane) are analyzed. The transition takes place at low (°–°) values of the order parameter θ in the middle layer of the film.  相似文献   

14.
Using data collected from 1992 to 1995 with the ALEPH detector at LEP, a measurement of the colour factor ratios C A/C F and T F /C F and the strong coupling constant α s = C F α s(MZ)/(2π) has been performed by fitting theoretical predictions simultaneously to the measured differential two-jet rate and angular distributions in four-jet events. The result is found to be in excellent agreement with QCD, {fx4-1} Fixing C A/C F and T F/C F to the QCD values permits a determination of α s(MZ) and η f, the number of active flavours. With this measurement the existence of a gluino with mass below 6.3 GeV/c2 is excluded at 95% confidence level.  相似文献   

15.
The dependence on temperature of the layer magnetization of a Heisenberg ferromagnetic ultrathin film in presence of magnetocrystalline single-ion anisotropy was theoretically investigated in the framework of a Green's function approach using the random phase approximation (RPA). The effect of surface orientation and of film thickness N on the Curie temperature TC was carefully investigated in the case of face centered cubic (FCC) films: the steepest increase of TC(N) was found in the case of the FCC(1 1 1) orientation and the smoothest in the FCC(1 1 0) one. Our results for TC(N) were successfully fitted by a finite-size scaling relation [TC(∞)−TC(N)]/TC(N)=(N/N0)λ, giving a shift exponent λ≃1.5, irrespectively of the surface orientation. Finally, the temperature evolution of the magnetization profile was analyzed, as well as its limiting shape at TC.  相似文献   

16.
The 11 800-14 380 cm−1 frequency range has been scanned for rotationally resolved rovibronic transitions in the A2B2-X2A1 electronic band system of the symmetric (C2v) 16O14N16O and 18O14N18O isotopologues and in the corresponding electronic band system of the asymmetric (Cs) 18O14N16O isotopologue. The rotational analysis—reflecting minor differences in mass—in combination with symmetry induced spectral differences allows an identification of 68 16O14N16O vibronic levels, 26 18O14N18O vibronic levels and 51 18O14N16O vibronic levels. The bands are recorded using near infrared fluorescence spectroscopy and a piezo valve based pulsed molecular beam expansion of premixed 18O2 and 14N16O in Ar. The majority of the observed bands is rotationally assigned and can be identified as transitions starting from the vibrational ground state of one of the isotopologues. Numerous hot bands have also been identified. A comparison of the overall spectroscopic features of C2v vs. Cs symmetric species provides qualitative information on symmetry dependence of vibronic couplings.  相似文献   

17.
Results of step fluctuation experiments for Mo(0 1 1), using low-energy electron microscopy, are re-examined using recently developed procedures that offer accurate coefficients of surface mass diffusion. By these means, surface diffusion Ds is documented at T/Tm ∼ 0.5, while the crossover to relaxation driven by bulk vacancy diffusion is inferred for T/Tm ∼ 0.6. Here, Tm is the melting temperature Tm = 2896 K. We obtain Ds = 4 × 10−4 exp(−1.13 eV/kBT) cm2/s for the temperature interval 1080-1680 K. Possible indications of diffusion along step edges appear for T/Tm ∼ 0.4. The same measurements of step fluctuation amplitudes determine also the step stiffness, which by symmetry is anisotropic on Mo(0 1 1). It is shown that three independent procedures yield mutually consistent step stiffness anisotropies. These are (1) step fluctuation amplitudes; (2) step relaxation rate anisotropies; and (3) the observed anisotropies of islands in equilibrium on the Mo(0 1 1) surface. The magnitude of the step stiffness obtained from step edge relaxation is consistent with earlier measurements that determine diffusion from grain boundary grooving.  相似文献   

18.
By employing the perturbation formulae of the spin Hamiltonian parameters (SHPs) (g factors gxx, gyy, gzz, hyperfine structure constants Axx, Ayy, Azz and superhyperfine parameters Axx׳, Ayy׳, Azz׳) for a 3d1 ion in orthorhombically elongated octahedra and tetrahedra, the defect structures and the experimental EPR spectra are theoretically and systematically investigated for the two orthorhombic Ti3+ centers C1 and C2 in ZnWO4. Center C1 is ascribed to the impurity Ti3+ at host W6+ site associated with two nearest neighbor oxygen vacancies due to charge compensation. The resultant tetrahedral [TiO4]5– cluster is determined to undergo the local orthorhombic elongation distortion, characterized by the axial distortion angle Δθ (=θθ0≈–6.84°) of the local impurity-ligand bond angle θ related to θ0 (≈54.74°) and the perpendicular distortion angle Δε (=εε0≈2.5°) related to ε0 (≈45°) of an ideal tetrahedron because of the Jahn–Teller effect. Center C2 is attributed to Ti3+ on Zn2+ site, and this octahedral [TiO6]9– cluster may experience the local axial elongation ΔZ (≈0.001 Ǻ) and the planar bond angle variation Δφ (≈9.1°) due to the Jahn–Teller effect, resulting in a more regular oxygen octahedron. All the calculated SHPs (i.e., g factors for both centers, the hyperfine structure constants for center C2 and superhyperfine parameters of next nearest neighbor ligand W for center C1) show good agreement with the observed values. However, the theoretical results based on the previous assignment of center C1 as Ti3+ on W6+ site with only one nearest planar oxygen vacancy (i.e., five-fold coordinated octahedral [TiO5]7– cluster) show much worse agreement with the experimental data. The defect structures and the SHPs (especially the g anisotropies) are discussed for both centers. The present studies on the superhyperfine parameters of ligand W6+ for center C1 would be helpful to further investigations on the superhyperfine interactions of cation ligands which were rather scarcely treated before.  相似文献   

19.
The 2D resonant Fermi gas with p-wave pairing is considered n the BCS-BEC regime. For the 2D analog of the superfluid A1 phase, the Leggett equations [1] for superfluid gap Δ and chemical potential μ are analytically solved at T = 0 and the spectrum of the collective excitations (acoustic waves) is analyzed in the BCS regime (μ > 0), where the triplet Cooper pairs emerge; in the BEC regime (μ < 0), where the triplet local pairs (molecules) emerge; and in the transition region, where μ → 0. At low temperatures, the contribution of the superfluid Fermi quasiparticles of the resonant gas to heat capacity C v and the density of normal component ρn is also calculated. At μ = 0, the fermionic contribution to ρn and C v are represented as power functions of temperature (ρnT 3 and C v T 2). However, similar power contributions to these quantities are related to phonons (bosonic acoustic oscillations). The possibility of the experimental observation of the nontrivial topological term with the charge Q = 1 in the BCS regime of the 2D A1 phase is briefly discussed.  相似文献   

20.
Advancing contact angles, θ, for aqueous solutions of the anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) were measured on glass and poly(methyl methacrylate) (PMMA) surface. Using the obtained results we determined the properties of aqueous AOT solutions in wetting of these surfaces. It occurs that the wettability of glass and PMMA by these solutions depends on the concentration of AOT in solution. There is almost linear dependence between the contact angle (θ) and concentration of AOT (log C) in the range from 5 × 10−4 to 2.5 × 10−3 M/dm3 (value of the critical micelle concentration of AOT—CMC) both for glass and PMMA surface. For calculations of AOT adsorption at solid (glass, PMMA)-solution drop-air system interfaces the relationship between the adhesion tension (γLV cos θ) and surface tension (γLV) and the Gibbs and Young equations were taken into account. From the measurement and calculation results the slope of the γLV cos θ  γLV curve was found to be constant and equal 0.7 for glass and −0.1 for PMMA over the whole range of AOT concentration in solution. From this fact it can be concluded that if ΓSV is equal zero then ΓSL > 0 for the PMMA-solution and ΓSL < 0 for glass-solution systems. It means that surfactant concentration excess at PMMA-solution interface is considerably lower than at solution-air interface, but this excess of AOT concentration at glass-solution interface is lower than in the bulk phase. By extrapolating the linear dependence between the adhesion and surface tension the value of the critical surface tension (γc) of wetting for glass and PMMA was also determined, that equaled 25.9 and 25.6 mN/m for glass and PMMA, respectively. Using the value of the glass and PMMA surface tension as well as the measured surface tension of aqueous AOT solutions in Young equation, the solid-liquid interface tension (γSL) was found. There was a linear dependence between the γSL and γLV both for glass and PMMA, but there were different slope values of the curves for glass and PMMA, i.e. −0.7 and 0.1, respectively. The dependence between the work of adhesion (WA) and surface tension (γLV) was also linear of different slopes for glass and for PMMA surface.  相似文献   

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