Depth dependence of the anomalous thermal behavior of Cu(110)

We report the results of a mirror electron microscope low energy electron diffraction (MEM-LEED) study of anomalous thermal behavior caused by enhanced thermal vibrational motion on a clean Cu(110) surface. The enhanced vibrational amplitudes cause diffracted intensities to deviate from simple Debye-Waller behavior. These deviations become apparent between 550 K and 600 K; i.e. at about 0.45 of the bulk melting temperature. Because of the finite penetration of low energy electrons, LEED intensities contain information about both surface and subsurface order. Our LEED results are analyzed to extract this information using a kinematical model in which the electron attenuation and the depth and temperature dependent vibrational amplitudes are parametrized. For a large range of model parameters, we conclude that by 850 K large anharmonic vibrations are present in several layers and at least two subsurface layers may have melted.     

The Pt(110) phase transitions: A study by rutherford backscattering,nuclear microanalysis,LEED and thermal desorption spectroscopy

The adsorbate induced (1×2) (1×1) (2×1)p1g1 phase transitions on Pt(110) have been studied by Rutherford backscattering (RBS), nuclear microanalysis (NMA), LEED and thermal desorption spectroscopy. RBS data indicate that any displacement of the surface atoms from their expected bulk-like lattice sites in the (1×2) phase is ? 0.002 nm laterally and ? 0.007 nm vertically. This contraint eliminates models for the reconstruction which involve significant lateral displacements (e.g., the paired-atom or hexagonal overlayer models). The RBS data are consistent with both the rumpled model with up/down displacements not exceeding ~0.007 nm and the missing row model with an unrelaxed surface in which the out-of-plane vibrational amplitude is slightly enhanced. A c(8×4) phase, produced by CO (or NO) exposure at T?250 K, has also been characterized by RBS which demonstrated that 0.92×10¹⁵ Pt cm^?2 move on average by ~0.017 nm laterally out-of-registry with the bulk upon formation of this phase. The values of the saturation adsorbate coverages at T?200K were determined by NMA to be 0.92 ± 0.05×10¹⁵, 1.0 ± 0.06×10¹⁵ and 1.07 ± 0.10×10¹⁵ CO molecules, NO molecules and D atoms, respectively, per cm². The value of the saturation coverage by CO (θ = 1.0) supports recent models of the (2×1)p1g1 overlayer. The isosteric heat of adsorption of CO is 160 ± 15 kJ mol^?1 in the range 0.2?θ?0.5.     

Kinetics of domain wall pinning in an incommensurate Pb overlayer on a Cu(110) surface

We have used grazing incidence X-ray scattering to study the kinetics of domain wall pinning during the transition between an incommensurate phase and a domain wall glass phase. The growth of both domain wall density and domain size are described by power-laws. The results are discussed in the context of recent theoretical models of growth.     

Saturation coverage of deuterium on Ni(110)

The absolute deuterium coverages associated with the (2 × 1) and (1 × 2) phases on Ni(110) at 175 K have been measured by nuclear reaction analysis to be 1.0 and 1.5 monolayers, respectively. Our measured values are consistent with the previous models but are substantially larger than those reported recently by Stensgård and Feidenhans'l.     

Surface relaxation of clean,and hydrogen covered Pt(111)

Backscattering of MeV ⁴He⁺ ions has been used to show that the interplanar spacing between the topmost and second layers of a clean Pt(111) crystal is identical to that in the bulk (to ± 0.4%); i.e. the surface is unrelaxed. Adsorption of hydrogen at 185 K to a coverage of 1.10 × 10¹⁵ H-atoms cm^?2 (measured by the D(³He,p)⁴He nuclear reaction) produces an outward relaxation of 0.003 ± 0.001 nm and also appears to reduce the vibrational amplitudes of the platinum surface atoms.     

用蒙特卡罗方法计算光通过薄层随机分布粒子的散射

本文利用蒙特卡罗方法计算准直光束通过薄层随机分布粒子散射的透射和反射光强,并和输运理论的扩散近似结果做了比较。当散射接近各向同性时两者符合良好。当散射明显地成为各向异性时,蒙特卡罗方法的结果是合理的,而输运理论的扩散近似失效。     

Surface parameters of clean nickel (001) determined by LEED averaging

The low-energy electron diffraction from clean Ni(001) has been studied in detail over a large range of temperature and diffraction geometries with particular emphasis on the reproducibility of the data and internal consistency of the analysis. Averaging the data at constant momentum transfer extracts the kinematic intensity for both the specular and non-specular beam. The results are analyzed to obtain both structural and electron diffraction parameters. The electron total attenuation coefficient and its energy dependence are determined from both the widths of peaks in the averaged data and from the integrated elastic intensity and are consistent with one another and with previous determinations for Ni(111) and Ni(001). While both the inner potential and the relaxation of the interplanar spacing cause shifts in peak positions, their effects can be separated in the analysis because the latter also causes changes in the relative peak intensities and in the peak shapes. The resulting inner potential, as well as its energy dependence, agree with earlier determinations for Ni(111) and with theoretical calculations to within an uncertainty of about 2 V. From analysis of peak shapes in the averaged data, it is concluded that the surface interplanar spacing is within ±4% of the bulk interplanar spacing at 380 K. This result agrees with estimates obtained from dynamical theories. It is shown that the excess atomic thermal vibrations near the surface, and not experimental uncertainties, limit the accuracy to which this spacing can be determined.     

Integration of enzyme kinetic models and isotopomer distribution analysis for studies of <Emphasis Type="Italic">in situ</Emphasis>cell operation

A current trend in neuroscience research is the use of stable isotope tracers in order to address metabolic processes in vivo. The tracers produce a huge number of metabolite forms that differ according to the number and position of labeled isotopes in the carbon skeleton (isotopomers) and such a large variety makes the analysis of isotopomer data highly complex. On the other hand, this multiplicity of forms does provide sufficient information to address cell operation in vivo. By the end of last millennium, a number of tools have been developed for estimation of metabolic flux profile from any possible isotopomer distribution data. However, although well elaborated, these tools were limited to steady state analysis, and the obtained set of fluxes remained disconnected from their biochemical context. In this review we focus on a new numerical analytical approach that integrates kinetic and metabolic flux analysis. The related computational algorithm estimates the dynamic flux based on the time-dependent distribution of all possible isotopomers of metabolic pathway intermediates that are generated from a labeled substrate. The new algorithm connects specific tracer data with enzyme kinetic characteristics, thereby extending the amount of data available for analysis: it uses enzyme kinetic data to estimate the flux profile, and vice versa, for the kinetic analysis it uses in vivo tracer data to reveal the biochemical basis of the estimated metabolic fluxes.     

Growth and properties of oxide films on Zn(0001)

The initial stages of the formation of ZnO films on cleavage planes of Zn single crystals have been studied. The growth rate is found to be linear in pressure and independent of temperature over the range from 77 to 425 K. The growth law is consistent with the rate limiting step being the adsorption of oxygen rather than ion transport through the oxide layer. Both the atomic and electronic structure have been monitored during oxygen exposure. The LEED patterns and intensity-voltage data indicate that at low temperatures the oxide is either amrophous or a fine grained polycrystal while above room temperature it is single crystal ZnO epitaxially oriented on Zn(0001). The changes during oxygen exposure of the Zn M₂₃M₄₅M₄₅, M₂₃M₄₅V, M₂₃VV and corresponding M₁ Auger spectra have been studied in some detail. The details of the spectra are identified through comparison with calculated and measured band structures and with previous observations of LMM transitions. The relaxation energy associated with the two d-band holes in the final state (i.e., δE(M₄₅M₄₅)] is found to be ～9 eV in good agreement with a recent calculation. The corresponding relaxation energy for the oxide is ～13.5 eV. The development of the electron energy loss spectra with oxygen exposure has been followed for a range of primary energies. A growth model which is consistent with the LEED observations, Auger peak heights and lineshapes and energy loss spectra is that the oxide grows heterogeneously on the Zn surface with no distinguishable precursor adsorbed state. However, contrary to previous models, it is shown that the surface is completely covered by oxide at a mean thickness of 2–3 monolayers. LEED, Auger and energy loss data for the two polar cleavage faces of ZnO are presented for comparison with those from the oxide overlayer.