Coverage dependence of the kinetics for H2 desorption from Rh(111)

Coverage-dependent sticking probabilities and second-order rate constants for recombinative desorption of hydrogen from Rh(111) have been measured using molecular beam relaxation spectroscopy (MBRS) and time-resolved specular helium scattering. The sticking probability follows second-order Langmuir coverage dependence, with s₀ equal to 0.01 ± 0.005. Under isothermal and nearly isosteric conditions over the coverage range 0.2–0.7 ML, the second-order rate constant for desorption is essentially independent of hydrogen coverage, in contrast to kinetic parameters determined from thermal desorption spectra.     

Low temperature formation of benzene from acetylene on a Pd(111) surface

Experiments with UPS, metastable noble gas deexcitation spectroscopy (MDS) and thermal desorption demonstrated that C₂H₂ adsorbed on Pd(111) at 140 K undergoes cyclotrimerisation to C₆H₆ after higher (? 100 L) exposures. If the surface is intermediately warmed up to 300 K, the low temperature state of adsorbed acetylene transforms irreversibly into another species which is unreactive. The surface species formed by reaction was identified by comparison with the electron spectroscopic data of C₆H₆ adsorbed from the gas phase as well as with those of free C₆H₆. The molecules are only weaky held on the surface and start to desorb already at about 150 K.     

Angle-resolved photoemission study of Pd(111) and Co/Pd(111)

Electronic structure of epitaxial Co films on Pd(111) has been investigated as a function of the Co overlayer thickness, by using angle-resolved photoemission spectroscopy (ARPES). It is found that experimental band structures for Pd(111) and work functions for Co/Pd(111) agree reasonably well with the results of band structure calculations. Compared to Pd(111), valence band ARPES spectra of Co/Pd(111) support the existence of the induced magnetic polarization of Pd atoms near the interface, suggesting large hybridization between Co 3d and Pd 4d states near the interface.     

Direct observation of rotational cooling in thermal desorption: No/Pd(111)

The rotational energy distribution of nitric oxide desorbed from a Pd(111) surface has been determined directly at high surface temperature by means of laser heating of the substrate. For a surface temperature of T_s = 1100 ± 100 K, the rotational energy distribution of the desorbed NO could be described as thermal, but with a significantly lower temperature of T_r = 640 ± 40 K.     

Electronic and rotational excitation of H2 by positron impact

The body frame fixed nuclei and the laboratory frame rotational close coupling approximations have been employed to calculate electronic excitation of H₂ (X¹ _g ⁺ B¹ _u ⁺ ) by positron impact. Two electronic states (X¹ _g ⁺ and B¹ _u ⁺ ) and three rotational states in both the elastic and excitation channels are retained in the calculations. Results are reported for electronically elastic, excitation and rotational cross sections up to the incident energy 75.0 eV. The elastic cross sections are found to be influenced significantly with the addition of B¹ _u ⁺ state in the expansion scheme, in both the models used. Above the incident energy of 25.0 eV, the electronic excitation cross sections using the laboratory frame differ significantly from those of the fixed nuclei model.     

The temperature dependence of rotational tunnelling

A novel approach to the temperature dependence of rotational tunnelling is presented. By means of a projection operator technique a generalized Master equation is derived, where the effects of coupling to lattice vibrations are embraced in a memory function non-local in time. After an expansion in a power series in the coupling Hamiltonian, the Master equation is used for the calculation of two-times correlation functions of scattering operators, the Fourier transforms of which give the scattering function. The theory allows for all features of the spectrum obtained by neutron scattering methods, in particular for those of the central peak and the librational excitations. Furthermore, it is not confined to the low temperature regime, but rather covers the whole range of temperatures of experimental interest.     

The temperature dependence of rotational tunneling

In the last few years tunneling transitions have been observed for the highly symmetric groups CH₄, CD₄, NH ₄ ⁺ , and CH₃ rotating in various environments. Typically the tunneling lines shift to lower energies with increasing temperatures. In this paper the shift of the tunneling energy is calculated in a microscopic approach to the problem. The coupling of the rotating groups to the lattice modes is studied in two stages. First the rotating group is coupled to a single oscillator, then to the modes of a Debye crystal. The first calculation leads to a set of discrete tunneling lines with an energy that diminishes as the oscillator is excited into higher levels. The second approach yields a single tunneling line shifted down-wards with increasing phonon population. The shift is proportional toT ⁴. The calculation explains the energy shift of the tunneling lines with reasonable values for the coupling parameters. In some cases also a broadening has been observed which does not follow from our calculations.     

Magnetic properties of (Co Pd ) superstructures on Pd(100) and Pd(111)

The magnetic properties of (Co_nPd_m)_r superstructures on Pd(100) and Pd(111) are evaluated using the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only in the case of a Pd(111) substrate such superstructures exhibit perpendicular magnetism, while on a Pd(100) substrate the magnetization is oriented in-plane. Also investigated is the effect of interdiffusion in repeated superstructures. By using the inhomogeneous coherent potential approximation (CPA) for layered systems the effect of ordering into (repeated) superstructures can be described in an ab-initio-like manner. It is found that already small amounts of interdiffusion can be decisive for the actual value of the magnetic anisotropy energy. Received 3 November 1999 and Received in final form 18 January 2000     

Effect of surface temperature on the sorption of hydrogen by Pd(111)

Temperature programmed desorption (TPD) subsequent to various hydrogen exposure conditions indicates the formation of chemisorption, solid solution, and hydride phases of hydrogen in the near surface region of Pd(111). Variation of the sample exposure temperature (T_e) between 80 and 300 K has a strong effect on the subsequent TPD spectra. At T_e = 80 K a single desorption peak, β, appears at 310 K. Coverage variation of the β peak is consistent with second-order recombinative desorption of chemisorbed hydrogen. For T_e between 90 and 140 K a slight enhancement of the β peak occurs and a new peak, α, appears initially near 170 K. It does not saturate, exhibits near-zeroth-order desorption kinetics, and is assigned to the decomposition of a near surface palladium hydride phase. Population of the α peak is thermally activated with a maximum at T_e ≈ 115 K. For T_e, greater than 140 K, α disappears while the total amount of absorbed hydrogen increases significantly. At these temperatures, the concentration of absorbed hydrogen decreases significantly if the sample is held in vacuo at T_e after completion of the hydrogen exposure. At all exposure temperatures there is also a broad desorption feature near 800 K which is enhanced by higher T_e and is associated with hydrogen in solid solution.     

Two-dimensional oxide on Pd(111)

The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.     

When Langmuir is too simple: H2 dissociation on Pd(111) at high coverage

Recent experiments of H2 adsorption on Pd(111) [Nature (London) 422, 705 (2003)]] have questioned the classical Langmuir picture of second order adsorption kinetics at high surface coverage requiring pairs of empty sites for the dissociative chemisorption. Experiments find that at least three empty sites are needed. Through density functional theory, we find that H2 dissociation is favored on ensembles of sites that involve a Pd atom with no direct interaction with adsorbed hydrogen. Such active sites are formed by aggregation of at least 3 H-free sites revealing the complex structure of the "active sites."     

The hydrogenation of CN on Pd(111) and Pd(100)

Adsorbed CN may be produced on Pd(111) and Pd(100) surfaces at RT by dissociative adsorption of cyanogen. HREELS measurements show that adsorbed CN forms adsorbed HCN or DCN on these Pd surfaces by reaction with H adsorbed from the residual gas or by dosing with H₂ or D₂. The reaction temperature is slighly lower for Pd(100) than for Pd(111), and the range of temperatures over which the reaction takes place much narrower. The reaction occurs on a time scale easily monitored with HREELS.     

Anisotropic interactions of hydrogen molecules from the pressure dependence of the rotational spectrum in the Ar(H2)(2) compound

We report the pressure evolution, up to 70 GPa, of the fine structure of the S0(0) rotational excitation in the high-pressure Ar(H(2))(2) compound (with almost 100% para-H(2)) at about 30 K. A perturbative theoretical analysis is developed to calculate intensities and frequency shifts of the active Raman rotational components, on the basis of the intermolecular anisotropic interaction. The comparison between experimental results up to 35 GPa and calculation allows a reliable determination of the anisotropic intermolecular potential in the solid, both for H(2)-H(2) and H(2)-Ar at short range. Such results are important for the interpretation of the high-pressure orientational properties of solid hydrogen.     

CO在Pd掺杂Rh(111)表面吸附的理论研究

采用密度泛函理论与周期性平板模型相结合的方法,对CO在Rh(111)表面top、fcc、hcp、bridge四个吸附位和Rh-Pd(111)表面Rh-top、Pd-top、Rh Rh-bridge、Rh Pd-bridge、Pd Pd-bridge、Rh2Pdhcp、Rh Pd2-hcp、Rh2Pd-fcc、Rh Pd2-fcc九个吸附位的13种吸附模型进行了构型优化、能量计算,得到了CO较有利的吸附位;并对最佳吸附位进行总态密度分析.结果表明:CO在Rh(111)和Rh-Pd(111)表面的最稳定吸附位分别为Rh-hcp和Rh-top位,其吸附能的大小顺序为Ph(111)Rh-Pt(111);CO与金属表面成键,属于化学吸附.     

Surface temperature effects in the dissociative adsorption of D2/Cu(111) revisited

We have calculated the dissociation probability of H₂ and D₂ on low dimensional potential energy surfaces (PESs) including substrate motion by attaching the PES to a simple harmonic oscillator or to a free mass. For all potentials examined, we find that the dissociation curve has a fixed midpoint while the width increases with surface temperature as observed experimentally. This is in agreement with earlier calculations by Hand and Harris and in disagreement with more recent ones. We conclude that the explanation of Hand and Harris, which ascribes the broadening to the approaching (receding) part of the oscillator phase increasing (decreasing) the relative velocity of the incident atom, is correct. This has the further consequence that the broadening is greater for D₂ than for H₂. We speculate that yet more broadening may be caused by thermally increased corrugation of the surface.