Selective adsorption of 4He on clean LiF (001) surfaces

The selective adsorption of ⁴He on in-situ cleaved LiF surfaces has been studied under improved resolution. The main results are as follows: (1) There are four bound states in the surface potential well, at energies of ?5.8, ?2.2, ?0.6 and ?0.1 meV. The lowest three levels were reported previously. (2) Most of the structure previously designated as “fine structure” is due either to transitions to these four levels via various small reciprocal lattice vectors or to the opening of diffraction channels. (3) The transitions involving the [01] and [01?] reciprocal lattice vectors (i.e., the ones nearly perpendicular to the incident wave vector) are strong; as much as 85% of the specular intensity may be removed. Transitions via the other small reciprocal lattice vectors are much weaker. (4) The widths of the lines are consistent with the velocity distribution, which has a half-width of about 2%. (5) The observed energies agree fairly well with those calculated by Tsuchida for a zeta-function potential, but are not consistent with a Morse potential.     

An IR study of the adsorption of water on Ru(001)

The strong OH stretch at 3400 cm^?1 (fwhm ～ 230 cm^?1) in the IR reflection absorption spectra of the system $\text{H}{}_2\text{O}\text{Ru(001)}$ at 85 K indicates the presence of hydrogen-bonded clusters. These clusters appear to form even at the lowest coverages. On the clean surface there is a linear relationship between integrated absorption intensity and coverage. The presence of small quantities of preadsorbed oxygen delays, however, the onset of absorption. It is thought that the oxygen atoms “bind” the water molecules, thus preventing cluster formation and in turn eliminating the intensity enhancement due to hydrogen bonding. Flash desorption spectra also indicate a second binding state when oxygen is coadsorbed. The relevance of these results to models of the electric double layer at the metal-electrolyte interface is discussed.     

Theoretical consideration on dimer vacancy images in the STM observations of Si(001) surfaces in terms of the adsorption of O2 molecules

Geometric and electronic structures of dimers on the Si(001)-2 × 1 reconstructed surfaces before and after O₂ adsorption have been investigated with an ab initio molecular orbital (MO) calculation at the Hartree-Fock level. The theoretically predicted electron tunneling current in the dimer before and after O₂ adsorption has strongly suggested that the O₂-adsorbed dimer on clean Si(001) surfaces is observed as dark in the scanning tunneling microscope (STM) images. The dark spot images which have so far been recognized to be a dimer vacancy type defect could be due to the O₂ adsorption in parallel to a Si dimer.     

Dynamic low-temperature scanning force microscopy on nickel oxide (001)

We present atomically resolved images of nickel oxide (001) obtained with low temperature non-contact atomic force microscopy. Using iron coated silicon cantilevers, it is possible to distinguish defects with a vertical resolution of less than 10 pm and to obtain atomic resolution across step edges on the upper and lower terrace within 1 nm of the edge. The noise level in these images could be reduced to ƹ.5 pm.     

TiC-和TiN-(001)表面吸附性能的密度泛函理论研究

基于密度泛函理论系统研究了碳化钛(TiC)和氮化钛(TiN)非极性(001)表面吸附气体分子和原子的性能。鉴于这些材料拥有不同的电子结构特征,发现受电子的CO分子或未饱和的O和H原子在TiC(001)和TiN(001)表面吸附于不同的活性位点,而供电子的NH3和H2O气体分子或完全饱和的O2和H2分子仅倾向与两个表面的金属原子位点结合。这些吸附特性可能与此类材料表面的电子结构有关。     

Adsorption of water molecule on (001) and (110) surfaces of MgH2

First principle calculations have been performed to explore the adsorption characteristics of water molecule on (001) and (110) surfaces of magnesium hydride. The stable adsorption configurations of water molecule on the surfaces of MgH₂ were identified by comparing the total energies of different adsorption states. The (110) surface shows a higher reactivity with H₂O molecule owing to the larger adsorption energy than the (001) surface, and the adsorption mechanisms of water molecule on the two surfaces were clarified from electronic structures. For both (001) and (110) surface adsorptions, the O p orbitals overlapped with the Mg s and p orbitals leading to interactions between O and Mg atoms and weakening the O–H bonds in water molecule. Due to the difference of the bonding strength between O and Mg atoms in the (001) and (110) surfaces, the adsorption energies and configurations of water molecule on the two surfaces are significantly different.     

Alkali-metal-affected adsorption of molecules on metal surfaces

The study of coadsorption of alkali metals and simple molecules on transition metal surfaces has been a favored topic of research ever since the pioneering work by Langmuir in 1923. The main reasons for this continued interest are both of fundamental and applied nature. There are a number of interesting physical effects, such as work function changes, charge transfer, two-dimensional ordering, bond energy and molecule orientational changes, and altered surface reactive properties, which have been investigated by a large number of different surface analytical techniques. From an applied point of view, alkali metal covered surfaces are important in the areas of electron and ion emission and heterogenous catalysis, for example.In this paper we will give a short review of the adsorption of alkali metals on well-defined transition metal surfaces. The interaction of these adsorbed alkali metals with subsequently adsorbed atomic or molecular species will be treated more extensively. The emphasis will be on recent experiments dealing with well-characterized surfaces. In particular we will consider questions of adsorption energetics and kinetics, but also review in detail the vibrational, electronic, structural and reactive properties of the coadsorbed complex. Based on a wealth of experimental data, several models of the coadsorbed alkali metal-molecule complex will be introduced and discussed.     

Alkali adsorption on the Al(001) surface

The chemisorption of Na on the Al(001) surface has been studied by 2D bandstructure calculations on slab models using a density functional STO-LCAO method. Two slab models of three and five layers of substrate atoms have been used. Overlayers of the structuresp(2×2),c(2×2) andp(1×1), representing coverages of a quarter, a half and a full atomic monolayer of sodium atoms, respectively, have been investigated. The electronic structure of the adatoms and the charge transfer to the substrate are discussed. Satisfactory agreement with experiment is obtained for the adsorption induced change of the work function, correctly reproducing its monotonic character.     

Structural phase transition on Si(001) and Ge(001) surfaces

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, there have been many conflicting arguments about the atomic structure on these surfaces. We first present experimental evidence indicating that the buckled dimer is the basic building block and that the structural phase transition between the low-temperature c(4x2) structure and the high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction parameters are derived from total-energy calculations for different arrangements of buckled dimers. The calculated critical temperature agrees reasonably well with the experimental one. It is pointed out that the nature of the phase transition is crucially affected by a small amount of defects on the real surfaces.     

The effect of relaxation and rumpling on the point ion potential at NaCl(001) surfaces

The effect of surface relaxation and rumpling on the potential in the vicinity of a (001) surface of the NaCI structure is calculated within the point-ion approximation. Zero order (the Madelung term), first, and second order kubic harmonics are considered, and it is shown that it may be necessary to go beyond simple Stark splitting to interpret the energies of cationic surface excitations.     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

Low temperature adsorption of water on cleaved GaAs(110) surfaces

The adsorption and desorption of water on UHV-cleaved GaAs(110) surfaces was studied using synchrotron-excited photoelectron spectroscopy. Water was adsorbed at T = 100 K. Desorption was studied during heating to room temperature. At low coverages, dissociated species are observed followed by molecular adsorption. Molecular water is desorbed at T = 160 K. The dissociated species are also mainly desorbed after heating to room temperature. The chemical changes are accompanied by substrate binding-energy shifts, reflecting the movement of the Fermi level at the surface.     

Extrinsic nature of point defects on the Si(001) surface: dissociated water molecules

Point defects on a Si(001)-(2 x 1) surface were examined by scanning tunneling microscopy and ab initio pseudopotential calculations. The residual water molecules in the ultrahigh vacuum chamber are found to be the sole origin of the type-C defects. Most of the apparent dimer vacancies in the filled-state images were found to show a distinct U-shaped triple-dimer footprint in the empty-state images, which also originate from water adsorption. These two defects were identified as a single dissociated water molecule forming Si-OH and Si-H bonds in the interdimer (type-C defect) and the on-dimer (dimer-vacancy-like or U-shape defect) configurations.     

Oxygen adsorption and oxide formation on Cr(100) and Cr(110) surfaces

The adsorption of oxygen on (100) and (110) clean chromium surfaces was studied by electron diffraction (LEED and RHEED) and scanning microscopy. The beginning of the oxidation process was investigated using LEED technique. The successive patterns, observed with increasing oxidizing atmosphere, allow us to propose an oxidation mechanism for these chromium surfaces. For both orientations, rhombohedral chromium oxide grows on the surface, as shown by RHEED diagrams. The behaviour of the two planes is markedly different: the oxide film is polycrystalline on the (100) surface, while the pseudo six-fold symmetry of the (110) plane permits a parallel oriented growth.     

Si(001)表面硅氧团簇原子与电子结构的第一性原理研究

在周期性边界条件下的k空间中,采用基于密度泛函理论的第一性原理广义梯度近似方法,对建立的规则对称型结构（A）、周期性非对称型结构（B）、周期性非对称型结构（C）、不规则型结构（D）四种可能的Si（001）表面硅氧团簇的结构模型进行了优化计算.结果表明优化后的表面结构呈无定形状,并且优化后的B,C,D三种模型的表面结构具有类似SiO₂的四面体结构的几何特征.此外,通过电子局域函数图以及Mulliken布居分析发现硅氧团簇中的Si—O键既有明显的离子键成分,也有一定的共价键成分. 关键词： Si（001）表面 硅氧团簇 密度泛函理论 第一性原理