Theoretical model of chemisorption on metals: I. Formalism

A theoretical model of chemisorption on narrow band transition metals is presented. The model is based on the KKR formalism, and concentrates on calculations of quantities which may be associated with UPS measurements. These include for instance changes in density of states ΔN(?) induced by adsorption of an atom on a clean surface, or localised density of states ρL(?) of a given angular momentum L. Results are presented in a form reminiscent of the old Anderson model of chemisorption, thus clarifying considerably the nature of the bond in these systems.     

The role of HH interactions in the formation of ordered structures on Ni and Pd single crystals

The interaction between H adatoms on a surface is calculated within the embedded cluster model of chemisorption. The model is first applied to the case of two H atoms on a free electron surface. The interaction energy is found to be an oscillatory function of the H-H separation R_ab. Application of the free electron model to the problem of chemisorption on transition metal surfaces leads to unphysical results with the prediction of formation of ordered H overlayers which are not observed in LEED experiments. We next include the l = 2 TM muffin tins. Results for H adsorption on the low index faces of Ni and Pd substrates are presented. Graphitic structures are predicted for the (111) faces of both Ni and Pd with the H atoms occupying both types of three-fold hollow sites on the surface. This agrees with the results of LEED experiments for H/Ni(111). Comparison with experiment is not possible in the case of H/Pd(111) owing to the lack of low temperature studies for that system. Zig-zag chains with the H atoms adsorbed in sites of three-fold coordination on alternate sides of the TM(110) rows are predicted for both Ni and Pd. This is in agreement with the results of He diffraction experiments for H/Ni(110). No structure determination has been done for H/Pd(110). Adsorption in the four-fold centre sites for H on the (100) faces of Ni and Pd is found to be unfavourable. The H atoms are expected to adsorb in sites of three-fold symmetry below the (100) surface for H on Pd with formation of a c(2 × 2) structure in agreement with the LEED observations. For H/Ni(100) the H atoms are believed to adsorb above the surface, away from the centre site and to bond to two surface Ni atoms. No short-range ordered structures are predicted in this case.     

CHARGE STATES OF SPUTTERED ATOMS/IONS

The charge states of sputtered cesium and oxygen ions/atoms from metal surfaces have been discussed with the time-dependent Newns-Anderson model. The final charge state distribution has been calculated as a function of work function of the surfaces. The calculated results fit the experimental data well. Besides, the dependence of the final charge states of the sputtered oxygen particles on the surface temperature is also predicted.     

Structural and electronic properties of Danzer's model of a three-dimensional quasicrystal

By using deflation rules, the coordinates of the atoms in Danzer's model of a 3d quasicrystal are computed. The resulting atomic positions in a cut through the model well agree with experimental findings. Furthermore, for Danzer's model the static structure factor, the pair correlation function, and properties of a tight binding electron model are studied. The electron density of states shows a characteristic peak at energyE=O, which may be associated to confined states.     

CHARGE STATES OF SPUTTERED ATOMS/IONS

The charge states of sputtered cesium and oxygen ions/atoms from metal surfaces have been discussed with the time-dependent Newns-Anderson model. The final charge state distribution has been calculated as a function of work function of the surfaces. The calculated results fit the experimental data well. Besides, the dependence of the final charge states of the sputtered oxygen particles on the surface temperature is also predicted.     

Electron correlations in the chemisorption theory: Self-consistent calculations of the adatom charge

The single hydrogen-like adatom chemisorption on transition metal surfaces is studied by using the generalized model Hamiltonian. This Hamiltonian includes the possibility of influence of the adatom orbital occupancy on the charge transfer between the adatom and the substrate metal. The correlation effects were included up to second order in V (the single particle coupling strength). The numerical calculations of the charge transfer between an adatom and a substrate metal, as well as the comparison with results obtained for the standard Newns-Anderson model indicate that this generalized Hamiltonian can be more efficient in describing the chemisorption process.     

GaAs(100)表面(4×1)结构的UPS研究

用偏振的紫外光源测量了GaAs(100)表面(4×1)结构的UPS谱,从清洁表面和吸附氧以后UPS的差谱中辨别出了在价带顶以下2eV以内的表面态峰,根据用偏振光所得到的谱和跃迁选择定则的讨论,认为表面态包含了三个峰,价带顶以下0.5eV处有一个对应于表面Ga原子桥键态的峰,在0.7eV处有对应于表面As原子桥键态的峰,而在1.3eV处的峰则同表面原子的悬键态相联系。 关键词：     

THE ADSORPTION OF Ge ON GaAS(ll0)

In the present work, the chemisorption of Ge on GaAs(1lO) was studied by a cluster model using the charge self-consistent extended H ückel method (EHT) and the results thus obtained were compared with those derived from the experiments. It was shown from the calculation that after chemisorption, the substrate tends to relax back to the ideal unrelaxed state, after cleavage. The tilted angle of the As-Ga bond was reduced from the well khown 27° to 10°.The gap states which were driven into both the valence band and the conduction band due to relaxation will-be introduced into the gap after chemisorption. From the calculation of the total energy of the cluster, it was found that the Ge atom could bind to both Ga and As atoms although the binding of the latter was somewhat stronger. The gap states were derived from the p states of Ge and the s and p states of Ga and As atoms. The states will serve as donors and acceptors for p and n type materials respectively . From a comparison with the previous results concerning studies of group Ⅲ and Ⅴ metals on GaAs(110) surface, it seems that the change of surface relaxation after chemisorption might be the origin of gap states and the cause for Fermi level pinningr. The explanation was first put forward by Chen et al . from their experimental studies . The present calculation also gives the core shifts of Ga(3d) and As(3d) of the clean relaxation surfaces with respect to that of the bulk as well as the core shifts of Ga(3d) and As(3d) after the chemisorption of Ge atom. A chemisorption induced state which is attributed to a precursor state for a heterojunction interface state is also obtained at a binding energy near 7eV which is in fairly good agreement with the experimental result.     

Model calculations of chemisorption on transition metal clusters

Alloying of a transition metal with a non-transition metal affects the nature of the chemical bonds between the transition metal atoms in the surface and hydrogen or other atoms chemisorbed on them. The resultant changes in strength of the chemisorption bonds are due to (1) changes in delocalization of the metal electrons involved in the chemisorption bonds, (2) changes in the number of electrons available for such bonds. Our calculations have shown that the effects of alloying on chemisorption are different for monocoordinated adsorbates (ad-atom on top of metal atom) and tricoordinated adsorbates (ad-atom equidistant to three transition metal atoms) respectively. With localized bonds, alloying increases the strength of the former bond, but weakens the latter. As a result the ratio of monocoordinated to tricoordinated adsorbates, present in equilibrium at high coverage, is drastically increased by this type of alloying. The increase is much larger than expected on a geometrical basis only.     

Au钝化Si(100)表面的电子特性

用TB-LMTO方法研究单层的Au原子在理想的Si(100)表面的化学吸附.计算了Au原子在不同位置的吸附能,吸附体系与清洁Si(100)表面的层投影态密度, 以及电子转移情况.结果表明, Au原子在吸附面上方的A位(顶位)吸附最稳定, Au钝化Si(100)表面可以取得明显的钝化效果, 这一结论与实验事实相符合.     

Chemisorption and decomposition reactions of oxygen-containing organic molecules on clean Pd surfaces studied by UV photoemission

We have used uv photoeinission (primarily at a photon energy hv = 40.8 eV) to study chemisorption and decomposition reactions of small oxygen-containing organic molecules on clean polycrystalline Pd surfaces at 120 and 300 K. These molecules include methanol (CH₃OH), dimethyl ether (CH₃OCH₃) formaldehyde (H₂CO), acetaldehyde [H(CH₃)CO], and acetone [(CH₃)₂CO]. Chemisorption bonding of these molecules to the Pd surface occurs primarily through the lone-pair orbitais associated with the oxygen atoms, as evidenced by chemical bonding shifts of these orbitais toward larger electron binding energy relative to the other adsorbate valence orbitals. At 300 K all the molecules studied decompose on the surface, resulting in chemisorbed CO. Since chemisorbed (as well as condensed) phases of some of these molecules (CH₃OH and H(CH₃)CO) are observed at low temperature, the decomposition to CO is a thermally-activated reaction. The observed orbital shifts associated with chemisorption bonding are used to make rough estimates of interaction strengths and chemisorption bond energies (within the framework of Mulliken's theory of electron donor-acceptor complexes as applied to chemisorption by Grimley). The resulting heats of chemisorption are consistent with the observed surface reactions.     

UPS spectra of H2O,CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)

The present communication presents ultraviolet photoemission spectra (UPS) of three different “alcohols”; water (H₂O), methanol (CH₃OH), and cyclopentanol (C₅H₉OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing r_s/a_o of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa₃ enable correlations to be made between UPS intensity peaks and one electron orbitals.     

AI在GaAs(110)面上的吸附

用电荷自洽的EHMO方法研究了Al在GaAs(110)面上的吸附问题。比较了两种吸附构型,从能量极小的观点定出了稳定的吸附应为Al取代表面Ga原子,使Ga落在表面As的悬挂键上。还计算了电荷转移、成键情况和状态密度。 关键词：     

Analytic conditions for the existence of localized electron states in linear chains with two impurities

The negative eigenvalue theorem is used to obtain an analytic result for the integrated density of electron states in several types of low-dimensional lattices with two impurities in the tight binding approximation. From this expression, analytic conditions for the existence of localized electronic states are obtained. The result is directly applicable to the problem of surface states and that of chemisorption on a linear chain.The author would like to thank Prof. Dr. L.Valenta for suggesting the problem and for constant encouragement.     

Influence of the electronic structure on chemisorbed CO

The chemisorption of carbon monoxide on amorphous and crystalline alloys such as Pd-Si, Ni---Zr, Ir---Si, and Pd---U---Si has been studied by ultraviolet photoelectron spectroscopy (UPS). Systematic trends in the CO chemisorption behavior have been found for the various alloy systems. A correlation between the position of the valence band d-electron states of the LTM (late transition metals) and the molecular orbitals of the chemisorbed CO molecules is reported. The influence of the substrate electron states on the C-O bond strength is explained in a qualitative model involving the symmetry and the binding energy of the substrate valence electrons and those of the molecular orbitals.     

Ab initio study of water adsorption on TiO2-terminated (100) surface of SrTiO3 with and without Cr doping

Water molecule adsorption on TiO₂-terminated (100) surface of SrTiO₃ with and without Cr doping is investigated by first principle calculation based on density functional theory. The band gap is shrunk compared with that of bulk due to the existence of defect states on the surface and 3d states of dopants. As a result the absorption energy edge is reduced and locates in the visible region. When adsorbed on the surface, energy levels of water molecules as a whole are lowered with respect to the Fermi energy, but the higher levels are split and electrons are transferred from low levels to high levels due to the decrease of the density of states in low energy region. Weak bonding is formed between water hydrogen atoms and surface oxygen atoms. This bonding causes the electron transferring from substrate to molecule and the occupation of the corresponding states.     

Electron energy-loss spectroscopy study of oxygen chemisorption and initial oxidation of Fe(110)

The initial stages of the interaction of oxygen with an Fe(110) surface have been studied at 300 K by electron energy-loss spectroscopy with in-situ combined low energy electron diffraction, Auger electron spectroscopy and work-function change measurement. From all the results, four different stages of the oxygen interaction are distinguished: (I) a first dissociative chemisorption up to 3 L, characterized by the c(2×2)-O structure, (II) a second dissociative chemisorption between 3 and 7 L, characterized by the c(3×1)-O structure, (III) incorporation of O adatoms into the selvage between 7 and 30 L, and (IV) oxidation above 30 L leading to the formation of FeO(111), characterized by the diffuse hexagonal diffraction pattern. The sticking probability was found to be initially near unity and fall off rapidly to a minimum value of ≈0.05 at ≈1 L. Particular emphasis was placed upon the investigation of the change in surface electronic properties from those characteristic of them metal to those of the oxide. In stage (I) an energy-loss peak, being attributed to the transition from the 2p orbital of the chemisorbed oxygen, was observed at 6.0 eV, while in stage (II) two additional peaks of the same origin appeared at 7.5 and 9.3 eV due to the formation of the O 2p band. The energy-loss spectrum in the oxide phase was characterized by the peaks at 4.8 and 7.5 eV due to the O²⁻ 2p → Fe²⁺3d charge-transfer transitions and by a peak at 2.4 eV due to the ligand-field d → d transitions of an Fe²⁺ ion in FeO. It is shown that the Fe 3d_yz,zx and 4sp electrons play a major role in the chemisorption bond (O adatoms located in the long-bridge site), and that for the incorporation process the Fe 3d_y² electrons are also involved in bonding by the symmetry breaking. The change in the Fe 3p-excitation spectrum during oxidation was also investigated. The differences between the experimental results on Fe(100) and (110) surfaces are summarized.     

On the bond lengths reported for chemisorption on metal surfaces

The bond lengths and geometrical arrangements reported with multiple-scattering analyses of LEED intensities for chemisorption on metal surfaces are assessed with Pauling's bond length-bond order relation. The approach here is partly empirical and it depends on information on the atomic valencies; the hybridisation model proposed for metals by Altmann, Coulson and Hume-Rothery is also used to guide the allocation of bonding electrons for the purposes of estimating the orders of surface bonds. It is shown that this framework allows estimates of surface bond lengths to within 0.1 Å of the values reported with LEED, and often the correspondence is substantially closer. Other factors that are likely to influence surface bond lengths have been recognised, but refinements to the present analysis should probably await clarifications of uncertainties in the structures determined with LEED including, for some chemisorption systems, consideration of possible displacements of metal atoms.