Alkali-induced effects on metal substrates and coadsorbed molecules

We present results of ab initio electronic structure calculations based on density functional theory which show in detail several effects of alkali adsorption on metal substrates and on molecules coadsorbed on the substrate. First, calculations of the isoelectronic reactivity index demonstrate a dramatic enhancement of the electronic polarizability of the metal substrate extending it several angstroms into the vacuum. This phenomenon is traceable to an unusual feature induced in the surface potential on alkali adsorption. The effect appears to be general and helps explain the observed substantial decrease in the vibrational frequency of molecules such as CO and O₂ when co-adsorbed with alkalis on metal surfaces. Next, for the oxidation of CO on Pd(111), we illustrate the changes in the reaction pathway and activation energy barriers induced in the presence of coadsorbed K. PACS 73.20.-r; 71.15.-m     

CO分子与碱金属原子在Fe(110)表面上的相互作用

本文介绍了CO分子分别与K原子或Na原子在Fe(110)表面上共吸附的研究。角分辨率紫外光电子能谱(ARUPS)的结果表明,碱金属原子在Fe(110)表面上的存在,紫外光电子能谱中出现了结合能低于通常CO分子1π能级的新峰。该峰位于费密能级以下6.3eV处,是C0在碱金属影响下电子结构改变的结果。该峰的出现对应着CO分子处于C—O键明显减弱的状态。偏振紫外光电子能谱的研究表明:6.3eV处的谱峰联系的电子轨道有一个对称平面和一个反对称平面,分别平行于<001>晶向和<110>晶向。CO分子轨道的对称性不因 关键词：     

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.     

Electronic structure of transition metal dichalcogenides PdTe_2 and Cu_(0.05)PdTe_2 superconductors obtained by angle-resolved photoemission spectroscopy

The layered transition metal chalcogenides have been a fertile land in solid state physics for many decades.Various MX2-type transition metal dichalcogenides,such as WTe2,Ir Te2,and Mo S2,have triggered great attention recently,either for the discovery of novel phenomena or some extreme or exotic physical properties,or for their potential applications.Pd Te2 is a superconductor in the class of transition metal dichalcogenides,and superconductivity is enhanced in its Cuintercalated form,Cu0.05 Pd Te2.It is important to study the electronic structures of Pd Te2 and its intercalated form in order to explore for new phenomena and physical properties and understand the related superconductivity enhancement mechanism.Here we report systematic high resolution angle-resolved photoemission(ARPES) studies on Pd Te2 and Cu0.05 Pd Te2single crystals,combined with the band structure calculations.We present in detail for the first time the complex multi-band Fermi surface topology and densely-arranged band structure of these compounds.By carefully examining the electronic structures of the two systems,we find that Cu-intercalation in Pd Te2 results in electron-doping,which causes the band structure to shift downwards by nearly 16 me V in Cu0.05 Pd Te2.Our results lay a foundation for further exploration and investigation on Pd Te2 and related superconductors.     

Structure of nano-objects through polarizability and dipole measurements

The electric polarizability and the electric permanent dipole are important quantities for understanding the electronic properties of a cluster. Experimental techniques, the simulations necessary to interpret the experimental results, and a review of measurements on atomic and mixed clusters are presented. For atomic clusters, the polarizability is related to the type of bonding. In simple metal clusters such as alkali clusters, the results are well interpreted by the electron delocalization characteristic of the metallic bonding. In other metal clusters, the polarizability reflects the difficulty of establishing a clear and regular picture of the size evolution of electronic properties. The size evolution observed for covalent and semiconductor clusters is different from the evolution for metal clusters, and the influence of the geometry is preponderant, as demonstrated in the case of fullerenes. For mixed clusters, the measurements of the electric dipole allows one to deduce the charge transfers and the geometric arrangement. This is illustrated in the case of the metal-fullerene system and alkali halide clusters. To cite this article: M. Broyer et al., C. R. Physique 3 (2002) 301–317.     

Platinum and palladium as substrates for surface enhanced Raman spectroscopy

Surface enhanced Raman scattering (SERS) has been observed from the electrogenerated I₂ and I^-₃ coadsorbed on Pt and Pd electrodes. Theoretical models on the SERS process that emphasize the important role of the optical properties of the metal substrate cannot be used to explain this observation. The electronic interaction between the adsorbate and the substrate is believed to be the most important mechanism for the observed SERS.     

Analysis of polarizabilities of alkali halides using Narayan-Ramaseshan repulsive potential

The repulsive potential in ionic crystals recently proposed by Narayan and Ramaseshan (NR) can be expressed as the sum of the contributions from the individual ions. In the present paper we show that using this repulsive potential it is possible to divide the polarizability arising from the relative displacement of ions into its ionic constituents. NR have also derived the ionic radii in alkali halides which we have used to estimate the electronic polarizabilities of ions with the help of polarizability-radius cube relation. The electronic polarizabilities of alkali and halogen ions thus evaluated show a good agreement with those deduced from the experimental refraction data.     

The effect of potassium on the molecular orientation of CO chemisorbed on Ni(111): An esdiad study

Using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) techniques, we have determined that coadsorbed potassium systematically quenches the O⁺ ion yield from CO on the Ni(111) surface for 1000 eV electron excitation energies. The quenching appears to be a short range K-CO interaction; 3 or 4 CO molecules are affected for each K atom adsorbed on the surface. The quenching effect of K on CO indicates that a significant electronic perturbation of CO is caused by its local interaction with K. This effect prevents ESDIAD observation of the K-quenched CO species. In addition, the CO molecules that are not quenched at a potassium coverage of 0.02 K/Ni exhibit a normally oriented C-O bond similar to that found for CO adsorbed on a K-free Ni(111) surface.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

Influence of alkali metal superoxides on structure,electronic,and optical properties of Be_(12)O_(12) nanocage:Density functional theory study

The effect of alkali metal superoxides M_3O(M = Li,Na,K) on the electronic and optical properties of a Be_(12)O_(12) nanocage was studied by density functional theory(DFT) and time-dependent density functional theory(TD-DFT).The energy gaps(Eg) of all configurations were calculated.Generally,the adsorption of alkali metal superoxides on the Be_(12)O_(12) nanocage causes a decrease of Eg.Electric dipole moment μ,polarizability α,and static first hyperpolarizability β were calculated and it was shown that the adsorption of alkali metal superoxides on Be_(12)O_(12) increases its polarizability.It was found that the absorption of M_3 O on Be_(12)O_(12) nanocluster improves its nonlinear optical properties.The highest first hyperpolarizability(β≈ 214000 a.u.) is obtained in the K_3O–Be_(12)O_(12)nanocluster.The TD-DFT calculations were performed to investigate the origin of the first hyperpolarizabilities and it was shown that a higher first hyperpolarizability belongs to the structure that has a lower transition energy.     

Frequency shifts in the spectra of molecules adsorbed on metals,with emphasis on the infrared spectrum of adsorbed CO

Two processes are considered which cause the frequency of the band maximum in the infrared absorption spectrum of molecules adsorbed on metals to shift with increasing coverage. The first arises from the coverage-dependent modification of the local field acting on an adsorbed molecule while the second originates from vibrational coupling arising from a through-metal interaction between molecules adsorbed on different metal atoms. Infrared absorption strengths are discussed and the method used heretofore to obtain the extinction coefficients of adsorbed molecules is questioned. In particular, a factor of 2 which results from the constructive superposition of the incident and reflected electric fields is shown to be often ignored. We show that dipolar (through-space) coupling is too weak to account for the magnitudes of the shifts recently observed for CO on single-crystal Pt and Pd as coverage progressed from low to high values. Vibrational coupling is also shown to account for the disparity in the intensities of the two bands observed when ¹²CO and ¹³CO are coadsorbed on supported metals. Similar experiments on single-crystal surfaces are suggested to help determine to what extent the observed spectral frequency shifts are derived from adsorbate—adsorbate coupling as opposed to other mechanisms.     

Scanning tunneling microscopy of adsorbates on insulating films. From the imaging of individual molecular orbitals to the manipulation of the charge state

Ultrathin insulating films on metal substrates are unique systems for using a scanning tunneling microscope to study the electronic properties of single atoms and molecules that are electronically decoupled from the metallic substrate. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. In the case of molecules on ultrathin NaCl films, the electronic decoupling allows the direct imaging of the unperturbed molecular orbitals, as will be shown in the case of individual pentacene molecules. PACS 68.37.Ef; 73.61.Ng; 73.20.Hb     

Magnetic 4d systems studied by implanted ions

By application of the perturbed -ray distribution method following heavy-ion reactions and recoil implantation techniques, we have found an experimental way of producing and investigating magnetic 4d states in metals. Strong 4d magnetism has been found for 4d ions in alkali metal hosts and in Pd hosts. In alkali metals, 4d ions reflect the phenomena of well-defined ionic ground states, orbital magnetism, mixed valence, and crystal field splittings smaller than theLS coupling. Magnetic 4d states in alkali metals cannot be described by one-electron approaches based on Anderson-type models, but requires an analysis in terms of many electron ionic configurations exhibiting basic features common to the physics of stable and unstable f stales in metals. In contrast, the local moment formation of 4d and 3d ions in Pd is governed by inter-atomic interactions of the magnetic d states with host d-band electrons, giving rise to spin magnetic behavior of the 4d impurity and to strong spin polarizations of the 4d electrons of the Pd host. Thus, the magnetism and electronic structure of 4d ions in metals exhibit qualitative differences in alkali metal hosts compared to Pd. The existence of magnetic 4d systems strongly depends on the 4d ion species and the host matrix, and on spin fluctuation rates or the corresponding Kondo temperatures. The results can be directly compared to theoretical work and also to the magnetic behavior of 3d ions in sp metal hosts and in hosts with d-band electrons.     

Structure Sensitive Reaction Channels of Molecular Hydrogen on Silicon Surfaces

The ability of the Si(001) surface to adsorb H2 molecules dissociatively increases by orders of magnitude when appropriate surface dangling bonds are terminated by H atoms. Through molecular beam techniques the energy dependent sticking probability at different adsorption sites on H-precovered and stepped surfaces is measured to obtain information about the barriers to adsorption, which decrease systematically with an increase in coadsorbed H atoms. With the help of density functional calculations for interdimer adsorption pathways, this effect is traced back to the electronic structure of the different adsorption sites and its interplay with local lattice distortions.     

Surface chemistry of carbon dioxide

The review discusses how CO₂ surface chemistry has developed since the early 1950s. Emphasis is given to studies of well-characterized surfaces of metals, oxides and some more complex systems involving in particular alkali modified surfaces and also of coadsorbed molecules.     

用Xα-DV方法研究CO,NO在Ⅷ族过渡金属表面的化学吸附(Ⅱ)——NO分子在Pd(111)表面的化学吸附

本文用X_α-DV方法计算了NO在Pd(111)表面化学吸附问题。得到了它的电子结构,包括分子丛轨道能量本征值谱、态密度、电荷转移等等结果。在计算中特别考虑了NO之间的相互作用,所得总态密度与实验UPS十分相符,从而支持了LEED所示的几何结构,决定了NO的吸附高度为1.27?,并得知吸附于Pd表面的NO分子之间的相互作用十分重要。从理论上探讨了NO分子在Pd表面吸附时的活化作用。计算了NO分子各个轨道上的占有数,发现其电荷转移情况与CO在过渡金属表面吸附的情况相似。另外,还发现NO的吸附对Pd的价电子能带无重大影响。 关键词：     

Real time chemical dynamics at surfaces

It is a major goal in surface science to make movies of molecules on surfaces, in which the reaction of the molecules on the surface can be followed on a femtosecond time scale, with sub-nanometer resolution. By moving the actors (the molecules) to precisely determined positions on the stage (the surface) at some well-defined moment in time, and subsequently making a space- and time-resolved documentary of what happens next, we would be able to understand the reactive interactions between molecules on surfaces in the greatest possible detail. This would enable us to set the stage and bring together the actors in such a way as to produce the chemical outcomes our society needs, by improving existing catalysts and designing novel catalysts, and by engineering novel reactions on surfaces. Any future director of such movies needs to know which techniques (i.e., which theoretical and experimental methods) hold promise for movie making, what has been done with these techniques, and what can be done with appropriate extensions. The methods we discuss are: (i) the time-dependent wave packet method, which is a theoretical method for simulating molecule–surface reactions with sub-nanometer resolution on a femtosecond time scale, (ii) molecular beam experiments, which allow detailed investigation of the molecule–surface interaction at a molecular level, and (iii) time-resolved laser pump–probe experiments, which allow reactions to be studied with femtosecond resolution. In particular, we discuss (i) theoretical studies of the dissociation reaction of hydrogen on metal surfaces, the reactive system presently understood at the greatest level of detail, (ii) the reactive and non-reactive scattering of heavy diatomics (NO,CO) from metal surfaces, and (iii) the competition between reaction of coadsorbed CO with O and desorption of CO, again on a metal surface. We examine possibilities to extend these methods to make movies at the desired level of detail. We also discuss which reactions are likely to provide good material for plots of movies that will be exciting for future generations of surface scientists.     

Study of adsorption states and interactions of CO on evaporated noble metal surfaces by infrared absorption spectroscopy: I. Silver

We have used infrared absorption spectroscopy to study the adsorption of CO at low temperature on evaporated silver films as a function of the coverage of CO and the deposition temperature of the silver. We observe two adsorption regimes when a cold silver film is exposed to CO gas. If the silver deposition temperature, (or the highest temperature at which the silver has been annealed), is above the threshold temperature of 150 K, then only physisorbed CO is observed. For sample temperatures below 25 K, these physisorbed molecules are oriented perpendicular to the metal surface. Films deposited at temperatures below 150 K, however, contain ≈ 0.01 monolayer of chemically active sites at which CO chemisorbs. The infrared band due to chemisorbed CO shifts to lower frequency with increasing coverage. We have analyzed this shift and separated the static and dynamic contributions. The static, chemical shift is caused in part by the change in the work function induced by surrounding adsorbates. The dynamics shift is fully explained by a dipole-dipole interaction; we find no evidence for a vibrational coupling through the metal. We have analyzed the vibrational polarizability and infrared absorption strength of the absorbed CO, and find no evidence for the infrared enhancement suggested by some theories of surface enhanced Raman scattering.     

Compound formation at Pd(100)/In interfaces

PAC spectroscopy has been used to study the compound formation at the Pd(100)/In interface. Indium films of various thicknesses have been deposited onto Pd(100) single crystals. The¹¹¹In PAC probes are either deposited on the Pd(100) surface before covering it with In or introduced into the In metal before producing the interface. Pd/In compounds with polycrystalline structure of various stoichiometry are identified from the detection of significant electric field gradient tensors after isochronal as well as isothemal annealing. In particular we find an ordered structure at the interface directly after In deposition.