Adsorbed Alkali Atoms Research Articles

Inner-shell promotions in low-energy Li+-Al collisions at clean and alkali-covered Al(100) surfaces.

Spectra of scattered ions and ion-induced electron emissions are used to investigate the electronic processes that occur during 0.4--5.0-keV Li[sup +] bombardment of clean and alkali-atom-covered Al(100). The results show that Li 1[ital s] electrons are promoted during hard Li-Al collisions, but not during Li-K or Li-Na collisions. Consequences of the inner-shell electron promotions are evident in the inelastic loss, neutralization behavior, and electron emissions of the scattered Li particles. Spectra of scattered Li[sup +] ions exhibit discrete-loss features, which are resistant to the usual increase in resonant neutralization that accompanies the deposition of alkali atoms on the surface. The loss features are due to Li 1[ital s] excitation via electron promotion, while the production of ions away from the surface via autoionization is responsible for their lack of response to alkali-atom adsorption. Spectra of ion-induced electron emissions confirm that Li[sup *](1[ital s]2[ital s][sup 2]) is produced and that it undergoes autoionization.

Alkali adsorption on GaAs(110): atomic structure, electronic states and surface dipoles

During the last six decades the theory of Schottky-barrier formation at metal/semiconductor interfaces has been and still is a matter of scientific controversy. In order to achieve a better understanding we feel that one of the key systems to be studied is the adsorption of alkali metals, since these adsorbates have a most simple electronic structure. An additional interesting aspect of such systems has been recently revealed by scanning tunneling microscopy and other experimental studies, which showed that Cs and Na form one-dimensional chains as well as two-dimensional layers. Both structures of the adatom overlayers are nonmetallic. In this paper we review the adsorption of alkali atoms on GaAs(110). We describe the state-of-the-art theory and, in particular, discuss theoretical results in comparison with experiments. We find that the interaction of the alkali metal with the surface is well sketched by that the alkali atom donates its valence electron into a surface Ga dangling orbital. The Ga dangling-bond character determines the key properties of the adsorbate system, such as the surface band structure, the Schottky barrier, and its dependence on the coverage for example, as well as the fact that the adatom-covered surface is insulating. Furthermore, we discuss the adsorbate-induced work-function changes and core-level shifts.

A reversal in dipole moment for adsorbed hydrocarbons on Pt(111) due to coadsorbed alkali

It is demonstrated here that an adsorbed alkali atom (Cs) can induce such a strong dipole in a coadsorbed hydrocarbon like cyclohexane or cyclopentene on Pt(111) that the direction of the dipole of the hydrocarbon/substrate complex is completely reversed. This results in attractive lateral interactions between the hydrocarbon and the alkali.

A reversal in dipole moment for adsorbed hydrocarbons on Pt(111) due to coadsorbed alkali

It is demonstrated here that an adsorbed alkali atom (Cs) can induce such a strong dipole in a coadsorbed hydrocarbon like cyclohexane or cyclopentene on Pt(111) that the direction of the dipole of the hydrocarbon/substrate complex is completely reversed. This results in attractive lateral interactions between the hydrocarbon and the alkali.

Alkali adsorption on W(110) studied by metastable impact electron spectroscopy

The adsorption of alkali atoms (Li, Na, K, Cs) on W(110) at room temperature has been studied by Metastable Impact Electron Spectroscopy (MIES) and UPS as a function of the alkali coverage. In the coverage range up to 0.5 ML (in units of the saturation coverage at room temperature) emission via the Auger de-excitation process involving substrate electrons is observed. Emission which can be associated with the alkali s-levels can be identified only at alkali coverages exceeding 0.4 ML. This conclusion is corroborated by results obtained for the W(110) surface exposed to NaCl and by the oxidation of alkali layers. An explanation is offered for the fact that the interaction of the He(2 1S) and He(2 3S) metastables with the surface is different. An attempt is made to analyse and interpret the line shapes of the alkali induced features.

Studies of adsorption of alkali atoms on A1(111) as a function of coverage

We have studied the adsorption of alkali atoms on Al(111) for different coverages θ. We calculate the total energy of the system in which the ionic interaction potential is described by pseudopotentials averaged over the parallel directions to the surface. This method allows us to consider all regimes in θ. We find that adsorbed alkali atoms evolve from ionic to metallic and from low to large coverages. In particular, we calculate for the Cs/Al(111) system that the equilibrium distance increases 0.12 Å when coverage changes from 0.4 to 1 ML. Work functions are also presented. We note that in order to obtain well-defined values of the distance, it is necessary to keep the substrate at a low temperature, because the energy minima are very shallow as a function of the adatom-substrate separation.

On the Promoting Effect of Alkali Metals on the Adsorption of Nitriles on the Gold (100) Surface

AbstractSubmolayer concentrations of (strongly electropositively polarized) Na and K on the Au(100) surface create three new acctonitrile (CH3CN) and benzonitrile (C6H5CN) binding states (the γ‐states) on the high‐temperature side of the corresponding molecular thermal desorption spectra (β‐state) observed for the clean, unpromoted Au surface. The additional states appear with both alkali metals and nitriles, whereby they are energetically about equally spaced (ΔT ≈ 30 K with K, and ΔT ≈ 50 K with Na) and sequentially populated, indicating a more general electrostatic modification of the nitrile's adsorptive bonding in the vicinity of an adsorbed alkali atom. The triplet of TD states is tentatively interpreted as being caused by a subsequent filling of a coordination shell formed by the nitriles around each individual alkali metal atom. Similar energetics involved in the formation of gas‐phase and surface cluster complexes of the kind Meδ+ {N  C–CH3}δ− is noted and discussed.

Hyperfine-interaction studies of surfaces

Applications of hyperfine-interaction techniques, like NMR, PAC and Mosbauer spectroscopy, to well-characterized surfaces are discussed and the present knowledge of surface hyperfine fields is reviewed. Measurements of nuclear spin relaxation permit to extract the local density of electron states at the Fermi level of adsorbed alkali atoms. From the observed electric-field-gradient properties surface probe sites and diffusion processes can be inferred; the experimentally determined magnetic hyperfine fields give access to the electron-spin behaviour at magnetic surfaces.

Work function of K and Rb submonolayers adsorbed on Al(111) and Mg(0001)

The work function variation due to K and Rb atom adsorption on Al(111) and Mg(0001) has been calculated self-consistently within the density functional formalism. To go beyond the jellium model for the substrate the effect of metallic ions on the conduction electrons is represented by a parametrized form of the perturbing potential. The adsorbed alkali atoms are represented by a jellium slab whose thickness varies with coverage. The computed work functions vs coverage curves, the work function minimum and initial dipole moments show good agreement with recent experimental data on work function change.

Local versus non-local character of the alkali-promoted oxidation of silicon

Photoemission of Adsorbed Xenon (PAX) and low-temperature PhotoElectron Spectroscopy (PES) data for K-Si (100)2 × 1 strongly support the existence of an important local contribution to the observed oxidation enhancement. PAX spectra taken on the K-covered Si surface reveal a local work function decrease in the vicinity of the adsorbed alkali atoms in addition to a non-local decrease of the surface potential at the clean Si patches. During exposures to molecular oxygen, the alkali adatoms bond immediately to oxygen forming K-O complexes and/or potassium suboxides (depending on the K-coverage) which act as mediators in the Si oxidation.

Alkali-metal-affected adsorption of oxygen on W(110)

The oxygen attack of a W(110) surface partially covered by sodium or cesium was followed by Auger spectroscopy (AES), work function measurements, and LEED studies. The work function exhibits remarkable changes as a function of the oxygen exposure. For small amounts of oxygen offered a work function decrease proportional to the alkali precoverage is observed. The results obtained in this range of oxygen exposures are explained by the formation of an oxygen sublayer underneath the prior adsorbed alkali atoms. A rapid work function increase occurs for longer oxygen exposure. The magnitude of this increase is also correlated with the degree of the alkali precoverage.

On the oxygen induced stabilization of adsorbed alkali atoms

Abstract Due to the dipolar repulsion interaction, adsorbed alkali atoms on transition metals are not thermally stable at the working temperature of some catalysts. It is shown that the coadsorption of oxygen and alkali atoms appreciably increases the desorption energy of the alkali adlayer which becomes thermally stabilized, in qualitative agreement with previously published experimental data.

On the oxygen induced stabilization of adsorbed alkali atoms

Due to the dipolar repulsion interaction, adsorbed alkali atoms on transition metals are not thermally stable at the working temperature of some catalysts. It is shown that the coadsorption of oxygen and alkali atoms appreciably increases the desorption energy of the alkali adlayer which becomes thermally stabilized, in qualitative agreement with previously published experimental data.

Self-consistent calculations of work functions and relaxations of metals upon alkali adsorption

We present self-consistent calculations of the work function of the clean aluminum (111), (100) and (110) faces, taking into account the surface relaxation and we study the changes of the substrate work function due to adsorption of alkali atoms. The calculations are done by using the local density approximation and including in a total way the ion pseudopotential in the self-consistent procedure. Our finding shows: (i) the relaxation of metal layers on the work function are not important, (ii) in order to explain the work function dependence on coverage it is necessary to introduce an adsorbate-metal substrate distance that increases with coverage. Our calculations show excellent agreement with experiment on Cs/Al(111).

Theoretical aspects of surface reactions

A short review is given of our present understanding of the trends in the chemisorption energies and activation energies for dissociation of simple gas molecules on the transition metals. The effect of adsorbed alkali atoms on the activation energy for dissociation is also discussed. This is then used to explain the trends in activity along the transition metal rows and the promoting effect of K for the ammonia synthesis reaction. The basis for the description is the development of a kinetic model for the ammonia synthesis which can describe quantitatively the macroscopic kinetics of a commercial catalyst under industrial conditions. The model relates the reaction rate directly to the properties of the chemisorbed reactants, intermediates and product as measured for model single crystal systems under ultrahigh vacuum conditions.

Work Function of Alkali-Atom Adsorbed Graphite

We propose a simple layer model in which real atomic configurations are simulated by multi-layers with homogeneous electron density on each layer, in order to investigate work function of alkali-atom adsorbed graphite. Charge transfer from an alkali-atom layer to graphite layers is determined within the model by minimizing total energy. The calculated decrease in work function of graphite upon alkali-atom adsorption agrees satisfactorily with the experimental data.

Charge transfer on alkali adsorption on transition metal surfaces

Abstract A simple model of the electronic charge transfer from adsorbed alkali atoms to a transition metal substrate is proposed and applied to experimental data. The model includes the dependence both on the type of alkali (through its electronegativity) and on the crystallographic orientation of the adsorbent (through its work function).

Charge transfer on alkali adsorption on transition metal surfaces

A simple model of the electronic charge transfer from adsorbed alkali atoms to a transition metal substrate is proposed and applied to experimental data. The model includes the dependence both on the type of alkali (through its electronegativity) and on the crystallographic orientation of the adsorbent (through its work function).

Electronic excitations in K monolayer adsorbed on Si(100) 2×1

Abstract K adsorption on Si(100) 2 sx 1 has been studied by using angle-integrated electron energy loss spectroscopy, LEED, AES, Δφ and core-level excitations as a function of K coverage. At coverages of 0.25 the insulator-to-metal transition in the K overlayer is observed and the intensity of the overlayer plasmon starts to increase. The plasmon has a one-dimensional character from the standpoint of the structural model of K adsorbed. The energy loss of the plasmon decreases slowly to 1.8 eV. In the case where alkali atoms are adsorbed on metal surface, the energy loss of the plasmon increases with increasing coverage. The different behaviour is discussed based on the structure of adsorbed alkali atoms.

Electronic excitations in K monolayer adsorbed on Si(100) 2 × 1

K adsorption on Si(100) 2 sx 1 has been studied by using angle-integrated electron energy loss spectroscopy, LEED, AES, Δφ and core-level excitations as a function of K coverage. At coverages of 0.25 the insulator-to-metal transition in the K overlayer is observed and the intensity of the overlayer plasmon starts to increase. The plasmon has a one-dimensional character from the standpoint of the structural model of K adsorbed. The energy loss of the plasmon decreases slowly to 1.8 eV. In the case where alkali atoms are adsorbed on metal surface, the energy loss of the plasmon increases with increasing coverage. The different behaviour is discussed based on the structure of adsorbed alkali atoms.