Alkali Coverage Research Articles

The structure of 2D charge transfer salts formed by TCNQ/alkali metal coadsorption on Ag(111)

The structure of coadsorption phases formed on Ag(111) by TCNQ (7,7,8,8-tetracyanoquinodimethane) with Cs are compared with previously reported coadsorption phases formed with K, following investigation by scanning tunnelling microscopy (STM), low energy electron diffraction, soft X-ray photoelectrons spectroscopy and normal incidence X-ray standing waves (NIXSW). For each alkali we identify two ordered phases, one with an alkali: TCNQ stoichiometry of 1:1 and the other 2:1. STM images show the molecular organisation is the same for Cs and K, although only the K2TCNQ phase is commensurate with the substrate. A previously-published detailed structure determination of the K2TCNQ phase, complemented by density function theory calculations that identify bonding strengths, showed that the binding within the layer is much stronger than that of the layer to the substrate. Insensitivity to commensuration is thus to be expected. The situation for KTCNQ and CsTCNQ is less clear; these ordered incommensurate overlayers clearly have strong intralayer bonding, but the relative strength of the average overlayer-substrate bonding is unknown. NIXSW data show that the alkalis in these phases occupy adsorption sites far more distant from the substrate than the TCNQ molecules when compared to the near coplanar alkali-TCNQ geometry of K2TCNQ and Cs2TCNQ. Ultraviolet photoelectron spectra show increasing bonding shifts of TCNQ orbital states with alkali coverage.

Time-resolved photoemission study of the electronic structure and dynamics of chemisorbed alkali atoms on Ru(0001)

We investigate the electronic structure and photoexcitation dynamics of alkali atoms (Rb and Cs) chemisorbed on transition-metal Ru(0001) single-crystal surface by angle- and time-resolved multiphoton photoemission. Three- and four-photon photoemission (3PP and 4PP) spectroscopic features due to the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ resonances arising from the $ns$ and $np$ states of free alkali atoms are observed from $\ensuremath{\sim}2$ eV below the vacuum level in the zero-coverage limit. As the alkali coverage is increased to a maximum of 0.02 monolayers, the resonances are stabilized by formation of a surface dipole layer, but in contrast to alkali chemisorption on noble metals, both resonances form dispersive bands with nearly free-electron mass. Density functional theory calculations attribute the band formation to substrate-mediated interaction involving hybridization with the unoccupied $d$ bands of the substrate. Time-resolved measurements quantify the phase and population relaxation times in the three-photon photoemission (3PP) process via the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ resonances. Differences between alkali-atom chemisorption on noble and transition metals are discussed.

Photoelectron spectroscopy for probing surface reactions at the CO/K/Cu(1 1 5) interface

The adsorption of carbon monoxide on the potassium modified Cu(1 1 5) surface was investigated using photoelectron spectroscopy based on synchrotron radiation. From detailed analysis of the 1 s core levels in combination with existing knowledge, the assignment of surface species is performed. It is demonstrated that in dependence of the alkali coverage, several adsorption states of CO are present on the interface at 135 K. From the temperature dependence of the C 1 s and O 1 s profiles it is established that surface reactions based on CO dissociation start from 223 K over an interface with a potassium coverage close to half a complete K overlayer. The role of potassium as a reordering environment of adsorbed CO, leading to molecule dissociation and disproportionation is proposed. It is observed that a higher density of potassium on the substrate surface blocks adsorption sites for incoming CO molecules and no dissociation takes place.

Adsorption Properties of the Cu(115) Surface: Basic Interfaces

The interfaces: K/Cu(115) and CO/Cu(115) have been characterized using surface sensitive techniques, including low energy electron diffraction and photoelectron spectroscopy. K adatoms show tendency to occupy the sites close to the step edges. At low temperature (near 125 K), after completion of two layers, potassium grows in 3D islands (the Stranski-Krastanov mode). At higher temperature, e.g. at room temperature, potassium introduces reconstruction of the substrate even at low coverages. Calibration of the alkali coverage, up to completion of the first layer, using the work function changes curve has been confirmed as a very convenient and precise procedure. The adsorbed state of CO at 130 K has been identified by registration of core levels obtained by the use synchrotron radiation photoelectron spectroscopy. The characteristics of the main is and satellite peaks have been analyzed in context of substrate geometry and compared with the ones of other copper planes. There are no indications of dissociative adsorption of CO, only residual carbon and oxygen were found after adsorbate desorption around 220 K. CO molecules show a strong tendency to on top adsorption in sites far from the step edges of the Cu(115) surface.

Vibrations of alkali metal overlayers on metal surfaces

We review the current progress in the understanding of vibrations of alkalis adsorbed onmetal surfaces. The analysis of alkali vibrations was made on the basis of availabletheoretical and experimental results. We also include in this discussion our recentcalculations of vibrations in K/Pt(111) and Li(Na)/Cu(001) systems. The dependence ofalkali adlayer localized modes on atomic mass, adsorption position and coverage as well asthe dependence of vertical vibration frequency on the substrate orientation isdiscussed. The square root of atomic mass dependence of the vertical vibration energyhas been confirmed by using computational data for alkalis on the Al(111) andCu(001) substrates. We have confirmed that in a wide range of submonolayercoverages the stretch mode energy remains nearly constant while the energy ofin-plane polarized modes increases with the increase of alkali coverage. It wasshown that the spectrum of both stretch and in-plane vibrations can be verysensitive to the adsorption position of alkali atoms and substrate orientation.

Vibrational measurements of Na/Ni(111) and (Na + CO)/Ni(111)

The vibrational properties of Na atoms and of Na coadsorbed with CO on Ni(111) have been studied by high-resolution electron energy loss spectroscopy. Loss measurements showed a significant weakening of the alkali–substrate bond as a function of the alkali coverage. Moreover, we found that coadsorbed CO molecules dramatically influence the vibrational properties of Na adatoms. The Na–Ni stretching frequency (22 meV) measured on the Na/Ni(111) system shifted down to 13 meV for the (Na + CO)/Ni(111) surface. This unexpected result was ascribed to a charge transfer from Na to CO. Present findings give new insights on the nature of the alkali–substrate and alkali–CO bond.

Alkali adsorption on Ni(1 1 1) and their coadsorption with CO and O

The adsorption of alkalis (Na, K) on Ni(1 1 1) and their coadsorption with CO and O were studied by high-resolution electron energy loss spectroscopy. Loss measurements of clean alkali adlayers provided the expected behaviour of the alkali-substrate vibration energy as a function of the alkali coverage. This result was achieved by eliminating any trace of CO contamination from the alkali adlayer. As a matter of fact, a significant softening of the alkali–Ni vibration energy was revealed in the alkali + CO coadsorbed phase. Moreover, alkali coadsorption with oxygen caused a weakening of the O Ni bond and a strengthening of the alkali Ni bond.

Nature of the Alkali Surface Bond at Low Coverages Investigated by Vibrational Measurements

High-resolution electron energy loss spectroscopy has been used to study the nature of the Na/Ni(111) and K/Ni(111) bond at very low coverages. Measurements taken for very clean alkali layers provided, for the first time, the expected behavior of the alkali−substrate stretching energy as a function of the alkali coverage. Moreover, we found that the Na−Ni (K−Ni) bond was dramatically influenced by coadsorbed CO molecules causing a red shift of the Na−Ni (K−Ni) vibrational frequency. We argue that this effect could be at the base of discrepancies existing between the calculated and measured behavior of the vibrational energies as a function of the alkali coverage.

Electronic properties of self-assembled quantum dots of sodium on Cu(1 1 1) and their interaction with water

The electronic properties of thin films of Na on Cu(1 1 1) and their interaction with water have been investigated at room temperature by high resolution electron energy loss spectroscopy. The first Na layer is characterized by two features tentatively assigned to charge density waves. The second Na layer grows as small islands. The loss spectrum of this layer shows a feature at 3.0 eV identified as a Mie resonance. Increasing alkali coverage, Na islands form a continuous film, as indicated by the appearance of a Na surface plasmon and by the disappearance of the Mie resonance. Water vapour strongly interacts with Na layers as shown by the OH–Na vibration whose frequency shifts from 36 meV to 53 meV as a function of alkali coverage.

Relative stability of missing-row reconstructed (1 1 0) surfaces of noble metals

The missing-row reconstruction has been observed experimentally on clean Au, Pt and Ir, while Cu and Ag are dormant cases in that the reconstruction can be provoked only by modest alkali coverage. The experimentally observed deconstruction and roughing transitions of Au(1 1 0) and Pt(1 1 0) are interesting realizations of two-dimensional Ising and XY models. The detailed understanding of the structures and energies of the missing-row configurations is also important for the constructions of many-atom model such as the embedded-atom potentials. We have carried out density-functional molecular-dynamics calculations for the missing-row reconstructed (1×2), (1×3), and (1×∞) surfaces. The calculation results show that the delocalization of d-electrons and the accumulation of sp-electrons near the surface associated with the multilayer relaxation are important factors in stabilizing the experimentally observed (1×2) missing-row structure for Au and Pt. For Ir(1 1 0), a faceted (3 3 1) surface is shown to be the lowest energy configuration, in agreement with experimental observation. These results shed light on the relative stability of the missing-row configurations, and are useful for the construction of improved semi-empirical potentials, especially for the long range interaction.

Dephasing and population decay of the Cu [formula omitted] and [formula omitted] image potential states induced by scattering on low coverage alkali adsorbates

Image states at surfaces are very sensitive to the presence of impurities on the surface. A theoretical study of the effect of alkali adsorbates on the image state dynamics is performed for the Cs/Cu(1 0 0) and Na/Cu(1 1 1) systems using a wave-packet propagation approach. Scattering of the image state electron by an alkali adsorbate is shown to induce large inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. Even very low alkali coverage in the 10 −3 monolayer range can effectively affect the image states, reducing their lifetime and broadening their photoemission peaks. The present theoretical results are discussed and compared with available experimental results on the perturbation of image states by various adsorbates.

Thermal and adsorbate induced plasmon energy shifts in graphite

Electron-energy-loss spectroscopy has been used to study plasmon excitations in pristine and alkali metal (Na, K, Cs) dosed highly oriented pyrolitic graphite in the temperature range 85–400 K. The energy and line width of the π-band E∥ c plasmon in pristine graphite was found to be extremely sensitive to substrate temperature and to the presence of adsorbates in qualitative accord with previously reported observations and theoretical calculations. After increasing monotonously with coverage (following square root dependence) the plasmon energy levels off above specific alkali coverages ( θ Na≈1.45, θ K≈0.25, and θ Cs≈0.2) at a common plasmon energy ℏ ω≈250 meV. The results are interpreted as evidence for formation of alkali-like bands in the topmost graphite layers.

Electrochemical promotion of catalytic reactions using alkali ion conductors

Electro-pumping of Na or K to the surfaces of metal catalyst films can lead to very large and reversible changes in both catalytic activity and selectivity for a range of different reactions. This is illustrated with respect to the catalytic reduction of NO by CO and by propene over Pt, Rh and Cu, the combustion of propene over Pt, and the selective hydrogenation of acetylene over Pt. The results are interpreted in terms of alkali-induced changes to the adsorption state of reactants (NO reduction, propene combustion) or intermediate species (acetylene hydrogenation). At low alkali coverages, changes in catalyst potential scale linearly with the measured work function change. XP and Auger spectroscopy shows unambiguously that the changes in catalytic behaviour are due to changes in the surface concentration of the alkali. These electron spectroscopic data, supplemented by X-ray absorption spectra, show that the chemical state of the alkali promoter is dependent on the reactive gas environment. The fundamental insight provided by electrochemical promotion experiments can be used to guide the synthesis of practical dispersed catalysts.

HREEL Study of the Coadsorption of Sodium and Oxygen on Ni(111)

The coadsorption of oxygen and sodium on Ni(111) was studied by high resolution electron energy loss spectroscopy. Measurements were performed for a definite sodium precoverage, as a function of oxygen exposure and temperature. For a coverage of two layers of sodium, the loss spectra show the vibrational features related to Na– O 2 and O–O bonds of superoxide NaO 2 molecules. The annealing up to 450 K causes the decomposition of these complexes and the reappearance of the stretching frequencies of isolated Na–O and O–Ni species. At 550 K, only the oxidation of the Ni substrate occurs. The spectra are correlated to work function measurements, and the behavior of the work function versus oxygen exposure, for high alkali coverages, is ascribed to a reionization of the metallic Na layer, rather than to a penetration of oxygen atoms underneath the Na layer.

H 2O adsorption on alkali (Li, Na and K) precovered Ni(775)

The coadsorption of H 2O and alkalis (Li, Na and K) on a stepped Ni(s)(111) surface with nominal (775) orientation was studied by UPS (UV-light induced photoelectron spectroscopy), LEED, TDS (thermal desorption spectroscopy) and work function change measurements (Δ Φ). On clean Ni(775) five H 2O desorption states denoted as A ( T m=160 K), B (175 K), C (225 K), D (260 K) and E (340 K) were detected. The A and B states are also observed at similar desorption temperatures on flat Ni(111) and are therefore attributed to H 2O adsorption on the (111) terraces of Ni(775) ( B state) or to the desorption of ice multilayers ( A state). Consequently, the C, D and E states are ascribed to the adsorption and a partial dissociation at step sites. The C state is associated with a “five peak” UPS spectrum interpreted as due to the overlap of the 1b 1, 3a 1 and 1b 2 orbitals of molecular water and 2 σ and 1 π emission from OH groups. A preferred adsorption of the alkalis at step sites for low coverages is deduced from the Δ Φ changes. For Li adsorption the saturation of the step sites is accompanied by a significant change in the slope of the Δ Φ versus coverage curve. The increase of the effective dipole moment (less negative for low coverages, adsorption at step sites) is caused by screening of the positively charged alkalis in a step-down adsorption geometry. For the coadsorption of H 2O and alkalis (at low alkali precoverages) only H 2O desorption states C, D and E from step sites are influenced. The C state shifts to lower peak temperature, whereas the D and E states (recombination from dissociation products) decrease in intensity with increasing alkali coverage. Alkali coadsorption on Ni(775) is accompanied by the dissociation of water, depending on the nature and the precoverage of the alkali. The tendency to induce dissociation is highest for Li and decreases with increasing ionic radius from Na to K.

Li + neutralisation in back-scattering from alkali/Al(100) surfaces: comparison between the various alkalis

The Li + ion neutralisation by back-scattering from an Al(100) surface partially covered by alkali adsorbates is theoretically studied, using a previously developed model representation of the Li + ion–alkali adsorbate–Al substrate system [A.G. Borisov, G.E. Makhmetov, D. Teillet-Billy, J.P. Gauyacq, Surf. Sci. 350 (1996) L205]. Three alkali adsorbates are studied: Na, K and Cs and the differences between their effects are discussed. In the back-scattering geometry, the neutralisation probability is found to be quite different for lithium bouncing from the alkali adsorbate or from the Al substrate; it is also different for the various alkali adsorbates. The present results are found to be in quantitative agreement with the recent experimental data of Weare and Yarmoff [C.B. Weare, J.A. Yarmoff, Surf. Sci. 348 (1996) 359]. These results confirm the importance of the local variation of the charge transfer process on the surface, as well as the capability of the charge transfer process to probe the local electronic potentials. A large probability for Li(2p) excited atom formation by collision on the adsorbate sites is also found in the present study, even in the limit of very low alkali coverages.

Interactions between H, CO and CO 2 on an K-modified Cu(110) surface

This paper presents new results, in terms of reactions, dissociation and site blocking, on the interactions between H and CO 2, H and CO on an alkali-modified copper surface. In addition, H adsorption on the K/Cu(110) interface was performed. The experimental methods used include photoelectron spectroscopy based on synchrotron radiation, high-resolution electron energy-loss spectroscopy (HREELS), work-function measurements and low-energy electron diffraction (LEED). In order to leave free Cu sites, the alkali coverage chosen was mainly half a monolayer. The adsorption of hydrogen on Cu(110) and the K/Cu(110) interface gives rise to two valence-band states. The binding energies and populations of these states show different behaviour with increasing temperature. It is found that when the H/K/Cu(110) interface is exposed to CO 2 at 130 K, formate is synthesised and carbonate is formed. No CO is present on the surface. The reactions depend on the adsorption sequence. CO 2 in an amount corresponding to saturation was admitted to the K/Cu(110) interface at the same temperature. Both carbonate and CO were present on the surface. It is concluded that when the CO 2/K/Cu(110) interface is exposed to H, carbonate is decomposed to CO. Hydrogen adsorption on the CO/K/Cu(110) interface at 146 K results in a weakening of the internal CO bond, as shown by the increase in the 4σ satellite intensity in the UPS spectrum and the increase in the binding energies of the CO orbitals. The HREEL spectra show indications of hydroxides and carbon-hydrogen species. Adsorption in the reverse order (i.e. CO on H/K/Cu(110)) gives no CO adsorption due to site blocking.

Rare gas coadsorption with Cs or K on Ag(111)

Rare gas (RG) coadsorption with submonolayer amounts of Cs or K on Ag(111) was studied using low-energy electron diffraction (LEED). A crossover in the alkali-RG interaction from repulsive to attractive was observed as a function of alkali species and of alkali coverage. The KRG interaction was observed to be repulsive at all coverages, while the CsRG interaction was observed to be attractive at low Cs coverages and apparently repulsive at high Cs coverages. For the K + RG adsorption system, desorption data were analyzed to determine the spreading pressure in the alkali layer, thus showing that RG can be used as a 2D manometer in some coadsorption systems. From the spreading pressure it is possible to obtain some information about the properties of the adsorbed alkali such as the energy differences between commensurate and incommensurate phases. We also demonstrate that work function measurements from such coadsorption systems do not necessarily have a simple interpretation.

Theoretical and experimental study of N −( 1D) formation in nitrogen collisions with a metal surface

The dynamics of nitrogen collisions with metals partially covered by alkali atoms is studied both experimentally and theoretically. Our attention focuses on the formation of N −( 1D) metastable ions and their interaction with the surface. We present the electron energy spectra induced by slow collisions of N + ions with partially cesiated Pd(111) surfaces under grazing incidence. These spectra display, as a function of Cs coverage, a sharp feature which is due to the autodetachment of N −(2p 4, 1D) to the N(2p 3, 4S) ground state. Our calculations, performed with the coupled angular mode (CAM) method on the basis of the resonant electron exchange between the nitrogen atom in states of the 2p 3 configuration and the metal surface, consistently explain how negative ions formed close to the surface can survive against electron loss to the metal during the outgoing trajectory and can later decay as free ions. In order to understand the alkali coverage dependence of the N −( 1D)-N( 4S) peak intensity, the local character of the nitrogen interaction with the surface partially covered by adsorbate atoms has been taken into account.

The effects of charge promotion on the measured charge state of low-energy Li + scattered from alkali/Al(100)

Charge promotion of an inner-shell electron during ion bombardment of a surface can strongly affect the neutralization of scattered particles. In this work, the neutral fractions for 2.0 keV 7Li + scattered from clean and alkali-covered Al(100) are measured with time-of-flight spectroscopy. During hard collisions, a fraction of the Li particles that scatter from Al atoms undergo a promotion of their 1s electrons above the Fermi level, with subsequent Li 1s hole creation. Differences are observed between the neutral fractions of the scattered promoted and non-promoted projectiles, and these neutral fractions depend on the alkali coverage. Secondary electron spectra collected during Li bombardment are used to identify the decay channels of the Li 1s hole. The results indicate that the Li 1s hole decays both near the surface via an Auger process and in vacuum via auto-ionization. Therefore, the lifetime of the 1s hole is on the order of the scattering time.