Stig-Åke Lindgren

Energy shifts of a Cu(111) surface band upon adsorption of cesium and oxygen

Abstract Angle resolved photoelectron energy spectra show that a Cu(111) surface band shifts rigidly to lower energy upon cesium adsorption and to higher energy upon exposure of a Cs covered Cu(111) surface to oxygen. For Cu(111)/Cs the energy shift at low coverage corresponds to a transfer of 0.5 electrons per adsorbed Cs atom to the surface state. We associate the shift upon oxygen exposure with the formation on the Cu(111) surface of cesiumoxide molecules which are interacting weakly with the substrate.

Surface state energy shifts by molecular adsorption : CO on clean and Na covered Cu(111)

Angle resolved photoelectron energy spectra recorded in the near UV show that the surface state 0.4 eV below EF of Cu(111) shifts to higher energy upon CO adsorption. The surface band related emission intensity is reduced by the adsorption at a rate suggesting that each adsorbed molecule wipes out the surface state over an area corresponding to seven surface layer Cu atoms. The surface state energy shift is not as closely related to the adsorbate induced workfunction change as found theoretically and experimentally for alkali adsorbates. The influence on the shift of the workfunction and initial energy of the surface state is studied by preadsorbing Na on the Cu(111) surface.

Sodium induced structure in UPS spectra of Cu(111)/Na

Abstract UPS Spectra of sodium monolayers evaporated onto Cu(111) are presented. We interpret sodium induced structure below the Cu d-band as due to photoemitted 3d electrons which have experienced an energy loss by exciting a sodium monolayer plasmon. An experimental difficulty is that even small amounts (

Dilution of the electron gas in a Cs monolayer on Cu(111) by adsorption of oxygen

Abstract The characteristic loss energy of plasmons in a close packed Cs p(2 × 2) monolayer on Cu(111) is found to decrease gradually upon increasing the amount of oxygen adsorbed, the oxygen appearing to act as a thinner of the electron gas in the overlayer. We observe no dispersion for the monolayer plasmon.

Some vibrational properties of clean and water vapour exposed Cu(111)/Na,K

Abstract When Na (K) is adsorbed on Cu(111) and the coverage is below half of a full monolayer electron energy loss spectra show a peak at 21 meV (13 meV) ascribed to the excitation of Na (K) vibrations perpendicular to the surface. From the intensity of the loss peak at low alkali metal coverage a dynamic charge of approximately 0.5e (0.6e) is obtained assuming dipolar scattering. The value agrees well with theoretical estimates obtained from the dependence of the induced dipole moment on the distance from the substrate surface and provides strong support for the description of the adsorbate as partly ionic at low monolayer coverages. If the Na covered Cu(111) surface is exposed to water vapour there is a rapid uptake only if there is more than 10% of a full monolayer of Na on the surface. The existence of a Na precoverage threshold is revealed by the exposure induced vibrational losses and work function changes. Above threshold there is, upon H 2 O exposure, an increase of the work function and strong changes in the low loss energy region of the electron energy loss spectra with peaks appearing at 18 meV and 36 meV loss energy.

Oscillatory photoemission cross section for alkali monolayer quantum well states

A strong oscillatory photon energy dependence, observed in the photon energy range between 5 and 26 eV, for the intensity of photoemission peaks due to quantum well states in Na and Cs monolayers on Cu(111) is ascribed to interference between the contributions to the outgoing photoelectron wave from the two tails of the quantum well state.

Submonolayer Na coverage threshold for water vapour adsorption on Cu (111)/Na

Abstract When Na-covered Cu(111) is exposed to water vapour the adsorption proceeds rapidly only if the amount of Na on the surface is greater tha 10% of full monolayer coverage. The adsorption is monitored via the electron energy loss spectrum which for the coadsorption system shows vibrational excitations at 18 and 36 meV while for Na on Cu (111) the adsorbate vibrations perpendicular to the surface produce a loss peak at 21 meV.

The Cu conduction band gap at the L point determined by photoemission and electron reflectivity measurements

Abstract The energies of the Cu conduction band gap edges around the Fermi level at the L point of the Brillouin zone are determined by angle resolved photoemission and electron reflectivity measurements on Na covered Cu(111).

Ba adsorption on Cu(111) studied by photoemission

Abstract As increasing amounts of Ba is adsorbed on Cu(111) a surface state shifts to progressively lower energy relative to the Fermi energy. No photoemission peak due to a partly filled Ba 6s resonance is found at low Ba coverages. Low photon energies are used (

STRUCTURE CHANGES FOR Cu(111)/Li OBSERVED VIA DISCRETE QUANTUM WELL STATES

Via photoemission in the near UV from discrete quantum well states, four different types of atom rearrangements are observed for Cu(111)/Li. In the monolayer range a gradual energy shift reflects a gradual compression of the Li layer as more atoms are adsorbed. The onset of Li substitution and formation of a (2×2) structure when, at RT, monolayer Li coverage is exceeded is monitored via the appearance of a quantum well state characteristic of the new phase. A small energy shift indicates that, if it is cooled (170 K), the new structure is unstable with respect to Li adsorption. Finally, an energy downshift with increasing coverage observed for a state characteristic of 2 ML Li is ascribed to an increasing area for one-atomic-layer-high islands formed by Li atoms on top of the first full layer.