Photoemission studies of Ar,Kr and Xe adsorbed on Al(111): Dipole moments,polarizabilities and spatial distributions

Submonolayers of rare gases adsorbed on Al(111) have been studied using photoemission techniques. Coverage-dependent core level binding energy shifts and work function changes have been measured; polarizabilities and dipole moments are deduced. Results indicate that adatom spatial distributions for Xe submonolayers adsorbed on Al(111) at 40 K are best described by a random 2-dimensional distribution, and there is negligible charge transfer from the Al substrate onto the Xe adatoms in the photoionization process.     

A LEED investigation of Xe adsorption on W(110)

Four ordered LEED patterns are observed for Xe adsorption on W(110) for temperatures between 77 and 90 K. A (2 × 2) structure with an area per Xe atom of 28.3 Ų is transformed into two coincidence structures which correspond to a disordered (100) Xe layer. The area per Xe atom in these structures is 17.6 and 20.2 Ų. Xe adsorption on oxygen covered W(110) leads to one-dimensional disorder in the structures observed on clean W(110) without the formation of new structures.     

Electron impact desorption of Xe from the tungsten (110) plane

The electron stimulated desorption of Xe adsorbed on the clean and on oxygen and CO covered tungsten (110) surfaces has been investigated. Only neutral Xe desorption was observed; for Xe on clean W a very small initial regime with cross section 10^?17cm² is followed by a slow decay with cross section 3×10^?19cm². The Xe yield varies nonlinearly with coverage, suggesting desorption from edges of islands or from sites with less than their full complement of nearest neighbor Xe atoms. Desorption from oxygen or CO covered surfaces results in an apparent desorption cross section identical to that of the underlying adsorbate. This results from a kicking off of Xe by electron desorbed O or CO. The true cross sections for these processes are ～10^?14cm² for Xe-0 and ～10^?15 cm² for Xe-CO. Some speculations about the mechanism, particularly the absence of ions are presented.     

Adsorption of molecular nitrogen by the W(110) plane

Characteristics of the adsorption of nitrogen on the (110) plane of tungsten were determined by thermal desorption and work function measurements. The low temperature γ-N₂ state desorbs with first order kinetics and an activation energy of 6 kcal mole^?1. The absence of isotope mixing between ¹⁴N₂ and ¹⁵N₂ demonstrates γ-N₂ is adsorbed molecularly. Monolayer coverage shows a decrease of 0.19 eV in work function. A Topping model plot indicates the layer is immobile at 123 K.     

LEED,Auger and work function study of iodine adsorbed on W(110)

The adsorption of iodine on a W(110) surface has been studied by LEED, Auger and work function changes. LEED has revealed several phases which desorb in different temperature regimes and are accordingly designated γ, α, β₁, β₂, and β₃. The γ and α phases exhibited p(2 × 2) and p(2 × 1) surface nets respectively with coherently positioned antiphase boundaries which produced a splitting of selected LEED beams. The separation between the antiphase boundaries of the α phase increased with decreasing coverage. Both the γ and α phases were associated with molecularly absorbed iodine. The three β phases were associated with dissociatively adsorbed iodine which formed chain structures on the surface with the arrangement of iodine atoms within each chain being unique to the particular phase. Continuous changes in coverage then occurred by sheets of these chains shearing to produce packing faults at the resulting shear lines. This shearing process occurred coherently in the β₁ and β₃ phases and incoherently in the β₂ phases. In the former cases, the effect was seen by the continuous movement of coherent LEED beams with changing coverage. A phase diagram was constructed to describe the relative coverages and thermal stability of the phases. The characteristic Auger electron emission of iodine was observed at 495 eV and used to estimate the surface coverage. The work function was found to decrease by 0.4 eV with the adsorption of the first half monolayer and remained unchanged with further adsorption.     

Angle-resolved investigation of Auger electrons from Cu and Au adsorbed on W(110)

The angular distribution of Cu M_2,3VV and Au N_6,7VV Auger electrons from Cu and Au mono- and double layers on W(110) is measured with the goal of obtaining information on the contribution of the backscattered wave on the angular distribution of Auger electrons from adsorbed atoms.     

Temperature dependence of the work function of Cu layers on a W(110) plane

Temperature coefficients of the work function for Cu layers in the range 0 ≤ n ≤ 6 where n is the number of monolayers were measured from 90 to 500 K by a Kelvin probe method. dφ/dT is negative at all coverages and has a strong maximum of −1.5×10⁴eV/K at n=1 at 90 K. A qualitative discussion of these findings is given. The design and operating characteristics of the Kelvin probe are also described.     

Xe and Kr adsorption on the Si(111) 7 × 7 surface

A study of adsorption of Xe and Kr on the Si(111) 7 × 7 surface is presented. Low energy electron diffraction indicates the substrate structure is not appreciably modified by the adsorption. The data consist primarily of isobars, i.e. the amount adsorbed as a function of temperature at fixed pressure. The stepwise adsorption of each of the first several adatoms per surface unit mesh is resolved. This indicates a sequence of distinguishable adsorption sites in the mesh each of which can be occupied by a single atom. There are appreciable differences in the measured binding energies between the various sites. Pairwise sums of 6–12 potentials have been used to estimate binding energies expected for sites presented by proposed structural models of the surface. The observed sequence of sites seems to exclude buckled models and those models which involve only arrays of identical entities. No presently published model is consistent with these data in detail. The triangular island models with some modifications might present sufficiently large binding energies and rich sequences of sites. Whatever the actual structure, it must be more complex than those which have usually been considered.     

Isotope effects in electron impact desorption of CO and O2 adsorbed on the (110) plane of tungsten

Results on the isotope effect for total and ionic desorption cross sections in the electron impact desorption of various binding states of CO on the (110) plane of tungsten, and of oxygen on this plane are presented and discussed. It is shown that the observations allow a dissection of cross sections into excitation cross sections and escape probabilities, and that the latter can be used to estimate lifetimes of excited or ionic states. It is found that excitation cross sections for total desorption are of the order of 10^?16–10^?17 cm², but seem to be significantly smaller in some cases for excitation to ionic states, suggesting that different excitations are involved. In all cases examined here the isotope effect for total desorption is much smaller than for ion production. This can be explained by the fact that ion lifetimes are somewhat shorter than those of excited neutrals. Lifetimes are estimated, in the cases examined, to be of the order of 10^?14s.     

The ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT _c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB _hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μ_B could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.     

Photoemission study of alkali/GaAs(110) interfaces

A detailed core-level photoemission study of interfaces between thin alkali films andn-orp-type GaAs (110) formed at different substrate temperatures 85 K and 300 K) is reported. All the interfaces grown at 85 K (with Na, K, Rb, and Cs) were found to be non-reactive, while at 300 K, the interface with Na is reactive and that with Cs remains non-reactive. In case of the non-reactive interfaces, a strong band bending of 1.0 eV is observed forp-GaAs at alkali coverages as low as 0.01 monolayers, but practically none forn-GaAs. This striking asymmetry in band bending is interpreted as a consequence of the donor character of the alkali atoms. On the other hand, an approximately symmetric band bending at low coverages is observed for the reactive interfaces of Na withn- andp-GaAs and assigned to defect states. For high alkali coverages (>2 monolayers), the final band bending is characterizeds by the same Fermilevel position forn- andp-GaAs, independent of the reactivity of the interface, and assigned to metal-induced gap states. Furthermore, systematic trends along the alkali series in Fermi-level position ionization energy, plasmon-loss features, and layer-dependent binding-energy shifts of alkali core levels are discussed.