Plasmons in monolayer Na,K and Rb films adsorbed on Ni(100)

Electronic excitations in denser monolayer Na, K and Rb films and Na duolayer films adsorbed on a Ni(100) surface have been investigated using Electron Energy Loss Spectroscopy (EELS). Lateral adatom distributions were monitored by LEED. Angular integrated EEL spectra from the ordered c(2 × 2)Na, coverage θ = 0.5, and the ordered hexagonal structures of K and Rb, θ = 0.29, show prominent losses at 3.1, 1.9 and 1.7 eV, of presumably collective nature. The loss energies shift with coverage as ∝ θ^0.4 and as ∝ θ^0.8 for the Na and K, Rb respectively. Angular resolved EEL spectra indicate an only weak dependence of the loss energies on the momentum transfer, Q. In particular the K and Rb losses seem to pass through shallow energy minima, which is predicted by the “box model”. Low energy losses observed at ?1.3 and ?1.0 eV for the c(2 × 2)Na and the hexagonal K and Rb, respectively, are tentatively identified with interband excitations. The observed interband energies yield, when introduced in the “box model”, 3.1., 2.3 and 2.4 eV for the Na, K and Rb, Q = 0 plasmon energies, which is in fair agreement with the observed plasmon loss energies.     

Monolayer K-films on Ni(100) — II. Electronic excitations

Electronic excitations in a K layer adsorbed on Ni(100) have been observed by means of electron energy loss spectroscopy. The observed excitation energy depends on the density of K atoms in the layer. In the low density range the loss energy decreases from 3.5 to 1.5 eV as the density increases from 6.10¹³ to 3.10¹⁴ K atoms per cm². This loss is interpreted to be due to an excitation from below the Fermi level to the shifted K valence level Increasing the density further from 3.10¹⁴ to 6.10¹⁴ K atoms per cm² yields a loss peak that increases in energy from 1.5 to 2.3 eV. This loss peak merges into the surface-bulk plasmon complex for a thick K film and the energy vs density dependence is compatible with a plasmon excitation in the K layer.     

The adsorption of oxygen and carbon monoxide on cleaved polar and nonpolar ZnO surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) with primary energies e₀ ? 80 eV has been performed on ultrahigh vacuum (UHV) cleaved nonpolar (11?00) and polar zinc (0001) and oxygen (0001?) surfaces of ZnO to study the adsorption of oxygen and carbon monoxide. Except for CO on the nonpolar surface where no spectral changes in ELS are observed a surface transition near 11.5 eV is strongly affected at 300 K on all surfaces by CO and O₂. At 300 K clear evidence for new adsorbate characteristic transitions is found for oxygen adsorbed on the Zn polar surface near 7 and 11 eV. At 100 K on all three surfaces both CO and O₂ adsorb in thick layers and produce loss spectra very similar to the gas phase, thus indicating a physisorbed state.     

Observation of interface plasmon excitation in photoemission spectroscopy from the heterostructures Cu/RbF/GaAs(100) and Cu/RbF/Ge(100)

Synchrotron radiation photoelectron spectra taken at 100 eV photon energy have been measured to characterize the interface reactions of the metal-insulator-semiconductor systems Cu/RbF/GaAs(100) and Cu/RbF/Ge(100). In comparision, similar sequences are studied on the Cu/GaAs(100) and Cu/Ge(100) interfaces without the RbF interlayer. After Cu-deposition of 1–4 Å on RbF-covered (10–14 Å) GaAs and Ge surfaces, shoulder peaks appear on both the Ga 3d and Ge 3d core levels. The shoulder peaks are shifted 1.1 and 1.4 eV to higher binding energy for the Ga 3d and Ge 3d levels, respectively. The Rb 4p and F 2p peak positions are slightly shifted between 0.25 and 0.5 eV. The broad second Ga 3d and Ge 3d peaks can be correlated to plasmon loss of electrons from these levels in a two-dimensional Rb metal-like layer formed at the Cu/RbF interface. The excitation energy of a Rb surface plasmon in a Cu-Rb-RbF system is calculated to be 1.3 eV.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Investigation of intrinsic plasmon energy losses associated with the Fe 1s core level in metallic iron

A recently developed Cu Kα₁ (hν = 8047.8 eV) X-ray source/ESCA300 electron spectrometer combination has been used to investigate the intrinsic plasmon energy losses associated with the Fe 1s core level (binding energy = 7111 eV) in metallic iron. The surface and bulk intrinsic plasmon energy losses were separated and it was found that using the theoretically calculated extrinsic energy loss cross-section to represent the bulk intrinsic energy loss cross-section gave an overall intrinsic loss probability which is approximately the same as if a Lorentzian type cross-section is used. However, this approach does not separate the surface and bulk intrinsic losses properly and is not a good approximation for peak shape analysis in the near peak region. A more realistic approximation is provided by using a Lorentzian type energy loss cross-section to represent the bulk intrinsic energy losses. It has also been shown that for the Fe 1s core level of metallic iron the probability that a photoelectron will suffer an intrinsic energy loss is higher at the surface than in the bulk. Also for this core level the excitation probability for the intrinsic plasmons is greater than that of the extrinsic plasmons. Hence ignoring the intrinsic plasmons would cause considerable error in peak shape analysis in the near peak region.     

Low energy photoelectron spectroscopy on alkali graphite intercalation compounds

The conduction band of various stages of alkali graphite intercalation compounds has been studied by low energy photoelectron spectroscopy (hv ? 6.55 eV). The dissimilar behaviour of the width β of the conduction band peak as a function of photon energy for C₆Li and C₈M (M = K, Rb, and Cs) is discussed in terms of different band types in the vicinity of the Fermi level. The stage dependence of β is measured and interpreted for the system C_xK (for stages 1, 2, 4, and 5).     

The 2π-derived level in the adsorption system CO/Cu(110)

The bremsstrahlung isochromat (or inverse photoemission) spectrum of CO chemisorbed on a Cu(110) surface has been investigated. An empty level derived from the CO 2 π orbital was found at an energy of 0.9 eV below the vacuum level E_vac and to have a halfwidth varying from 1.9 to 2.6 eV as a function of increasing coverage. The energetic position of the empty level is discussed in connection with photoemission data for occupied bands and excitation energies from electron energy loss measurements.     

Variation of electron energy loss spectra of Ni (100) with increasing primary energy

Electron energy loss spectra of clean Ni(1 0 0) show for the first time a 17 eV peak, which is attributed to an interband transtiion. All the observed peaks are shifted to higher energies as the primary electron energy E_p increases from 102 to 2045 eV. This shift is explained by a continuous decay in energy of the primary electrons inside the crystal. At E_p ? 700 eV, the decay takes place in the surface region of the crystal, while at E_p > 700 eV it takes place mainly in the bulk. The rate of decay increases with increasing temperature of the crystal between 300 and 900 K.     

Electron energy loss spectroscopy study of the initial stage of lanthanum oxidation

The electron energy loss spectra (EELS) of a pure metallic lanthanum surface and variations in these spectra at the initial stages of surface oxidation were studied. The measurements were performed at primary-electron beam energies E _p from 200 to 1000 eV. A very pronounced peak at a loss energy of about 7.5 eV arises due to transitions from the La4d electronic states of the valence band into the empty La4f electronic states of the conduction band at 5.0–5.5 eV above the Fermi level. Marked changes are observed in the EELS during the oxidation of lanthanum: the peak at an energy of 7.5 eV disappears, and the peak at 13.5 eV corresponding to bulk collective energy loss in lanthanum oxide becomes more pronounced. The results obtained are discussed in terms of the electronic structure of lanthanum and lanthanum oxide.     

X-ray photoemission study of physically absorbed SF6

The physical adsorption of octahedral SF₆ on Ru(001) has been studied with X-ray photoelectron spectroscopy (XPS) in an attempt to see effects on the energy levels resulting from the conformation of the molecule on the surface. Near 80 K surface coverages up to a monolayer have been studied at various steady state pressures of SF₆. Kinetic studies, core level binding energies, and peak areas indicate that the surface species studied was a physically adsorbed monolayer of sf₆. The sticking coefficient of SF₆, at ? 80 K is approximately unity. Also, a multilayer structure was observed at the highest pressures of SF₆. The binding energy of the F(ls) peak for monolayer coverage is centered at 688.2 ± 0.2 eV relative to the Ru Fermi level. while the multilayer F(ls) peak is shifted more than 3.5 eV to higher binding energy. The F(ls) linewidth for one monolayer has a full width at half maximum of 1.75 ± 0.1 eV. The F(ls) linewidth of the multilayer peak narrows with increasing coverage. Its narrowest observed linewidth was 1.35 eV ± 0.1 eV or approximately the same as that found in the gas phase. One of the mechanisms which may account for the F(ls) linewidth with monolayer coverage is a difference in F(ls) binding energy between those F atoms in contact with the substrate and those further away. This may be due to the variation in chemical environment and relaxation effects as a function of distance from tlie substrate. A classical image force calculation including finite screening effects of the substrate indicates that there is a differential binding energy, ΔW. between the F ligands; ΔW = 0.85 ± 0.25 eV, for realistic ranges of adsorption distances from the substrate and screening lengths in the substrate. The observed broadening of the monolayer F(ls) level is consistent with a ΔW of 0.7 ± 0.1 eV, indicating the possible existence of such a mechanism. Adsorption of a monolayer of SF₆ onto the Ru covered with a monolayer of oxygen shifts the F(ls) peak to lower binding energy by 0.8 eV. Similar effects due to oxygen have been observed previously in the physical adsorption of Xe on W(111).     

Giant core-exciton effects on Si(111) 7 × 7 surfaces

We present the first direct experimental evidence for a large surface influenced core-exciton effect on silicon. The Si(111) 7 × 7 L_2,3 absorption edge has been studied with core-level electron energy loss (ELS) and X-ray photoemission spectroscopy (XPS). An excitonic shift of ～1–2 eV have been found for transitions from Si(2p) to empty surface states.     

Surface plasma resonances in small spherical silver and gold particles

With 51 keV electrons surface plasma losses have been investigated on small spherical Ag and Au particles embedded in a medium with dielectric constant? _a=2.37. The surface loss of particles with radii of about 50 Å is found at 2.99 ±0.03 eV for silver and 2.34±0.03 eV for gold being in good agreement with calculated values. For larger radii the loss shifts to higher energy values which agrees qualitatively with the theory of Fujimoto and Komaki for the free electron gas. The optical extinction bands of the particles have been measured, too. Their maxima are shifted to lower energies, in accordance with Mie's theory, if the particle size increases sufficiently. Comparison of the energy values of the “electric” extinction bands with those of the surface plasmons shows, that they correspond in theory and experiment, if the particles are small. This confirms, that the optical colloid extinction of Ag and Au may be interpreted as plasma resonance absorption and emission of the particles.     

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E _e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p _3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E _e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E _e ≤ 66 eV. Deposition of Sm atoms at room temperature (～300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.     

Photodetachment measurements of alkali negative ions

Cross sections of alkali negative ions from 0.45 eV up to about 2.5 eV photon energy were measured. Electron affinities of the corresponding atoms are derived from the low energy onsets: EA(Li)=0.611 eV, EA(Na)=0.539 eV, EA(K)=0.497 eV, EA(Rb)=0.490 eV and EA(Cs)=0.470 eV with an estimated uncertainty of ±0.020 eV.     

Thermodynamics of melting of lithium,potassium and rubidium at high pressures

Volume measurements of liquid and solid Li, K and Rb in the vicinity of their melting curves have been carried out using a piston piezometer method. From the experimental results the volume, entropy and internal energy changes upon melting of the metals are calculated. The obtained data are analyzed along with the results for Na and Cs.It is stated that the anomalies in the melting curves of K, Rb and Cs are most probably connected with a s-d electron transition.     

The trapping of K and Na atoms by a clean W(110) surface trajectory calculations

The fraction of K and Na atoms initially trapped by the W(110) surface has been measured as a function of the incident energy (0.5–15 eV) and as a function of the incident angle. The trapping probability equals one at low incident energies (E_i ? 0.5 eV) and decreases with increasing energy. The measurements show an increase of trapping with increasing angle of incidence θ_i (measured from the surface normal). Simultaneously the desorption energies Q_i were determined from the temperature dependence of the measured mean residence time on the W(110) surface. We obtained for K: Q_i = 2.05 ± 0.02 eV, and for Na: Q_i = 2.60 ± 0.04 eV.The trapping phenomenon at a solid surface was approximated in a theoretical way by calculating the in-plane trajectory of a projectile scattered from a diatomic surface-molecule. The important feature which showed up was the conversion of tangential to normal momentum of the projectile, and thus the inapplicability of cube models. As a function of the angle of incidence two regimes can be distinguished: at the smaller angles the scattering is governed by simultaneous interaction of the projectile with two neighbouring surface atoms, and at the higher angles of incidence the single particle interaction contributes most to the momentum transfer.     

Elastic and inelastic scattering of slow positrons from a LiF(100) surface

The intensity profile for the elastic specular reflection of 5–100 eV positrons from a LiF(100) surface (ang1e of incidence 45°) has been measured using a simple time-of-flight spectrometer. The profile exhibits strong maxima below 25 eV and a smaller peak at 57 eV. Positron energy loss spectra have also been measured for a range of incident energies by retarding field analysis of the scattered beam. The mean energy loss appears to increase with increasing incident beam energy. Both the elastic and inelastic results are compared with similar data for slow-electron scattering obtained with the same apparatus.     

Inner core neutral atom binding energies — Comparison of relaxed-orbital theory with experiment

Neutral atom binding energies from relaxed-orbital relativistic Hartree-Fock-Slater calculations are compared with empirical values for Na, K, Rb, Cs, An and Cd atoms. The method is precise (within ～ 1 eV) for the highest l for a given n, but there are errors of several eV for s levels other than 1s.     

Interband transitions in ZnO observed in low energy electron spectroscopy

Using a high resolution two-axis-spectrometer energy loss spectra of electrons specularly reflected from (1¯100) ZnO-surfaces have been investigated. The electron impact energy was varied between 25 and 60 eV. The spectra show a strong dependence on the orientation of thec-axis with respect to the plane of incidence. The dependence of the loss intensities on the angle of incidence and the shape of the spectra can be qualitatively described by the “Dielectric Theory”. Contrary to optical reflection measurements the excitation of excitons could not be observed in agreement with the existence of a surface barrier for excitons.