Photoemission of Xe and Kr adsorbed on the W(110) plane

The UPS and XPS spectra of Xe adsorbed on clean, O, CO, and Xe covered W(110) surfaces and the UPS spectrum for Kr on clean and O covered W(110) surfaces have been investigated. On clean W, Xe and Kr show a splitting of the $\text{5p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{4p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ hole states respectively. For Xe the coverage dependence of this splitting was investigated in detail; neither the positions nor the intensity ratio of the substates are coverage dependent for θ ? 0.04, suggesting that splitting is due to differences in the image interaction of the $\text{m}{}_{\mathrm{j}}\text{= ±}\text{3}\text{2}$ and $\text{m}{}_{\mathrm{j}}\text{= ±}\text{1}\text{2}$ components. For Xe equal shifts, relative to vacuum, of ~1.0 eV were observed for 5p, 4d, and 3d levels, suggesting that initial state effects are small. Image interaction for Xe and Kr on clean W could best be fitted by assuming an increase, rather than a decrease in the effective hole-image separation from the nominal value, suggesting that the image plane is moved back into the metal by a screening length. For Xe adsorbed on $\text{Xe}\text{W}$(110), or on virgin-$\text{CO}\text{W}\text{(110)}$ polarization of the intermediate layers was found to contribute significantly to relaxation. Coadsorbed oxygen broadened Xe 5p and Kr 4p peaks. There was an almost linear relation between O 2p UPS intensity at the energies of the various peaks and the amounts of broadening, suggesting that the latter results from resonance neutralization by electrons from the O 2p states.     

Site-selective adsorption of xenon on a stepped Ru(0001) surface

The adsorption of xenon has been studied with UV photoemission (UPS), flash desorption (TDS) and work function measurements on differently conditioned Ru(0001) surfaces at 100 K and at pressures up to 3 × 10^?5 Torr. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) served to ascertain the surface perfectness. On a perfect Ru(0001) surface only one Xe adsorption state is observed, which is characterized by$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 5.40 and 6.65 eV, an adsorption energy of E_ad≈ 5 kcal/mole and dipole moment of μ'_T ≈ 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the step-height and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference in adsorption energy ofΔE_ad ≈ 3 kcal/mole and finally by their strongly deviating dipole moments of μ_S = 1.0 D and μ_T = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kink-sites are distinguishable via E_ad. The E_ad heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. —The observed effects suggest selective xenon adsorption as a tool for local surface structure determination.     

Interaction of inert gases with a nickel (100) surface: I. Adsorption of xenon

The adsorption of Xe on a Ni(100) surface has been studied in UHV between 30 and 100 K using LEED, thermal desorption spectroscopy (TDS), work function (Δφ) measurements, and UV photoemission (UPS). At and below 80 K, Xe adsorbs readily with high initial sticking probability and via precursor state adsorption kinetics to form a partially ordered phase. This phase has a binding energy of ~5.2 kcal/mole as determined by isosteric heat measurements. The heat of adsorption is fairly constant up to medium coverages and then drops continuously as the coverage increases, indicating repulsive mutual interactions. The thermal desorption is first order with a preexponential factor of about 10¹² s^?1, indicative of completely mobile adsorption. Adsorbed Xe lowers the work function of the Ni surface by 376 mV at monolayer coverage. (This coverage is determined from LEED to be 5.65 × 10¹⁴ Xe molecules/cm^-2.) For not too high coverages, θ, Δφ(θ) can be described by the Topping model, with the initial dipole moment μ₀ = 0.29 D and the polarizability α being 3.5 × 10^?24 cm³. In photoemission, the $\text{Xe 5p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{5p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ orbitals show up as intense peaks at 5.56 and 6.83 eV below E_f which do not shift their position as the coverage varies. Multilayer adsorption (i.e. the filling of the second and third layers) can be seen by TDS. The binding energies of these α states can be estimated to range between 4.5 and 3.5 kcal/mole. The results are compared and contrasted with previous findings of Xe adsorption on other transition metal surfaces and are discussed with respect to the nature of the inert-gas-metal adsorptive bond.     

Coverage and adsorption-site dependence of core-level binding energies for tin and lead on Al(111) and Ni(111)

Thin layers (0.2–10 monolayers) of Pb and Sn were prepared on Al(111) and Ni(111) surfaces and characterized by means of LEED, AES, UPS and work-function measurements. The binding energy of the shallow Sn 4d and Pb 5d core-levels was investigated with respect to coverage and adsorption-site-dependent changes. On Al(111) the Sn and Pb monolayers exhibit ordered, two-domain, aligned but not in registry structures. For these layers core-level binding energies were found identical to those of the bulk metals. On Ni(111), Pb gives rise to a 3 × 3 structure, followed then by a 4 × 4 structure at higher coverages. The Pb 5d core-level binding energies shift continuously to higher values. A final shift of 0.42 eV is reached after about 2 monolayers. Sn on Ni(111) exhibits two welll separated peaks lying at 23.70 and 23.97 eV for the $\text{4d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ line. These two lines can be correlated with two different adsorption sites which have to be assumed for the $\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ and the (2 × 2) structure found at different coverages. The binding energy shifts are discussed in a model based on a Born-Haber cycle.     

XPS studies of adatom-adatom interactions: I/Ag(111) and I/Cu(111)

We have studied submonolayer adsorption, at room temperature, of iodine on the (111) faces of silver and copper, using LEED and XPS. In both systems the √3 × √3 LEED pattern appears at ～0.2 monolayer (ML) coverage; no other superlattice pattern was observed. The $\text{I}\text{4}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ core electron binding energy in both cases decreases by ～0.15 eV between very dilute coverage and 0.33 ML. The leveling-off of the binding energy for I/Ag(111) for coverages >0.2 ML is shown to be a unique experimental manifestation of an indirect, substrate-mediated adatom-adatom interaction, an attraction of several meV between next-nearest neighbor iodine atoms. The more nearly linear decrease in the I binding energy on Cu(111) is shown to imply a significantly weaker next-nearest neighbor interaction on this surface. The appearance of the √3 × √3 LEED pattern at low coverages on Cu is shown to be consistent with short-range order produced merely by a size effect, that is, by nearest neighbor exclusion. These conclusions are reached with the help of Monte Carlo calculations of a triangular lattice gas.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Author index

The mean adsorption lifetimes of F, Cl and Br on (100) and (111) Mo surfaces have been obtained from the first order desorption kinetics observed in low converage conditions (θ < 10^?2 of a monolayer) using a pulsed ionic beam method. The mean adsorption lifetimes τ fit a general expression $\text{τ = τ}{}_0\text{exp}\text{(}\text{E}\text{kT}\text{)}$ in a large temperature range (1700–2400 K) allowing the determination of the binding energies E. The main results of this study are (1) the binding energies decrease from F through Br; (2) the binding energies on both (100) and (111) orientations are similar, E(F)～4.65 eV, E(Cl)～4.15 eV, and E(Br)～3.65 eV. These results are discussed and compared with those previously reported on (100) and (111) Nb and W surfaces. The close binding energies of F, Cl and Br on (100) and (111)^?Nb and Mo surfaces suggest that halogens have a different chemisorption behaviour with respect to O and N.     

Proprietes optiques et structure electronique de MnAu2

The optical spectrum of the helical antiferromagnetic compound MnAu₂(t_N = 90°C) has been measured, using a scanning ellipsometric method, in UHV, between 0.47 and 5.7 eV, at temperatures ranging from 88 to 700 K. Below 0.6 eV the experimental data can be fitted to a Drude-like intraband model. The maximum of the interband absorption occurs at 5.1 eV, while the onset of interband absorption may be placed at 0?.4eV as is suggested by the rapid rise of $\text{ε}{}_2\text{(ω)}\text{λ}$. below 0.5 eV. In the absence of theoretical work, the analysis of the optical spectrum leads to a preliminary rough model of the electronic structure; the proposed local density of d states is represented. The 5.1eV peak is attributed to d → E_F transitions (parabolic edge at 2.7 eV similar to Au), originating in the lower part of the band, associated mainly with Au sites. To account for the moment $\text{(}\text{3.6μ}{}_{\mathrm{S}}\text{at Mn}\text{)}$, the upper d band (mainly Mn sites) is split: the d↑ band is below E_F (interband edge at 0.4eV), while the d↓ band contains 1.4 electrons. ESCA measurements tend to confirm this model. An important unusual fact is the sharp anomaly of /~ε(ω) in the infrared, around T_N; attempts to correlate this with magnetic (s-d) interactions have been initiated.     

Anomalous two-electron auger resonance in thorium near the 5d(O5) photothreshold

The photoexcited $\text{O}{}_5\text{P}{}_3\text{V (5d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{6p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{valence}\text{)}$ Auger emission line for thorium metal shows an anomalous increase in kinetic energy of ～ 1 eV as the photon energy hv is increased through the atomic-like 5d → 5f resonant excitation at hv_r = 89 eV. Possible mechanisms for this anomalous behavior are discussed, and it is suggested that it can be interpreted as a two-electron resonance involving the O₅P₃V Auger excitation and a shake-up satellite of the $\text{6p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core level excitation.     

Empty electronic states in the sodium tungsten bronzes: a study by inverse photoemission spectroscopy

Empty electronic states in the sodium tungsten bronze $\text{Na}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{WO}{}_3$ have been studied by inverse photoemission spectroscopy using a novel narrow-bandpass photon detector. The empty density of states in the t_2g(W:5d) band peaks about 3 eV above the Fermi energy, implying an overall t_2g bandwidth greater than found in recent bandstructure calculations.     

Activation energy of field emission flicker noise power due to adsorbed potassium on tungsten

The temperature dependence of the field emission flicker noise spectral density functions has been investigated for potassium adsorbed on tungsten (112) planes by a probe hole technique. By integration of the spectral density functions $\text{W(?) = B}{}_{\mathrm{i}}\text{?}{}^{\mathrm{?ge}}\text{i}$ the noise power $\text{(δn}{}^2{}_{\mathrm{\Delta ?}}$ for different frequency intervals Δ? is obtained. From the exponential temperature dependence of $\text{(δn}{}^2{}_{\mathrm{\Delta ?}}$ noise power “activation energies” q_Δ? are determined. Plots of these energies versus coverage show a similar “oscillating” behaviour as recently found for $\text{W(?}{}_{\mathrm{j}}\text{)}$ or which indicates phase transitions of the adsorbed potassium submonolayers. The noise activation energies are discussed in terms of existing models and a comparison is made between the experimental q values and surface diffusion energies E_d as determined by conventional methods.     

Kerr-effect and dielectric tensor elements of magnetite (Fe3O4) between 0.5 and 4.3 eV

The complex polar Kerr effect (rotation and ellipticity) of magnetite single crystals has been measured at room temperature between 0.5 and 4.3 eV. From the magneto-optical data and the optical constants, the off-diagonal elements (?_xy) of the dielectric tensor has been derived. Three well separated magneto-optical transitions have been indentified. At about 0.75 eV one strong magneto-optical structure with a diamagnetic line shape is assigned to a $\text{3d}{}^6\text{→3d}{}^5\text{(}{}^6\text{A}{}_{\mathrm{1g}}\text{) 4s}$ transition from Fe²⁺ in octahedral sites. Two other structures with paramagnetic line shapes near 1.85 and 2.90 eV are assigned to the orbital promotion processes $\text{3d}{}^6\text{(Fe}{}^{\mathrm{2+}}{}_{\mathrm{oct}}\text{)→3d}{}^5\text{(}{}^4\text{T}{}_{\mathrm{1g}}\text{) 4s}$ and $\text{3d}{}^5\text{(Fe}{}^{\mathrm{3+}}{}_{\mathrm{tet}}\text{)→3d}{}^4\text{(}{}^5\text{T}{}_2\text{) 4s}$, respectively, which take into account Fe 3d^n?1 final state effects.     

Experimental evidence of low frequency defect vibrations in the fcc metal lead observed by superconducting tunneling

Superconducting tenneling measurements of Al-Al_xO_y-Pb junctions taken after low temperature irradiation with 10 MeV oxygen ions are reported. The normalized differential conductance $\text{σ}{}_{\mathrm{norm}}\text{= (}\text{d}\text{I}\text{d}\text{U}\text{)}{}_{\mathrm{s}}\text{/ (}\text{d}\text{I}\text{d}\text{U}\text{)}{}_{\mathrm{n}}$ shows additional low frequency structures to an enhancement of the Eliashberg function α²F(ω) in the range from 1.5 to 3.2 meV. The position in energy and the annealing behaviour corroborate that vibrational modes or irradiation induced self-interstitials are observed.     

The A-dependence of J/ψ-particle inclusive distributions

The differential cross sections $\text{d}\text{σ}\text{d}\text{x}$ and $\text{d}\text{σ}\text{d}\text{p}{}_{\mathrm{t}}{}^2$ of inclusive J/ψ production by 43 GeV/c π^? off Be, Cu and W nuclei have been measured. Fitting $\text{d}\text{σ}\text{d}\text{p}{}_{\mathrm{t}}{}^2\text{～ A}{}^{\mathrm{\alpha }}\text{(p}{}_{\mathrm{t}}{}^2\text{)}$ we observed the increase of α with p_t².     

Chemisorption-induced Pt 4f surface core level shifts

Soft X-ray photoemission experiments have led to the unambiguous observation of a metal surface core level ($\text{Pt}\text{4}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$) shift, due to an adsorbate (CO), to a binding energy larger than the bulk binding energy. The $\text{4}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ clean (110) surface component, with a binding energy 0.35 ± 0.02 eV lower than the bulk, is shifted by 1.06 ± 0.04 eV towards higher binding energy upon CO chemisorption. The lack of significant changes in the bulk component indicates the localized nature of the CO-Pt surface bonding.     

Anomalous temperature dependence of third-order elastic constants in NH4Cl and NH4Br near the λ point

Second order elastic constants of NH₄Cl and NH₄Br crystals were measured under hydrostatic and uniaxial pressures near the λ-type phase transitions. Third order elastic constants were calculated from the pressure derivatives of the second order elastic constants. Results show that the temperature dependences of the third order elastic constants of NH₄Cl differ from those of NH₄Br dramatically. In addition, it is found that the coefficients $\text{1}\text{54}\text{(C}{}_{111}\text{+ 6C}{}_{112}\text{+ 2C}{}_{123}\text{)}$ and $\text{(}\text{1}\text{24}\text{√3)(C}{}_{111}\text{? 3C}{}_{112}\text{+ 2C}{}_{123}\text{)}$ of strains in the free energy of NH₄Cl show quite different temperature dependences from those of NH₄Br.     

The spectroscopic study of excitation and ionization in the collision of two metastable atoms

Collisions between two excited atoms leading to an increase in the excitation energy of the particles have been under investigation. All measurements were made in the afterglow of gas-discharge plasma. The cross sections of the following reactions have been determined: $\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) +}\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) →}\text{Hg}{}^7\text{+}\text{Hg}\text{(6}{}^1\text{S}{}_0\text{),}\text{He}{}_{\mathrm{m}}\text{(2}{}^{\mathrm{1,3}}\text{S}\text{) +}\text{Xe}{}_{\mathrm{m}}\text{(}{}^3\text{P}{}_{\mathrm{0,2}}\text{) → (}\text{Xe}{}^{\mathrm{+}}\text{)}{}^1\text{+}\text{He}{}_0\text{+}\text{e}$. The cross section of the first reaction for different transitions lies in the region (2?35) × 10^?15 cm² and the cross section of the second, in (0.2?2.4) × 10^?16 cm². Possible systematic errors and the role of cascade transitions are discussed. Cross sections of the Penning reaction He_m + Xe₀ → He₀ + Xe⁺ + e have also been measured. The result is σ (2³S) = (1.4 ± 0.2) × 10^?15 cm², σ (2¹S) = (1.2 ± 0.3) × 10^?15 cm².     

Soft X-Ray appearance potential spectroscopy of C,N and O adsorbed on Cr,Mo and W

The 1s level soft x-ray appearance potential spectra of C, N and O chemisorbed on polycrystalline Cr, Mo and W surfaces have been observed. The adsorbate ls spectra show the following characteristic features in the chemisorption regime: (i) Cr: one single peak; (ii) Mo and W: a doublet structure. We tentatively assume that the doublet structure is due to a splitting in the antibonding levels of the surface complex. Oxidation of Mo and W at high temperature causes changes in the metal $\text{(Mo 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{W 3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ and in the oxygen 1s spectra as compared with the spectra obtained after room temperature adsorption of oxygen.     

Relativistic and correlation effects in the anisotropy of near-threshold Kr 3d and Xe 4d photoionization

Angular distributions of the spin–orbit split components and their branching ratios have been studied experimentally and theoretically for the 3d photoelectrons of Kr and 4d photoelectrons of Xe. The focus was on the electron dynamics near the ionization threshold of each spin–orbit split component and its behaviour as a function of Z   in passing from Kr to Xe. The experimental spectra were measured with high photon and electron energy resolutions with photon energies at about 3–12 eV above the 3d_3/2$3{\text{d}}_{3/2}$ and 3d_5/2$3{\text{d}}_{5/2}$ thresholds for Kr and at about 5–12 eV above the Xe 4d_3/2$4{\text{d}}_{3/2}$ and 4d_5/2$4{\text{d}}_{5/2}$ thresholds. Experimental results for the angular distribution parameters have been compared with theoretical values obtained with relativistic Dirac–Fock method and results from independent particle approximation with a modified Hartree potential [A. Derevianko, W. Johnson, K. Cheng, Atom. Data Nucl. Data Tables 73 (1999) 153]. The branching ratios were compared with theoretical predictions from Dirac–Fock and relativistic random-phase approximation [K. Cheng, W. Johnson, Phys. Rev. A 28 (1983) 2820].     

The adsorption of xenon by W(111), and its interaction with preadsorbed oxygen

The physisorption of Xe on W(111) and of Xe on partial layers of oxygen chemisorbed on W(111) has been studied using flash desorption and work function methods. It has been found that xenon adsorbs up to monolayer coverages at 104K. Xenon desorbs from W(111) as a single binding state following first order kinetics. At low coverages (θ_Xe < 0.07) the binding energy decreases with increasing coverage possibly because of the presence of high energy adsorption sites due to crystal imperfections and edge effects. For θ_Xe > 0.07 the desorption data fit a first order rate expression with a desorption energy of 9.3 kcal/mol and preexponential ν = 10¹⁵s^?1. The observed work function change of ?1.1 ± 0.1 eV is consistent with monolayer estimates reported in field emission studies of physisorbed xenon on tungsten. The effect of preadsorbed oxygen layers on the physisorption of xenon on this surface is very striking. The energy of desorption shifts as much as 50% higher for a moderate exposure of oxygen. Several physisorption models are explored along with estimates of dispersion and electrostatic interaction contributions.