Electronic structure of a cobalt monolayer on Cu(100)

Angular resolved photoemission spectra using synchrotron radiation have been measured for different amounts of cobalt evaporated on Cu(100). At room temperature cobalt grows layer-bylayer forming well-ordered layers in registry with the substrate, as judged by AES, LEED and UPS measurements. The energy position and linewidth of the Cu peaks remain unchanged when cobalt is deposited onto the surface, suggesting a rather weak interaction between the d-bands of Co and Cu. The two-dimensional band structure of the monolayer of cobalt has been determined. We have obtained a value for the magnetic exchange splitting of ΔE_exch = 0.80 ± 0.15 eV, which is nearly identical to the bulk value. A shift in the energy positions of the critical points for the monolayer versus bulk of cobalt is interpreted in terms of a narrower 2D density of states in the monolayer as compared to the bulk. A resonant valence-band two-electron satellite has been found. The correlation energy and screening effects of the two d-holes are very similar to the corresponding bulk values, while the decreased intensity of the satellite at resonance compared to the one for Co(0001) suggests that there are more d-states relative to s-states in the monolayer than in a bulk cobalt single crystal, in agreement with recent models of the valence band electronic structure at surfaces.     

Effect of self diffusion on NMR linewidth in Beryllium

NMR linewidth and T₂ measurements in Be between 300 and 1200 K are presented. Adiabatic and non adiabatic parts of the linewidth are dependent on temperature through the static quadrupolar interaction and this precludes a simple deduction of the self diffusion activation energy from c.w. linewidth measurements. Analysis of T₂ measurements using a pulse technique gives a value of 1.4±0.1 eV for this energy.     

Energies and chemical shifts of the sulphur 1s level and the KL2L3(1D2) Auger line in H2S,SO2 and SF6

The sulphur 1s binding energies and KL₂L₃(¹D₂) Auger energies have been measured in gaseous H₂S, SO₂ and SF₆. The experimental data, including the chemical shifts, are compared with various theoretical ab initio results. Theoretical and experimental values agree within 1-2 eV for the chemical shift and the binding energy of the 1s level, provided in the latter case relaxation, relativistic and correlation corrections are applied. Likewise, Shirley's method²⁰, which uses empirical energies, predicts the Auger energies satisfactorily. The measured S 1s binding energies are 2478.5(1) eV, 2483.7(1) eV and 2490.1(1) eV, and KL₂L₃(¹D₂) Auger energies are 2098.7(1) eV, 2095.5(2) eV, 2092.6(1) eV for H₂S, SO₂ and SF₆, respectively. The chemical shift for the 1s electron is found to be greater than for the 2s or 2p electron and in better accord with the prediction of the potential model. Data suggest the molecular relaxation energy to be small compared with the atomic relaxation energy.     

High resolution study of anion formation in low-energy electron attachment to SF<Subscript>6</Subscript> molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF₆ molecules in helium carrier gas, nozzle temperatures T₀= 300–600 K, stagnation pressures p₀= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF₆ molecules at high energy resolution. Using the standard LPA method, the yield for SF₆^- as well as SF₅^- and F^- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF₆^- and SF₅^- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF₆^- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF₅^- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T₀= 300 K and shifting to lower energies with rising T₀) and a monotonical decrease towards higher energies. SF₅^- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF₆ molecules with rising pressure. The SF₅^-/SF₆^- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF₅^- spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at p₀= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F^- formation is similar to that for SF₆^-, but the F^- signals are three to four orders of magnitude lower than those for SF₆^-; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Photolysis of SF6 adsorbed on Si(111)-7 × 7 by monochromatic soft X-ray

Continuous-time photoelectron spectroscopy (PES) and photon-exposure-dependent photon-stimulated desorption (PSD) were employed to investigate the monochromatic soft X-ray-induced dissociation of SF₆ molecules adsorbed on Si(111)-7 × 7 at 30 K (SF₆ dose = 3.4 × 10¹³ molecules/cm², ～ 0.5 monolayer). The photon-induced evolution of adsorbed SF₆ was monitored at photon energies of 98 and 120 eV [near the Si(2p) edge], and sequential valence-level PES spectra made it possible to deduce the photolysis cross section as a function of energy. It was found that the photolysis cross sections for 98 and 120 eV photons are ～ 2.7 × 10^? 17 and ～ 3.7 × 10^?17 cm², respectively. The changes in the F^? and F⁺ PSD ion yields were also measured during irradiation of 120 eV photons. The photon-exposure dependencies of the F^? and F⁺ ion yields show the characteristics: (a) the dissociation of adsorbed SF₆ molecules is ascribable to the substrate-mediated dissociations [dissociative attachment (DA) and dipolar dissociation (DD) induced by the photoelectrons emitting from the silicon substrate]; (b) at early stages of photolysis, the F^? yield is mainly due to DA and DD of the adsorbed SF₆ molecules, while at high photon exposure the F^? formation by electron capture of the F⁺ ion is likely to be the dominant mechanism; (c) the F⁺ ion desorption is associated with the bond breaking of the surface SiF species; (d) the surface SiF is formed by reaction of the surface Si atom with the fluorine atom or F^? ion produced by scission of S–F bond of SF_n (n = 1–6) species.     

Hole lifetime and chemical shift of the 4f levels in Yb metal

UHV photoemission studies of Yb metal are reported using synchrotron radiation in the photon energy range 25 < hv < 180 eV, whereby maximum surface sensitivity can be reached. The 4f photoemission spectrum shows a quadruplet structure owing to a superposition of two 4f spin—orbit doublets. One doublet has a very asymmetric lineshape and an extremely narrow lifetime linewidth 2γ? 0.013 eV; this is attributed to bulk emission. The other doublet exhibits almost symmetric lines with a much larger linewidth (0.43 eV FWHM); it has also a larger binding energy by 0.6 ± 0.03 eV. This is attributed as being due to chemically bound Yb atoms located at the surface. The implications of these findings on recent high-resolution photoemission studies of rare earth (RE) metals and their analysis are considered.     

Electronic state of ruthenium deposited onto oxide supports: An XPS study taking into account the final state effects

The electronic state of ruthenium in the supported Ru/EO_x (EO_x = MgO, Al₂O₃ or SiO₂) catalysts prepared by with the use of Ru(OH)Cl₃ or Ru(acac)₃ (acac = acetylacetonate) and reduced with H₂ at 723 K is characterized by X-ray photoelectron spectroscopy (XPS) in the Ru 3d, Cl 2p and O 1s regions. The influence of the final state effects (the differential charging and variation of the relaxation energy) on the binding energy (BE) of Ru 3d_5/2 core level measured for supported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parameters determined for the Ru/EO_x samples with the corresponding characteristics of the bulk Ru metal. It is found that the negative shift of the Ru 3d_5/2 peak which is observed in the spectrum of ruthenium deposited onto MgO (BE = 279.5-279.7 eV) with respect to that of Ru black (BE = 280.2 eV) or ruthenium supported on γ-Al₂O₃ and SiO₂ (BE = 280.4 eV) is caused not by the transfer of electron density from basic sites of MgO, as considered earlier, but by the differential charging of the supported Ru particles compared with the support surface. Correction for the differential charging value reveals that the initial state energies of ruthenium in the Ru/EO_x systems are almost identical (BE = 280.5 ± 0.1 eV) irrespectively of acid-base properties of the support, the mean size of supported Ru crystallites (within the range of 2-10 nm) and the surface Cl content. The results obtained suggest that the difference in ammonia synthesis activity between the Ru catalysts supported on MgO and on the acidic supports is accounted for by not different electronic state of ruthenium on the surface of these oxides but by some other reasons.     

Generation of high-energy secondary pulsed molecular beams

A method is suggested for generating high-intensity secondary pulsed molecular beams in which the kinetic energy of molecules can be controlled by an intense laser IR radiation through the vibrational excitation of molecules in the source. High-intensity [≥10²⁰ molecule/(sr s)] SF₆ molecular beams with a kinetic energy of ?1.0 eV without carrier gas and of ?1.9 and ?2.4 eV with carrier He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) gases, respectively, were obtained.     

Site-specific interaction of H2O with ZnO single-crystal surfaces studied by thermal desorption and UV photoelectron spectroscopy

The adsorption and condensation of H₂O(D₂O) on ZnO(101&#x0304;0), (0001)Zn and (0001&#x0304;)O surfaces was investigated by means of thermal desorption (TDS) and UV photoelectron spectroscopy (UPS). The clean ZnO single-crystal surfaces were prepared by Ar-ion sputtering and annealing and characterised by Auger electron spectroscopy, LEED, UPS and work-function measurements. On all three surfaces six different adsorption states were found. In the monolayer regime there is a stronger bonding to Zn sites (desorption temperature 340 K) than to O sites (190 K), The bonding to the Zn sites seems to be accompanied by some clustering. Before the chemisorption layer is completed a first ice state is found whose desorption temperature shifts from 162 to 168 K with increasing exposures. At higher exposures the multilayer ice state is found at 152 K. On the (0001&#x0304;)O face defect-induced features were identified. The water lone-pair orbital 1b₁, whose energy falls between the O p and the Zn 3d emission of the substrate and which is known to show bonding shifts, was analysed using angle-resolved UPS. In the monolayer, the main chemisorption states are found at E_B^V(1b₁) = ?9.6 eV for the (0001)Zn face and at ? 10.6 eV for the (0001&#x0304;)O face and are compared with the multilayer ice emission at 1&#x0304;1.1 eV. The difference in binding energies shows the same trend as the TDS data. For the (101&#x0304;0) face the 1b₁ emission is very broad, indicating some overlap between different states.     

Coordination of acetonitrile (CH3CN) to platinum (111): Evidence for an η2(C,N) species

Acetonitrile (CH₃CN) coordination to a Pt(111) surface has been studied with electron energy loss vibrational spectroscopy (EELS), XPS, thermal desorption and work function measurements. We compare data for the surface states with known acetonitrile coordination complexes. For CH₃CN adsorbed on Pt(111) at 100 K, the molecule is rehybridized and adsorbs with the CN bond parallel or slightly inclined to the surface plane in an η²(C, N) configuration. The ν(CN) frequency is 1615 cm^?1 and the C ls and N ls binding energies are 284.6 eV and 397.2 eV respectively. By contrast, weakly adsorbed multilayer acetonitrile exhibits a ν(CN) vibrational frequency of 2270 cm^?1, and C ls and N ls binding energies of 286.9 eV and 400.1 eV respectively. Both the EELS and XPS results are consistent with rehybridization of the CN triple bond to a double bond with both C and N atoms of the CN group attached to the surface. In addition to this majority η²(C, N) monolayer state, evidence is found for a second, more strongly bound minority molecular state in thermal desorption spectra. As a result of the low coverage of this state, EELS was unable to spectroscopically identify it and we tentatively assign it as an η⁴(C, N) species associated with accidental step sites. By contrast to the surface complexes, almost all of the known platinum-nitrile coordination complexes are end-bonded via the N lone-pair orbital. Several cases of side-on bonding are known, however, and we compare the results with the known complex Fe₃(η²-NCCH₃)(CO)₉. The difference in the coordinative properties of a Pt(111) surface versus a single Pt atom must be due to the increased ability of multi-atom arrays to back-donate electrons into the π¹ system of acetonitrile. Previously published EELS and XPS results for monolayer acetonitrile on Ni(111) and polycrystalline films are almost identical to the present results on Pt(111). We believe that the monolayer of $\text{CH}{}_3\text{CN}\text{Ni}\text{(111)}$ is also an η²(C, N) species, not an end-bonded species previously proposed by Friend, Muetterties and Gland.     

The optical properties of ZnO/ZnMgO single quantum well grown by P-MBE

In this paper, ZnO/Zn_0.9Mg_0.1O single quantum well (SQW) structures were fabricated on c-plane sapphire (Al₂O₃) substrate by plasma-assisted molecular beam epitaxy (P-MBE). The photoluminescence (PL) peak of the SQW shifted from 3.31 to 3.37 eV as the well layer thickness was decreased from 6 to 2 nm. The spectral linewidth increases with temperature due to the scattering of excitons with acoustic and optical phonons. The transition energy of the localized exciton in the ZnO/Mg_0.1Zn_0.9O SQW with well width of 3 nm was found to be about 3.407 eV at 80 K, consistent with theoretical calculation. The first subband energies in the conduction and valence band were calculated to be 49 and 11 meV, respectively.     

Fundamental optical properties of α-RuCl3

The fundamental optical properties in the paramagnetic phase of α-RuCl₃ are studied at different temperatures in the photon energy interval 0.03 to 10 eV. Infrared reflectivity spectra show a transverse optical frequency at 0.038 eV (32 μm) for an E_u mode (E⊥c, in plane atomic displacements). The absorption spectra in the energy range 0.2 to 1 eV reveal three bands (0.29, 0.51, 0.71 eV) attributed to d-d electronic transitions. Reflectance and thermo-reflectance measurements indicate the onset of the charge-transfer transitions at 1.1 eV and show structure at 1.85, 2.55, 3.05, 4.5 eV. The marked reflectivity peak at 5.2 eV is probably related to p(Cl) → s(Ru) band-to-band transitions.     

SF6电子动量谱学研究中干涉效应的观测

利用非共面对称的高效率(e,2e)电子动量谱仪测量了SF₆分子外价分子轨道的二维电子能量- 动量密度谱. 通过理论计算与实验结果的比较,发现B3LYP密度泛函理论计算结果可以较好地解释实验测量的轨道电子动量分布. 此外,对于最外层的4个来自F2p孤对电子贡献的非键分子轨道,实验上观测到非常明显的多中心干涉图样.     

State and site selectivity in SF6 fragmentation studied via Auger electron-ion coincidence

The fragmentation of sulfur hexafluoride by 4 keV electron impact is studied via Auger electron-ion coincidence experiments in the range of 50 to 70 eV of the binding energy of the dication. Results are presented for the fragmentation following S 2p and F 1s ionization. The experimental data confirm that no bound SF₆²⁺ states exist at the studied energies and that the fragmentation pattern strongly depends on the dication final state and the site where the primary hole is located.     

Intensitäten myonischer Röntgenlinien in SF6 und H2 plus SF6

Muonic X-ray spectra from S and F in SF₆ and H₂+SF₆ have been measured with a high-pressure gas target. Strong differences between the observed intensities of corresponding lines were found for the two cases. The results are compared with cascade calculations. The per-atom ratio between Coulomb capture in F and S was determined to be 0.96±0.09 and 1.04±0.24 for SF₆ and H₂+SF₆, respectively.     

On the use of yttrium and sodium anodes in photoelectron spectroscopy

Low-cost anodes of the active metals yttrium and sodium were constructed and used under modest vacuum conditions. The yttrium Mζ line (132.3 eV) was used to determine np/ns cross-section ratios in the valence shells of three rare gases. The NaKα_{1 2} line and its satellites were characterized with a neon converter. It yielded a linewidth of 0.58(1) eV on the neon 1s line, from which an upper limit of 0.42 eV was set for the NaKα_{1 2} X-ray linewidth. On several “test cases” the NaKα _{1 2} X-ray fell between other characteristic X-rays (e.g., MgKα _{1 2}) and monochromatized sources in its ability to resolve fine structure. The methane C 1s peak was asymmetric and the correct vibrational spacing was obtained by fitting with three components of the correct intensity ratios. Similarly, relative chemical shifts of the unsubstituted carbons in m-difluorobenzene were determined almost completely by fitting the asymmetric peak. In the PF₅ (F 1s) and O₂ (O 1s) cases some new information was obtained.     

Gas phase and solid state X-ray photoelectron spectra of the substituted acetylenes (SF5)n C2 (CF3)2−n (n = 0, 1, 2): A comparison of the pentafluorosulfur and trifluoromethyl groups

The core level X-ray photoelectron spectra (XPS) of CF₃CCCF₃, CF₃CCSF₅ and SF₅CCSF₅ have been measured in the solid state. Gas phase spectra of CF₃CCCF₃ and CF₃CCSF₅ have also been obtained. The XPS data, interpreted with the point charge potential model and semiempirical MNDO (minimum neglect of differential overlap) molecular orbital calculations, indicate that the electron withdrawing effect of the ?CF₃ group is greater than that of the ?SF₅ group. Results further suggest that sulfur 3d orbitals do not play a detectable role in the bonding or charge distribution in these molecules. Carbon 1s linewidths of ?CF₃ carbon atoms are found to be much narrower than those arising from the acetylenic carbon atoms. The narrower lines correlate with the much higher binding energy of the ?CF₃ carbon atoms. Large shifts (nearly 1 eV) in heteroatom core level binding energy differences (for example, F 1s — C 1s) between the gas phase and solid state data are observed. These shifts are attributed to solid state effects (Madelung potential, intermolecular bonding interactions, and/or extramolecular relaxation contributions). From these comparisons it is clear that solid state effects are not uniform in their influence on the photoionized sites in these molecules.     

Spectrum and intensity of light scattered from SF6 along the coexistence curve

Our two groups have measured independently the “Rayleigh linewidth” (Γ), the reduced compressibility (??/?μ)_T, and the pressure (P) of SF₆ along the liquid and vapor sides of the coexistence curve. We find that (??/?μ)_T = 1.67 × 10^-10 (1 - T/T_c)^{-1.22 ± 0.015}g²erg^-1cm^-3 and (?P/?T)lχ = (7094 ± 0.1) × 10⁵ dyne/cm² ° We analyze our linewidth measurements in terms of the Kadanoff-Swift-Kawasaki mode-mode coupling theory using estimates for the viscosity and correlation range.     

Growth of germanium thin films on the W(100) surface

The growth of Ge thin films on the surface of a textured predominantly (100)-oriented tungsten ribbon is studied by thermal desorption spectrometry at different substrate temperatures over a wide range of coverages. The mechanism of growth of the Ge films at T = 300 K is similar to a layer-by-layer mechanism. For T > 300 K, the films grow through the Stranski-Krastanov mechanism, according to which the completion of the monolayer coverage is followed by the formation of three-dimensional crystallites; as a result, the desorption kinetics changes. For small coverages (i.e., in the absence of lateral interactions), the activation energy of Ge desorption from W(100) is E = 4.9 ± 0.2 eV. In a monolayer, this activation energy decreases to E = 3.9 ± 0.2 eV due to the repulsive lateral interactions. The energy of pairwise lateral interactions is determined to be ω = 0.3 eV.