An Auger and X-ray photoelectron spectroscopic study of sodium metal and sodium oxide

Photoelectron and Auger electron measurements have been made on polycrystalline films of sodium metal evaporated in ultra high vacuum, and on Na₂O produced by in-situ oxidation by dry oxygen. Most of the spectra were recorded using Mg Kα (1254 eV) radiation but excitation by 5 keV electrons or monochromatized Al Kα (1487 eV) X-rays was used for specific purposes. Core and valence electron binding energies, photoionization cross-sections relative to Na 1s, KLL and KLV Auger energies and transition probabilities are reported. Energy losses in the metal and oxide are discussed and the relative intensities of surface and bulk plasmon losses have been used to calculate mean electron escape depths in the metal. When corrections were made for experimental geometry, escape depths of 10 Å at 180 eV and 31 Å at 1200 eV were obtained. An escape depth of 23 Å at 980 eV was obtained by Na 1s-Na K-Auger intensity correlation and this is consistent with the plasmon data. Data on Auger satellite lines are presented and, in particular, evidence has been obtained which indicates that a high energy satellite should not be attributed to a plasmon gain mechanism. Valence band influences on the KLV Auger spectra are discussed with reference to the XPS spectrum and other sources of valence band information. Unexpected structure was found in the KLV spectra of the metal which, pending thorough interpretation, offsets the sensitivity and resolution advantages which these spectra otherwise offer for valence band studies.     

Energy dependence of electron energy loss processes in Ge 2s photoemission

Deep core Ge 2s photoelectron spectra from polycrystalline Ge films induced by monochromatic synchrotron radiation, of 4, 6 and 8 keV were measured and analysed using two different methods, the partial intensity analysis and the extended Hüfner method to determine the spectral contributions from different electron energy loss processes due to bulk extrinsic, intrinsic and surface excitations. The obtained photon energy dependence of the ratio of these contributions was compared as a function of the photoelectron kinetic energy. It was found that the relative contribution of intrinsic excitations increase with the photon energy.     

Ab initio calculation of correlation effects for the O 1s core electron binding energy in MgO

Wavefunction based ab initio embedded cluster calculations are employed to calculate the O 1s core electron binding energies (CEBEs) of bulk MgO and the MgO(001) surface. A quantum cluster consisting of 61 atoms in five layers and embedded in a large point charge field is used for bulk MgO, the cluster for the MgO(001) surface is chosen accordingly. The O 1s CEBEs are calculated at the Koopmans' theorem (KT) and ΔSCF levels and with inclusion of correlation effects by means of the MC-CEPA method (multi-configuration coupled electron pair approximation), which is an approximate multi-reference coupled cluster approach. The correlation contributions to the O 1s CEBE of the central O atom due to the Mg atoms in the first and the O atoms in the second coordination shell turned out to be additive to a large extent. Therefore, they could be evaluated in an incremental fashion by a series of smaller calculations, where only a few atoms are included in the correlation treatment rather than all atoms of the first coordination shells or of the full quantum cluster. This makes the calculations feasible, even if large basis sets are used. The final results for the O 1s CEBEs are 533.47 and 533.50 eV for bulk MgO and the MgO(001) surface, to which electron correlation contributes 0.77 and 0.70 eV, respectively.     

Improved SPR technique for determination of the thickness and optical constants of thin metal films

A novel method with single-wavelength light is developed to determine the optical constants and the thickness of a thin metal film. It bases on a new geometry that consists of a coupling prism, a transparent coating layer directly deposited onto the prism base, a thin metal film to be detected, and the air. The attenuated total reflection technique is employed in our configuration to excite two different kinds of surface plasmon waves simultaneously. As a result, the reflection curve shows two obvious surface plasmon resonant dips. Using the Chen's et al. (Opt. Soc. Am. 71 189, 1981) method, we can obtain two sets of () and thickness of the thin metal film through one resonant dip (surface plasmon), then the real value can be determined through the other resonant dip (modified long-range surface plasmon). Compared to conventional double wavelength method and the double-medium technique (Yang et al. Appl. Opt. 27 11, 1988), the present single-scan method avoids both the ambiguity of different conditions caused by two-scan technique, and the dispersion problem with different light wavelength.     

Metal (Cu; Pd) adsorption on MgO: investigations with MIES and UPS

MgO films (2-nm thick) were grown on W(110) while metastable impact electron (MIES) and photoelectron (UPS(HeI)) spectra were collected in situ; apart from the valence-band emission no additional spectral features could be detected. The oxide surface was exposed to metal atoms (Cu; Pd) (substrate at 300 K). For Cu, but not for Pd/MgO, a characteristic initial decrease of the surface work function by about 0.4 eV is observed for small exposures. Metal-induced intensity develops above the top of the O2p valence band in UPS caused by 3dCu (4dPd) emission. The emission seen for Cu/MgO in the MIES spectra above the 2pO valence band is attributed to the ionization of Cu4s states of neutrally adsorbed Cu species; the shape of the MIES spectra suggests island growth even at the lowest studied exposures. For Cu/MgO the critical coverage for the transition from 2D to 3D island growth, as determined with MIES, is estimated as 0.15 monolayers. PACS 79.20; 79.60.Dp; 73.22.-f; 82.80.Pv     

Plasmon loss structure in synchrotron radiation photoemission from Mg films

Synchrotron radiation excitation has been used to vary the kinetic energy of electrons photoemitted from the 2p core level of polycrystalline magnesium films and the energy-dependence of the accompanying plasmon loss structure has been observed. Difficulties associated with background subtraction are discussed, and it is shown that the method of background subtraction profoundly affects the conclusions drawn from such an experiment on the relative importance of plasmon and single particle excitation mechanisms for electron scattering. Very thin films of Mg have also been studied in an attempt to separate the intrinsic and extrinsic modes of plasmon creation. Although the plasmon loss structure is dependent on film thickness implying a considerable contribution from extrinsic processes, we are not able to observe plasmons excited in the thin film by electrons originating in the substrate core level. The implications of these results are discussed and further experiments suggested.     

Cl2 adsorption on MgO(001) surface supported sodium monolayers: a density functional theory study

The adsorption of Cl₂ Na monolayers supported on the MgO(001) surface has been studied by the density functional method using cluster models embedded in a large array of point charges (PCs). The value of PCs was determined by charge self-consistent technique. The results indicate that Na-promoted MgO(001) surface is an efficient catalyst toward Cl₂ adsorptive decomposition. Besides, it was found that the role of the MgO(001) surface is not passive, which is different from CO adsorption on MgO(001) surface supported Na metal monolayers. The analysis of band and projected density of states indicates that the electron transfer from the surface Mg 3s valence orbital and Na 3s valence orbital to the anti-bonding σ^∗ orbital of Cl₂ is the source of the Cl₂ bond weakening. This is also different from the CO adsorption on MgO(001) surface supported Na metal monolayers, where only the electrons from the Na valence orbital are transferred to the anti-bonding π^∗ orbital of adsorbed CO. Our study suggests that the essence of catalysis is different for CO and Cl₂ adsorption on Na metal monolayers supported an MgO(001) surface.     

Line shape analysis of high energy X-ray induced Auger- and photoelectron spectra of thin Cu and Ni films

A detailed study of the line shape of high energy Cu X-rays excited Auger- and photoelectron spectra of layered samples is presented. The samples consisted of polycrystalline Cu and Ni overlayers of various thicknesses deposited on a Si substrate. The raw data show that the contribution caused by bulk inelastic scattering in the spectra increases significantly with overlayer thickness. Employing a general deconvolution procedure, the contribution of bulk inelastic scattering was consistently eliminated. The resulting spectra are in excellent agreement. The contribution caused by surface excitations has a very moderate effect at these high energies, while intrinsic losses were estimated to make up ∼30–50% of the total intensity. The true energy distribution at the source as seen by the spectrometer was established by elimination of the features due to surface excitations and the contribution from intrinsic excitations of the delocalized electronic states was determined.     

Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H₂O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)₂ upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H₂O, OH and Ox thickness changes as a function of relative humidity.     

Characteristic energy loss structure of solids from x-ray photoemission spectra

Energy loss peaks in x-ray photoemission spectra of nine metals are presented. No strong evidence for intrinsic plasmon structure was observed. Spectra from the free electron-like metals, Al, Li, and Na under ultra-high vacuum (10^?11 torr) conditions show intense bulk plasmons and surface plasmons. Systematic variations in the characteristic energy losses are reported for the series Ag to Te. In addition to losses that may be attributed to plasma oscillations of the 5s5p bands, with N = 1 and 2 electrons, respectively, both Ag and Cd show additional high-energy losses that may arise through 4d-shell participation, with N = 11 and 12.     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

Polarization modulation spectroscopy of surface plasmon resonance

The features of surface plasmon resonance in gold nanofilms deposited on the surface of a total-internal-reflection prism have been investigated theoretically, using the Fresnel equation, and experimentally, with application of the polarization modulation technique. The angular characteristics of the polarization difference of the reflection coefficients for s-and p-polarized light, Δρ = R _s ² ? R _p ² , were measured in the wave-length range λ = 0.4–2.0 μm. It is shown that the characteristics of Δρ, in contrast to the results of standard measurements by the surface plasmon resonance method, have a resonance peak. Due to this, the characteristics of the polarization difference contain nonresonant components whose magnitudes are determined by the internal reflection coefficients for the metal and insulator; these parameters depend on the film thickness. The calculated and experimental data coincide when the model assumes exponential dependence of the refractive indices and extinction coefficients on the thickness of the metal film. It is established that the characteristic parameter of the exponential is a metal film thickness of 11.0 ± 0.5 nm, at which the film optical parameters correspond to the bulk characteristics.     

Electron loss spectra from a W(001)surface

A high resolution energy dispersion analysis of the electron loss spectra (ELS) from a W(001) surface has been done with a specular reflexion geometry. The non-dispersive ELS correlates well with a combination of the bulk and surface loss functions determined from recent optical data whereas the dispersive ELS showed significant differences. Multiple plasmon losses were demonstrated while for the interband losses a momentum selection method has been demonstrated to separate scattering in the surface region from that in the bulk.     

Peculiarities of the radiospectroscopy line shape in nanomaterials

The theory of the radiospectroscopy (nuclear magnetic resonance [NMR] and electron spin resonance [ESR]) line shape for nanomaterials is developed. The consideration was performed in the core and shell models which are, respectively, the nanoparticle regions unperturbed and perturbed by the surface influence. The shift of the resonance frequency by the surface tension was taken into account. The homogeneously broadened line shape was supposed to be Gaussian or Lorentzian. Inhomogeneous broadening of lines via the distribution of nanoparticle sizes was calculated for several forms of the size distribution function. The splitting of radiospectroscopy spectra into two lines decreases with particle sizes, which looks like that in the bulk and on the surface. It was shown to be the characteristic feature of nanomaterial spectra. The changing of these lines’ intensity and width with the change of the distribution function parameters and the particle size decrease was considered. The comparison of the theory with NMR spectra of¹⁷O and²⁵Mg observed recently in nanocrystalline MgO is performed. The calculations fit pretty good the observed size dependence of the line shape, intensity and width.     

ELS study on initial oxidation of Ni(100) surfaces

Electron energy loss spectra of clean and oxygen-covered Ni(100) surfaces were observed with concomitant measurements of LEED, work function change, and Auger peak height ratio O(KL_{2, 3}L_{2, 3})/Ni(L_{2, 3}VV). The observed electronic transitions are interpreted on the basis of primary election energy dependence, and of comparison with the loss spectrum for a UHV-cleaved NiO(100) surface and optical data of Ni. The observed loss peaks at 9.1, 14, and 19 eV in the clean surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electrons, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Three oxygen-derived peaks at 6.0, 8.0, and 10.3 eV in the low oxygen exposure region (?4 L) are ascribed to the O 2p(e) → Ni 3d, O 2p(a₁) → Ni 3d, and O 2p → Ni 4s transitions, respectively. In the high oxygen exposure region (?50 L), the spectra become quite similar to that of the UHV-cleaved NiO(100) surface. The oxidation process consistent with LEED, Auger peak height ratio and work function change measurements is discussed.     

Quantification of surface effects: Monte Carlo simulation of REELS spectra to obtain surface excitation parameter

The surface excitation effect is investigated by using the quantum mechanical frame work of complex self-energy of electrons which interact with a bounded semi-infinite medium. In the self-energy formalism, differential inverse inelastic mean free path (DIIMFP) has contributions from bulk and surface plasmons. Monte Carlo simulation of the interaction of electrons with a solid medium and surface has been performed. The surface excitation parameter (SEP) is then obtained from the simulated reflection electron energy loss spectroscopy (REELS) spectra. The calculated SEP results by Monte Carlo simulation are compared with the previous calculations of total surface excitation probability, which was estimated by a numerical integration of surface term of DIIMFP. The contribution merely due to surface excitations towards REELS spectra is extracted by subtracting the two Monte Carlo simulated REELS spectra that based on the two models of electron inelastic scattering, i.e. a full surface model (SM) and a pure bulk model (BM). The surface excitations found to be significant at low energy losses and diminish at higher energy losses whereas the bulk plasmon contributions show opposite behavior and are negligible at lower energy losses. The average number of surface excitations is then evaluated by the computation of ratio of the integrated surface contribution to the elastic peak. The calculated results for Ag are found to be reasonably in agreement with our previous results for total probability of surface excitations and other reported experimental data for SEP. Surface correction factor (SCF) is calculated using SEP for several metals and is compared with the reported ratio of SCF with Ni sample as reference.     

Generalized mechanism of plasmon excitations in nondiagram X-ray spectra

In the present work, we have studied a plasmon model for the specific contributions of intrinsic as well as extrinsic plasmon effects. The interference ‘plasmon-coupling’ have been obtained theoretically in terms of non-diagram line to diagram X-ray line for the low and high energy satellite profiles in the X-ray photoemission spectra. It is found that our results demonstrate generalized mechanism for the origin of X-ray photoemission satellite spectra.     

Plasmon response of a quantum-confined electron gas probed by core-level photoemission

We demonstrate the existence of quantized "bulk" plasmons in ultrathin magnesium films on Si(111) by analyzing plasmon-loss satellites in core-level photoemission spectra, recorded as a function of the film thickness d. Remarkably, the plasmon energy is shown to vary as 1/d2 all the way down to three atomic layers. The loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. With decreasing film thickness, spectral weight is gradually transferred from the plasmon modes to the low-energy single-particle excitations. These results represent striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures.     

金属氧化物包覆LiMn_2O_4的红外光谱及光电子能谱研究

本文采用溶液化学的方法,在锂离子电池正极材料LiMn2O4上包覆一层金属氢氧化物前驱体,再通过热处理得到稳定的金属氧化物包覆层。傅立叶红外光谱(FIIR)的研究表明,当包覆LiMn2O4中外来金属原子的实际百分比小于1.11%时,不能探测到包覆层红外信号。X射线光电子能谱(XPS)的研究表明,表面包覆层中Al或Mg的存在不仅削弱Mn-O键的作用,而且给体系引入过多的正电荷,导致Mn(2p)结合能的降低。包覆层中的Al2O3和MgO均在加热过程中向LiMn2O4体相中扩散;而包覆层中的SnO2在焙烧后并未进入LiMn2O4晶格内,而是附着于其表面。     

Structure and nonvolatile holographic recording of Mg:Ce:Cu:LiNbO3 crystals

A series of Mg:Ce:Cu:LiNbO₃ crystals has been grown by Czochralski method. Their infrared transmittance spectra and ultraviolet-visible absorption spectra were measured and discussed to investigate their defect structure. The nonvolatile holographic recording of Mg:Ce:Cu:LiNbO₃ crystals was characterized by the two-photon fixed method. We found that the recording time of Mg:Ce:Cu:LiNbO₃ crystals became shorter and nonvolatile diffraction efficiency decreases with the increase of Mg doping concentration, especially doping with Mg approaches and exceeds the so-called threshold. And the nonvolatility vanishes when the concentration of MgO exceeds 4 mol%. The intrinsic and extrinsic defects were discussed to explain the nonvolatile holographic properties in the Mg:Ce:Cu:LiNbO₃ crystals.