Electron spectroscopy applied to the study of reactivity at metal surfaces — A review

Photoelectron spectroscopy (PES) has become an important tool in the study of surfaces and the solid state. This review discusses information obtained on the interaction of gases with metal surfaces. The fundamentals of electron spectroscopy are briefly reviewed and experimental methods outlined. Approaches to the interpretation of both X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectra (UPS) are discussed. The chemisorption of carbon monoxide is examined in detail, PES distinguishes two situations, where CO retains its molecular identity and where dissociation occurs. Bonding of the molecule is considered, as are factors affecting dissociation. Some XPS investigations of metal oxidation are examined and the significance of the several oxygen 1s peaks observed is discussed. The adsorption and decomposition of organic molecules has been studied primarily by UPS. Several investigations are summarized, and the decomposition of formic acid considered in detail. The application of electron excited Auger electron spectroscopy is briefly discussed.     

Expanding the view into complex material systems: From micro-ARPES to nanoscale HAXPES

The analysis of chemical and electronic states in complex and nanostructured material systems requires electron spectroscopy to be carried out with nanometer lateral resolution, i.e. nanospectroscopy. This goal can be achieved by combining a parallel imaging photoelectron emission microscope with a bandpass energy filter. In this contribution we describe selected experiments employing a dedicated spectromicroscope – the NanoESCA. This instrument has a particular emphasis on the spectroscopic aspects and enables laterally resolved photoelectron spectroscopy from the VUV up into the hard X-ray regime.     

5f-electron localization in PuSe and PuSb

Thin layers of PuSb and PuSe were studied by photoelectron spectroscopy. X-ray photoelectron spectroscopy and high-resolution valence-band ultraviolet photoelectron spectroscopy spectra show localization of the 5f states and a low density of states at E(F) in PuSb. In PuSe, which can be classified as a heavy fermion system with low carrier density, we observed three narrow peaks in the valence band, which can be related to the 5f emission. These three features are very sensitive to stoichiometry deviations and disappear for PuSe prepared at T = 77 K.     

Photoelectron emission for layers of finite thickness

Basic tools that describe the energy and angular distributions of photoelectrons emitted by inhomogeneous targets are presented. These are the functions of reflection and transmission, the function of photoelectron emission, the equation for photoelectron flow, and the equation for the transmission function, which describes the dynamics of the angular and energy spectra of photoelectrons when they move in the tar-get. The determining influence of the effects of multiple elastic scattering on the angular distribution of photoelectrons is shown. It is demonstrated in which way and by how much the effect of brightness body rotation, well known in optics, influences the X-ray photoelectron spectroscopy signal. Along with analytical solutions, the results of simulation of the X-ray photoelectron spectroscopy signals are presented.     

Novel approach to tailoring the electronic properties of single-walled carbon nanotubes by the encapsulation of high-melting gallium selenide using a single-step process

A single-step method of filling the channels of single-walled carbon nanotubes with the melt of refractory GaSe is proposed and successfully implemented. The filled nanotubes are investigated by optical absorption spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It is found that charge transfer from the nanotube walls to embedded GaSe accompanied by lowering of the Fermi level in nanotubes occurs in the obtained nanocomposite; i.e., acceptor doping of nanotubes takes place.     

Electron spectroscopy as applied to the layer-by-layer analysis of hydrocarbon coatings produced by ion-plasma deposition

The results of the nondestructive layer-by-layer analysis of hydrocarbon coatings, which is carried out by means of the electron spectroscopy method, are presented. The coating is prepared using the ion-plasma-deposition technique. Analysis of the elemental composition is performed with the help of X-ray photoelectron and Auger electron spectroscopies. The hydrogen concentration is determined via elastic peak electron spectroscopy. The coating structure is investigated by means of electron energy-loss spectroscopy.     

Angular dependence in photoemission: from atoms to surfaces to atoms

Some aspects of atomic and solid-state contributions to the angular dependence of photoemission from core levels, and their relevance and utility in the study of surfaces, are reviewed from a personal and historical perspective. In particular, the role played by dipole angular dependence in photoelectron diffraction and X-ray absorption spectroscopy, and by both dipole and non-dipole effects in X-ray standing wave studies are highlighted.     

等离子体增强化学气相沉积法制备含氢类金刚石膜的紫外Raman光谱和X射线光电子能谱研究

利用脉冲辉光放电的方法,在硅片上采用不同的沉积工艺制备了含氢类金刚石膜层,并采用Raman光谱和X射线光电子能谱(XPS)对膜层进行表征.用Raman光谱仪在波长为325 nm的紫外光源的激励下观察膜层的键结构.紫外Raman光谱对含氢类金刚石膜是非常有用的,它能有效避免可见光Raman光谱测量时的荧光干扰,清晰地得到膜层的D峰和G峰.同时利用XPS分析得到膜层的sp3键含量,并与Raman光谱所得数据进行比较.通过Raman光谱和XPS分析可以发现,在紫外光源的激励下,膜层的G峰峰位向高频移方向移动,G峰峰位、I(D)/I(G),G峰半高宽和sp3键含量之间存在一定的关系.     

Ultrathin film growth and spectroscopic characterization of VOx (0.8x1.3) on Pt(1 1 1)

VO_x ultrathin epitaxial films (0.8x1.3) have been grown on Pt(1 1 1) by evaporating vanadium in a low and well-controlled water background (1×10⁻⁷ Pa). X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and X-ray excited auger electron spectroscopy data strongly indicate that vanadium(II) in the predominant oxidation state. Angle-scanned photoelectron diffraction data are compatible with a single domain film, bearing a rock-salt VO structure and exposing the (1 1 1) plane. Such films evolve toward more oxidized species within hours, probably due to oxidation by residual water in the chamber.     

Electron spectroscopic study of the iron surface and its interaction with oxygen and nitrogen

The interaction of oxygen and nitrogen with a clean polycrystalline iron surface was investigated using Auger electron spectroscopy, photoelectron spectroscopy and electron energy loss spectroscopy. Further evidence for the dissociative nature of adsorbed nitrogen on iron is shown by the photoelectron spectroscopy data in conjunction with preliminary thermal desorption work. The first electron energy loss data for the nitrided iron surface is presented and shown to be very similar to that for the clean and oxidized surfaces.     

Photoelectron imaging of helium droplets

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.     

Development of laser photoelectron spectroscopy based on resonantly enhanced multiphoton ionization

This review article is mainly concerned with laser photoelectron spectroscopy based on REMPI (resonantly enhanced multiphoton ionization), consisting of two parts. One is associated with the measurement of photoelectron kinetic-energy distribution and the other is associated with the measurement of zero-kinetic-energy (ZEKE) photoelectrons in very high resolution. These two techniques are complementary to each other. During the last three decades, the author and co-workers have carried out photoelectron spectroscopic studies of molecules in the gas phase, using the HeI 584 Å resonance line and a nanosecond tunable UV–visible laser. In this article, firstly the author’s background in the field of molecular photoelectron spectroscopy is described briefly, by mentioning three topics of HeI photoelectron spectroscopy. Secondly, the principle and characteristics of laser photoelectron kinetic-energy spectroscopy and its application to several typical topics, namely, one-photon forbidden states (O₂), autoionization from a super-excited state (NO), and small van der Waals molecules (NO–Ar and the NO₂ dimer), and some others. Thirdly, the principle and characteristics of ZEKE photoelectron spectroscopy are described, together with its application to the following several topics: (1) rotational spectra of NO⁺, (2) vibrational coupling of (naphthalene)⁺, (3) large-amplitude torsional motion of (tolane)⁺, (4) rotational isomers of (cis and trans n-propylbenzene)⁺ and structural isomers of (2-hydroxypyridine)⁺, (5) proton tunneling of (tropolone)⁺ and (9-hydroxyphenalenone)⁺, (6) intramolecular vibrational redistribution of trans stilbene, and (7) cation van der Waals molecules of aniline–Ar_n (n=1,2) and azulene–Ar. All the results mentioned as examples are those obtained in the author’s laboratory.     

X-ray photoelectron spectroscopy: Exact solution to the problem with internal sources

A fundamentally new model of multiple photoelectron scattering in solids is described in this paper. It takes into account the nonuniform depth distribution of photoelectron sources and the anisotropic character of single scattering and yields an exact solution to the problem of multiple elastic scattering. Highly accurate and fast algorithms for calculating X-ray photoelectron spectroscopy signals can be developed on the basis of this model.     

Nanoscale modification of Ni/Al interfaces by low-energy $\mathrm{O}_{2}^{+}$ reactive ion beam mixing

The kinetics of growth, composition and electronic structure of thin oxide films formed by reactive ion beam mixing (IBM) of Ni/Al interfaces bombarded with low-energy (3-keV) O₂⁺\mathrm{O}_{2}^{+} ions have been studied at room temperature using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and factor analysis. Initially, NiO species are formed but, later, with increasing ion dose, Ni–Al mixed oxide species appear due to Al incorporation in the near-surface region. These changes are accompanied by a slight increase of the oxygen concentration and a decrease of the Ni/Al ratio in the thin oxide films formed. Angle-resolved X-ray photoelectron spectroscopy shows that Ni–Al mixed oxide species are located nearer the surface than NiO species. Experimental results have been compared with Monte Carlo TRIDYN simulations, suggesting that processes driven by residual defects or the reaction with oxygen predominate over pure ballistic mechanisms during reactive IBM of Ni/Al interfaces.     

A new method to study complex materials in solid state chemistry: application to chalcogenide materials

We show that a combined application of Mössbauer spectroscopy and other experimental tools such as X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy and nuclear magnetic resonance provides a coherent picture of the local electronic structure in chalcogenide materials. In order to develop this idea we propose an analysis of the Sn, Sb and Te local electronic structures for three different systems of materials. The first example concerns the In–Sn–S system. We show that Li insertion in In₁₆Sn₄S₃₂ leads to changes of the Sn oxidation states from Sn(IV) to Sn(II). The second example concerns materials of the Tl–Sb–S system. We show that variations of the ¹²¹Sb Mössbauer isomer shift and surface of the first peak of the X‐ray absorption spectra at the Sb L_III edge can be linearly correlated because of the main influence of the Sb 5s electrons. This is explained by changes in the local environment of the Sb atoms. The last example concerns the crystalline phases of the Tl–Sn–Te system. The formal oxidation numbers of the Te atoms are determined from ¹²⁵Te Mössbauer spectroscopy and X‐ray photoelectron spectroscopy. They are related to the different types of bonds involving the Te atoms in the Tl–Sn–Te compounds.     

Magneto-optic spectrum and electronic structure of single-crystal MnBi

Single crystals of MnBi were used for magneto-optic Kerr-angle spectroscopy and photoelectron spectroscopy. The Kerr-angle peak at 3.35 eV was observed for our nearly oxygen-free bulk samples, but the role of the oxide overlayer, demonstrated by Auger spectroscopy, as a possible origin of this peak cannot fully be assessed. The energy bands from photoelectron spectroscopy were in general agreement with calculated band structures, but were not extensive enough, due to sample cleavage properties, to distinguish reliably between band-structure calculations that disagree.     

Ultrafast photoelectron spectroscopy: Femtosecond pump-probe coincidence detection of ammonia cluster ions and electrons

A new type of experiment is described, in which the femtosecond pump-probe method is combined with the photoelectron-photoion coincidence technique and time-of-flight photoelectron energy analysis. The experimental conditions for observing true coincidences are discussed. The performance of the new time resolved, ultrafast photoelectron spectroscopy is exemplified by studying the excited state dynamics of ammonia molecules and clusters.     

Surface termination of BaTiO3 (001) single crystals: A combined electron spectroscopic and theoretical study

The surface termination of oxide surfaces is of crucial importance for the growth of a second material like metals or other oxides. In this study we have investigated the surface of a BaTiO₃ (001) single crystal during sample preparation by various electron spectroscopic methods. It is shown by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES) that during sputtering a Ba rich overlayer is formed, in which the Ba²⁺ ions are under coordinated. Below this layer, an oxygen deficient BaTiO₃ layer is found. During annealing, we observe the reformation of the crystalline structure. UP and MIE spectra provide clear evidence of a BaO terminated surface. X-ray photoelectron diffraction studies support this result, comparing recorded polar angle scans with calculated intensity modulations using multiple scattering cluster models.     

Synchrotron-Radiation Photoemission Study of Growth and Stability of Au Clusters on Rutile TiO2(110)-1×1

利用同步辐射高分辨光电子能谱研究了金团簇在部分还原TiO₂-(1×1)表面的生长和稳定性．价带谱实验结果观察到非常少量金团簇的沉积导致了Ti3+的3d峰完全消失,表明金团簇成核在TiO₂-(1×1)表面的氧缺陷位．Au4f芯电子光电子能谱实验结果证明了TiO₂-(1×1)表面氧缺陷位向金团簇转移电荷．还对比研究了化学剂量比和部分还原的TiO₂-(1×1)表面上金团簇的热稳定性．当金团簇尺寸相近时部分还原的TiO₂-(1×1)表面上金团簇要比化学剂量比的TiO₂-(1×1)面上金团簇稳定;在相同的表面上尺寸大的金团簇要比尺寸小的金团簇稳定．     

Kinetic measurements by variable-temperature photoelectron spectroscopy. The kinetic rate of the bicyclopentadiene to cyclopentadiene reaction in the medium-vacuum region

Using an appropriate reactor and variable-temperature photoelectron spectroscopy (VTPES), the kinetic rate-constant for the duration of bicyclopentadiene to cyclopentadiene is determined as approximately 1.9 s^?1 at 328°C and 0.1 torr. The study constitutes the first application of photoelectron spectroscopy to the quantitative investigation of chemical reactions. The technique might prove useful for the study of thermal decomposition reactions in the medium-vacuum region.