Classical ultraviolet photoelectron spectroscopy of polymers

Although X-ray photoelectron spectroscopy of polymers was well established by Clark and coworkers in the 1970s, ultraviolet photoelectron spectroscopy of polymer films, was developed later. Previous to the 1970s, the first attempts to use ultraviolet light on polymer films took the form of appearance potential (valence band edge) measurements. Only some years later could the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. The development of what might be termed “classical ultraviolet photoelectron spectroscopy” of polymer films may be loosely based upon a variety of issues, including adapting thin polymer film technology to ultra high vacuum studies, the widespread use of helium resonance lamps for studies of solid surfaces, the combined advent of practical and sufficient theoretical–computational methods. The advent of, and the use of, easily available synchrotron radiation for multi-photon spectroscopies, nominally in the area of the near UV, is not included in the term “classical”. At the same time, electrically conducting polymers were discovered, leading to applications of the corresponding semiconducting polymers, which added technologically driven emphasis to this development of ultraviolet photoelectron spectroscopy for polymer materials. This paper traces a limited number of highlights in the evolution of ultraviolet photoelectron spectroscopy of polymers, from the 1970s through to 2008. Also, since this issue is dedicated to Prof. Kazuhiko Seki, who has been a friend and competitor for over two decades, the author relies on some of Prof. Seki's earlier research, unpublished, on who-did-what-first. Prof. Seki's own contributions to the field, however, are discussed in other articles in this issue.     

Investigating ultraviolet photoelectron spectroscopy of ice

Photoelectron energy distribution curves (EDCs) from ice excited by HeI (21.2 eV) and NeI (16.8 eV) radiation are presented. The strict connection between valence density of states and EDCs forces to rule out the previous suggestion by Shibaguchi et al. that conduction band density of states is of paramount importance in determining the EDCs excited by ultraviolet light. The results also allow a discussion of band calculations published till now; the need for a theoretical investigation on photoemission from ice is put forward.     

Adsorption studies by ultraviolet photoelectron spectroscopy

The application of ultraviolet photoelectron spectroscopy (UPS) for adsorption studies is illustrated using three examples. The observation of difference curves for various gases adsorbed on the same substrate allows those features due to the substrate or the adsorbate to be distinguished. The observation of the coverage dependence of UPS difference spectra allows one to relate the observed features to results from other techniques, as shown for oxygen adsorbed on the (100) face of tungsten. Finally, the measurement of difference spectra of the same species adsorbed on different faces of the same crystal gives information on the substate-adsorbate bonding as illustrated for the case of hydrogen on the (100) and (110) faces of tungsten.     

Vacuum ultraviolet photoelectron spectroscopy of atoms and molecules

For a complete study of the photoionization of atoms and molecules it is essential to make use of the technique of Photoelectron Spectroscopy and the continuum characteristics of synchrotron radiation. A brief review is given of the application of the above techniques in measuring partial photoionization cross sections and the angular distribution assymetry parameter β. Selected results are given, which are compared to theoretical values.     

Study of methyl chalcogen compounds with ultraviolet photoelectron spectroscopy

Dimethyl sulfide, dimethyl selenide, dimethyl telluride, dimethyl disulfide, dimethyl diselenide and dimethyl trisulfide have been studied by ultraviolet photoelectron spectroscopy. For the first time, the high resolution He(I) spectrum of dimethyl trisulfide is reported. However, the main thrust of this work is to examine trends among related compounds. We report the results of correlations of the first three ionization energies of (CH₃)₂X, X = O, S, Se, and Te and of the correlations of the first five ionization energies of (CH₃)₂X₂, X = O, S, and Se with the ionization energies of oxygen, sulfur, selenium, and tellurium. Where possible, orbital trends of isoelectronic molecules were also examined. Finally, we report the results of correlations of the FWHM of the first three bands of CH₃(S)_nCH₃ with the number of sulfur atoms and investigate convergence of the first two orbital bandwidths to the bandwidths of the two highest occupied bands in μ-sulfur.     

Angle-dependent ultraviolet photoelectron spectroscopy of sputter cleaned polycrystalline noble metals

Polycrystalline noble metal films are commonly used in practical applications across a variety of different fields. However, the surface electronic structure of the noble metals has primarily only been studied on single crystal substrates. In addition, sputter cleaned polycrystalline noble metal films are commonly used substrates in ultraviolet photoelectron spectroscopy (UPS) studies, but have yet to be systematically studied in terms of their photoemission anisotropy. The angle-dependence of the valence band spectra of sputter cleaned polycrystalline Au, Ag and Cu were studied using angle-resolved UPS. It is found that the photoemission is anisotropic with respect to photoelectron take-off angle. The results for Ag and Cu are in good agreement with previous reports of surface d-band narrowing in polycrystalline noble metal films. However, significant anisotropies in the d-band, s-band and Fermi edge of sputter cleaned Au are observed, which cannot be attributed to surface d-band narrowing alone. The unusual results for Au are attributed to drastic changes in the film morphology near the surface as a result of sputter cleaning.     

Initial oxidation of Mn surface studied by ultraviolet photoelectron spectroscopy and metastable induced electron spectroscopy

Metastable induced electron spectroscopy (MIES) was combined to ultraviolet photoelectron spectroscopy (UPS) to study the initial steps of manganese oxidation. Oxygen exposure directly led to the formation of MnO with no intermediate states. The MnO feature saturation observed by MIES and UPS techniques showed noticeable differences and proved the formation of several oxide layers. The oxidation kinetics was studied by measuring MnO features by UPS, which depend on the surface coverage by oxygen. We observe a decrease of oxygen adsorption probability with oxygen exposure. Oxidation proceeds by oxygen dissolution into the first layers to form a three-dimension MnO. This hypothesis was confirmed by our work function measurements.     

Intensity measurements of extreme ultraviolet radiation from helium plasma by photoelectron spectroscopy

Extreme ultraviolet radiation emitted from He plasma has been studied by means of photoelectron energy analyzer. Relative and absolute intensities of Lyman series of ionized helium have been measured. From these spectral intensities, the electron temperature of the plasma has been estimated at several tens of eV.     

Electronic band structure of graphite studied by highly angle-resolved ultraviolet photoelectron spectroscopy

The electronic valence band structure of graphite has been experimentally determined with considerable accuracy for the first time by the highly angle-resolved ultraviolet photoelectron spectroscopy. The photoemission measurements were performed on a synthesized single-crystalline graphite (kish graphite) with the He II resonance line as an exciting source. Some high symmetry points in the Brillouin zone have been successfully identified in the experimentally determined band structure. The present experimental result has been compared with early photoemission experiments and some band calculations presented so far.     

Doppler-free spectroscopy of the 8s state of Cs

Doppler-free two-photon spectroscopy and ion detection in a thermionic diode were used to measure the hyperfine splitting and absolute term energy of the 8S state of Cs. The results, a=219.3(2) MHz for the magnetic dipole coupling constant, and E(8S)=24317.1499(4) cm^-1 for the term value of the c.g. of the 8S state relative to the c.g. of the ground state, agree well with earlier, less precise measurements. The hyperfine coupling also agrees well with a recent relativistic Hartree-Fock calculation.     

Zn(0001) water interaction: An ultraviolet photoelectron spectroscopy investigation

The (0001) face of a Zn crystal has been exposed a ～300°K and at ～345°K up to 20.000L of water with no modification of the photoemission spectra at hν = 21.2 eV. Approximately one monolayer of reaction products is seen after condensing water at low temperature (～140°K) and desorbing it thermally. A broad peak at ～5 eV below Fermi level is seen and is attributed mostly to the formation of hydroxide.     

Angle-resolved ultraviolet photoelectron spectroscopy of expitaxial films of vanadyl-phthalocyanine grown on alkali halides

The molecular orientation of Vanadyl-Phthalocyanine (VOPc) grown on alkali halides was studied by Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS). To avoid charging up of the specimen during ARUPS measurment, thin epitaxial films of alkali halides grown on GaAs (0 0 1) were used as substrates. Photoelectrons emitted from the highest occupied molecular orbital band of VOPc showed strong angular distribution leading to the conclusion that the molecules stay almost parallel on the surface. This was in accordance with the expectation based on the reflection high-energy electron diffraction analysis of the film.     

Study of chemisorption on silver surfaces using ultraviolet photoelectron spectroscopy (UPS)

Ultraviolet photoelectron spectroscopy (UPS) was used to study the chemisorption of halogens on stepped [3(111) × (100)] and low-index (111) silver surfaces. The initial rate of halogen adsorption using CHCl₃ exposure on the silver stepped surface is approximately twice that on the low-index surface. This indicates that steps play an important role in chemisorption even on metals with a low density of states at the Fermi level. The adsorbate-induced levels on silver were correlated with halogen p valence orbitals using model extended Hückel calculations. Changes in the silver d band are interpreted as due to p?d orbital interactions.     

X射线光电子能谱

X射线光电子能谱（X-ray photoelectron spectroscopy，XPS）技术也被称作用于化学分析的电子能谱（electron spectroscopy for chemical analysis，ESCA）．XPS属表面分析法，它可以给出固体样品表面所含的元素种类、化学组成以及有关的电子结构重要信息，在各种固体材料的基础研究和实际应用中起着重要的作用．文章简要介绍了XPS仪器的工作原理和分析方法，并给出了XPS在科学研究工作中的应用实例．     

X射线光电子能谱

X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）技术也被称作用于化学分析的电子能谱（electron spectroscopy for chemical analysis,ESCA）.XPS属表面分析法,它可以给出固体样品表面所含的元素种类、化学组成以及有关的电子结构重要信息,在各种固体材料的基础研究和实际应用中起着重要的作用.文章简要介绍了XPS仪器的工作原理和分析方法,并给出了XPS在科学研究工作中的应用实例.     

Metal-insulator transitions in Cr-doped V2O3 by x-ray and ultraviolet photoelectron spectroscopy

X-ray and ultraviolet photoelectron spectroscopy have been employed to investigate the high temperature metal-insulator transitions in V₂O₃ and (V_0.99Cr_0.01)₂O₃. The high temperature transitions are associated with more gradual changes in the 3d bands compared to the low-temperature transitions. Communication No. 20 from Solid State & Structural Chemistry Unit.