The chemisorption of CO on Cu(100) studied with angle resolved photoelectron spectroscopy

Using angle resolved UV photoelectron spectroscopy, coupled with the continuum of polarized light available with synchrotron radiation, we make an interpretation of the photoelectron spectrum of CO adsorbed on Cu(100). We point out that the bonding of CO on Cu as observed by photoemission is considerably different from the bonding of CO on Ni. These differences do not seem to be caused by bonding orientation differences, however, as the CO molecular axis is found to be very close to the surface normal, as was the case for CO on Ni(100). No evidence is found for a second phase of CO on Cu(100).     

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.     

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.     

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.     

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.     

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.     

Surface electronic state on stepped Cu(755) studied by angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation

The surface electronic state on the stepped surface of Cu(755) has been investigated by means of angle-resolved ultraviolet photoelectron spectroscopy using synchrotron radiation(SR-ARUPS). We have observed a free-electron-like surface state below the Fermi level. In spite of the anisotropy of the atomic arrangement due to steps, the surface state is shown to be isotropic since the dispersion profile and the peak shape are almost identical in the directions parallel and perpendicular to the steps. This result makes a clear contrast with the previous SR-ARUPS results on Ni(755) surfaces which have the surface structure similar to Cu(755). Those experimental evidences are discussed based upon the electron configurations of both metal substrates.     

Ultraviolet photoelectron studies of the chemisorption and decomposition of propyne on Fe(110) and Fe(100) surfaces

The chemisorption and decomposition of propyne on clean Fe(110) and Fe(100) surfaces have been studied by ultraviolet photoelectron spectroscopy (UPS). The UPS spectra suggest that exposure at 78 K produces chemisorbed propyne on both these surfaces, while excess exposure of the adsorbate produces a condensed propyne phase at the same temperature. Upon warming to ～100 K, the condensed phase disappears and the chemisorbed phase appears again on both the surfaces. Further warming to ～170 K produces a dissociatively adsorbed propyne phase on Fe(100), and the spectral features then scarcely change for further warming to ～370 K. In the case of Fe(110), on the other hand, the chemisorbed phase is apparently unchanged by heating even up to ～370 K. All peaks due to chemisorbed propyne on both Fe(110) and Fe(100) disappear at ～470 K, with the formation of carbonated surfaces.     

A study of SO2 adsorption on CaO (100) surfaces using X-ray photoelectron spectroscopy

The adsorption of SO₂ on CaO (100) at 300 K has been studied using X-ray photoelectron spectroscopy. Under ultrahigh-vacuum conditions, the surface was exposed to 0–500 Langmuirs of SO₂. The resulting adsorption yields a single SO surface species with an S 2p peak at 168.2 eV and an O 1s_{$\text{sol1}\text{2}$} peak at 531.7 eV. Subsequent heating of the exposed surface to 673 K indicated no desorption or changes in the binding energies of the S 2p and O 1s_{$\text{1}\text{2}$} peaks. On the basis of these data and binding-energy data for standard compounds, the adsorbed species is identified as SO₄^2?. The surface coverage due to the SO₄^2? species was also measured as a function of SO₂ exposure. From these data, the initial adsorption is found to be first-order in surface coverage, and the initial sticking probability is found to have a value of 0.4.     

Angle-resolved photoelectron spectroscopy of geometrically nonuniform surfaces

The formation of angle-resolved photoelectron spectra for crystal surfaces with a complex geometric relief is considered. A simple model of a thin corrugated single-crystal film is proposed which illustrates the main specific features of this process. It is shown that photoelectron spectra of these surfaces exhibit features that, rather than reflecting the true electronic structure of the system, result from its superposition with the geometric structure. The validity of the model is demonstrated with the use of a physical system based on Ni(771) and Ni(755) stepped surfaces with periodic steps. It is revealed that, as a graphite coating grows to a monolayer thickness, these surfaces become faceted to form a geometrically nonuniform surface relief. The photoelectron spectra obtained for such a surface can be used to derive a quantitative characteristic of both the electronic structure of the surface and its geometric properties. The surface topography of the system is determined independently using scanning tunneling microscopy.     

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.     

Highly-angle-resolved ultraviolet photoelectron spectroscopy of C8Cs

Highly-angle-resolved ultraviolet photoelectron spectroscopy was carried out for a C₈Cs single crystal to study the electronic charge transfer in alkali metal graphite intercalation compounds. The dispersive π¹-band at the K̃ point in the Brillouin zone was observed for the first time. The electron occupation in the π¹-band was estimated to be 0.45±0.05 unit electronic charge. This strongly suggests that a substantial part of an interlayer band exists below the Fermi level at the γ point, forming a spherical Fermi surface on the center of the Brillouin zone.     

Optical spectroscopy of surfaces: Reflectance studies of chemisorption

A high precision technique has been used to study the small changes in near-normal incidence reflectance R caused by chemisorption of gases on an atomically clean metal surface maintained in ultrahigh vacuum. Examples are given for O₂, CO, and H₂ on Mo(100) in the photon energy range 1.9 <?ω < 4.8eV. The relative reflectance change $\text{ΔR}\text{R}$ is negative and reaches as much as 1%. Structure that appears in ΔR(e), where e=exposure (pressure × time), is attributed to different binding configurations of the adatoms as observed, e.g., in LEED experiments. The dependence of $\text{ΔR}\text{R}$ on ?ω suggests the presence of adsorbate-induced surface states within a few eV of the Fermi energy. Possible approaches to determining the dielectric function of the surface region from the data are discussed.