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The photoionization cross section cr of 4d 2 states of Mo in 2H-MoS2 in the photon energy range 65-200 eV is compared with the 4d cross section from the bonding states of bcc Mo and with theoretical values for isolated Mo atoms. The h v dependence of cr in MoS2 is very close to the atomic cr with some deviations at higher hp, whereas the afrom Mo metal is severely distorted. An interpretation scheme is suggested to connect these differences in the cross sections to the different arrangements of the electron states in the solids.%"e have shown recently that changes in the photoionization cross sections (o-) of 4d and Sd states on passing from isolated atoms to the solid can be qualitatively discussed in terms of the degree of atomiclike behavior of the d orbitals in the solid. ' This is particularly significant in connection with the modification of the so-called Cooper minimum, which is well known at the atomic level and which basically originates from a cancellation in the dipole matrix element.T he crucial problem now is to clarify to what extent the modifications of the o. (h v) with respect to the atomiclike behavior can be related to the non-atomic-like behavior of the electron states in the solid, i.e. , to what extent photoelectron cross sections can be used as a new probe of the chemical bond in solids. This can be of paramount importance in consideration of the relative simplicity of the measurements of a (hv) and of the increasing availability of synchrotron radiation. The present work adds important new information to our previous papers' and contributes to the understanding of this solid-state effect. The work is based on a comparison of the experimental cr of the same atomic orbitals in different solid structures, showing how the differences on the wave functions are reflected in the cross sections. We compare the ain the range 65-200 eV for Mo 4d states in a case of marked distortion' with respect to free atoms (bcc Mo) and in a case (2H-MoS2) which is a good candidate for an atomiclike behavior. We give here the first experimental cr4d in MoS2 measured for this purpose with the same method and apparatus used for Mo (Ref. 1), we compare the experimental results with theoretical o-for atomic Mo (Ref. 7), and we suggest an interpretation scheme.Experimental details of the present work performed at Stanford Synchrotron Radiation Laboratory (SSRL) were presented in our previous work and are not repeated here. It suffices to say here that the light was at near-normal incidence to the sample so that reflectivity corrections were negligible.The spectra were recorded with a double-pass cylindrical mirror analyzer (CMA), with the analyzer axis at right angles to the light (and so the measurements were integrated over a large solid angle). The MoS2 sample was natural 2H-MoS2 crystal, characterized with x rays, and cleaved in situ (base pressure 5&10 " Torr). Since a MoS2 single crystal was used, diffractive effects are also present in the measurement.Fortunately, these effects take place at energies different...

We have carried out synchrotron radiation measurements both from valence states and core levels from Si(111)–Cu, Si(111)–Ag, Si(111)–Au, Si(111)–Pd interfaces before and after exposure at room temperature to 30×106 L of oxygen and we compare the results with those for the oxidation of Si(111). In all cases the oxygen interacts with Si and not with the metal, and the Si reaction rate is strongly increased with respect to that of Si(111). The strongest oxidation enhancement is obtained with Cu and Pd. In the noble metal case the interaction with oxygen produces the overgrowth of a SiO2-like phase having a Si 2 p chemical shift of ∠3.8 eV as opposite to Si(111) which cannot be oxidized to SiO2 in these conditions. Moreover, in the Si–Pd case the oxidation number is lower and the chemical shift is ∠2.3 eV. We discuss the relevance of the results both in terms of the structure of the interface and of the nature of the chemical interaction between Si and d metals.

Experimental evidence is presented for solid-state effects on the Ad photoionization around the Cooper minimum for a few transition metals: Ag, Pd, Mo, and Zr. The Ad parital cross section appears to be very sensitive to the d-d interaction; thus, the antibonding d states have a more atomic like cross section than the bonding d states.PACS numbers: 79.60.Cn, 71.70.-d As part of the development of a better understanding of atomic photoionization processes, the behavior of the cross section in the Cooper minimum (CM) region has been receiving increased attention. 1 " 5 Also, a field of current interest is the transition from atomic physics 6 " 8 to. solidstate physics and what implications that has for the photoionization processes and cross sections. In this Letter, we will address the latter problem and present unambiguous evidence for solid-state effects in the partial photoionization cross section of the Ad valence-band electrons in transition metals (Ag, Pd, Mo, and Zr).The interest of CM effects is not limited to the basic questions of atomic to solid-state transition. The CM effect has recently been used in surface physics to obtain information about the local nature of the bonding between a transition metal (Ad or 5d electrons in the valence band) and a semiconductor substrate (in particular Si and Ge), i.e., in the research on transition-metal silicides and germanides. 9 " 11 The important aspect in this spectroscopy is that the transition-metal d cross section is sufficiently low in a certain photon excitation region that the Si (or Ge) sp valence states and their changes upon bonding to the transition metal can be observed. 10 Spectroscopy of solids based on the CM effect raises several questions of fundamental interest:(1) Do CM's always exist in solids composed of atoms having CM? (2) Is there, in general, a modification of the CM due to a solid-state effect?(3) Is there any correlation between such a solidstate effect and the nature of the Ad (or 5d) wave functions in the solid? It thus seemed timely to try to answer some of these fundamental questions. It should be noted that the investigation of CM in solids can have a strong impact on the understanding of CM in atomic systems; solid-state experimental results can be used to test models in a great variety of situations as far as symmetry, shape of the wave functions, and relevant many-body effects are concerned. 6 We selected four transition-metal elements along the Ad period in order to change some important parameters of the Ad initial states: the noble metal Ag (fee), the near-noble element Pd (fee), and two refractory elements at the beginning of the period characterized by strong d-d bonds and different crystal structures, Zr (hep) and Mo (bcc). Our data give a comparison of the shape of the experimentally determined Ad photoionization cross sections (a) in the photon energy range 70 to 200 eV.The experiments were done on the "grasshopper" monochromator 12 at the Stanford Synchrotron Radiation Laboratory with use of a cylindrical mirror analyze...

We present extensive results on synchrotron-radiation angle-integrated photoemission from Si(111)surfaces onto which increasing amounts of Pt (coverages 8 from 0.07 to 40 monolayers) were deposited. Both core lines (Si 2p and Pt4f) and valence-band states have been measured. In the latter case we present results taken at a photon energy of h v=80 eV where the Pt Sd contribution is dominant and at h v=130 eV where the Cooper minimum effect reduces the Pt 5d photoemission considerably so that information on the Si contribution to the valence states can be revealed. We show that submonolayer coverages (8 & 0.07) of Pt disrupt the surface sufficiently to introduce considerable changes in the photoemission spectra with respect to that of clean Si(111). An interface with a photoemission spectrum resembling that of a silicide has developed at about 2 -10 monolayers. At increasing 8 the region explored with photoemission shows enrichment of the metal, but the situation at 8=40 is still very far from that of the pure Pt metal, thus indicating a very strong chemical interaction at room temperature on the depth scale of tens of monolayers. Opposite chemical shifts of the Si and Pt core lines are seen (Pt towards lower and Si. towards higher binding energies with increasing 8) and, moreover, the shape of the Si 2p core lines is modified towards that typical of a metallic phase. All these results are discussed in terms of the nature of the chemical bond between Si and Pt.