Comparative Ab initio analysis of valence XPS data for acenes and paraphenyl oligomers: Application of the molecular-orbital intensity model

The electronic structures of acenes and paraphenyls are approached by ab initio calculations and XPS valence data. Particular attention is drawn to the molecular-orbital intensity model, which proves to be a quite successful means of following the evolution of all the valence orbitals as a function of system size.     

The XPS valence band spectra of NbC

The x-ray photoemission valence band spectra of NbC have been measured and are compared with the x-ray emission spectra and with the results from band structure calculations. This comparison leads to a large enhancement of theC2 _p photoabsorption cross section (at=1,487 eV) compared to the value calculated for the free atom. The effect of the nonmetal vacancy in the valence band can be described very well with vacancy cluster calculations.     

Surface effects in the valence band XPS spectrum of copper

Comparison of valence band XPS spectra of Cu samples with theoretical density of states shows a high intensity near the upper edge of the d-band. This is a feature common for most d-band metals. However, if the difference in the density of states of the first two atomic layers with respect to the bulk is taken into account, good agreement between theory and experiment is found.     

The XPS valence band of nickel metal

The XPS valence band spectrum of evaporated nickel is compared with a theoretical density of states curve and with a synthetic density of states curve obtained by adding two XPS valence band spectra of copper shifted by 0.3eV with respect to each other. Both calculated curves agree with the main features of the XPS valence band spectrum.     

Theoretical study of EXAFS-like modulation observed in valence XPS spectra

Dynamic theory developed previously by the authors is applied to photoionization from delocalized molecular orbitals. EXAFS-like oscillation is observed in any case, but the origin, period and magnitude differ from those of EXAFS usually observed in core photoionization. Numerical calcu- lations are performed in the energy range k = 3–6 a.u. for three molecules: H₂O, CO and CO₂. Analysis of such oscillatory behavior aids in the characterization of photoionized orbitals.     

An XPS study of the valence bands in solid and liquid bismuth

High resolution XPS measurements on bismuth show the valence band structure of the liquid phase to be very similar to that of the solid and very different from that of free electrons. The spectra are in general accord with recent calculations of the densities of states for crystalline and amorphous group V elements.     

XPS study of the valence band spectrum of amorphous Fe-B alloys

XPS spectra of Fe 3s and Fe 3p levels and Fe 3d valence band of amorphous iron-boron alloys were measured as a function of boron concentration over the range from 12 to 25 at% B. It was found that a characteristic hump appeared at lower energy region in Fe 3d band spectrum of every amorphous Fe-B alloy. The origin of the hump was discussed in terms of the formation of a new bonding state between Fe and B atoms in amorphous Fe-B alloys.     

X-ray photoelectron spectroscopy (XPS) of adsorbate valence bands

The CO valence levels for a monolayer of CO adsorbed on the basal (001) face of ruthenium have been observed by XPS. The assignment of the observed peaks is discussed.     

Evidence for localizing properties of XPS applied to valence bands

X-ray photoelectron spectroscopy (XPS)-measurements of the valence bands of Cu, Ag and Au have been performed and their integrated intensities have been compared. A model calculation which considers the influence of the x-ray absorption length as well as the electronic escape depth on the obtained intensities was established and applied to the measurements. An additional electron microscopic study on the surface structure of the evaporated samples has been performed. Its influence on the obtained photoelectron yield is discussed. The results support a marked dependence of the integrated valence band intensity on the atomic number. The implications for the interpretation of XPS valence band spectra of alloys are discussed and compared with corresponding x-ray isochromat spectroscopy (XIS)-measurements. Furthermore we deal with the important question whether XPS-as well as XIS-spectra can be described mainly in terms of the electronic band structure or whether atomic features are also present, though both methods are free from participation of core levels. The results are discussed in terms of localizing properties of XPS.     

Giant enhancement of the valence band photoemission intensity in γ-Ce

We have observed a giant enhancement of the Ce valence band emission above the 4d absorption threshold. It is interpreted as being due to autoionization decay following 4d¹⁰4f¹ → 4d⁹4f² transitions. By taking advantage of this effect we have been able to make a determination of the location of the 4f levels in γ-Ce.     

High resolution core and valence band XPS spectra of non-conductor pyroxenes

High resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of the non-conductor pyroxene minerals, bronzite ((Mg_0.8,Fe_0.2)₂Si₂O₆) and diopside (Ca(Mg_0.8Fe_0.2)Si₂O₆) have been obtained with the Kratos magnetic confinement charge compensation which minimizes differential charge broadening. Observed Si 2p, O 1s, Mg 2p and Ca 2p total linewidths are all about 1.3 eV, very similar to those observed previously with the same instrument for SiO₂ and olivines ((Mg,Fe)₂SiO₄); and we consider that these widths are within 0.05 eV of the minimum room temperature linewidths for these samples with the experimental resolution of this instrument of 0.35 eV. These linewidths are all determined by vibrational broadening due to the M-O symmetric stretch in the ion states. The Si 2p binding energies (BE) are intermediate between the quartz and olivine Si 2p binding energies; but the O 1s spectra resolve the bridging oxygen (BO) and non-bridging oxygen (NBO) in the unit, with the NBO O 1s very close in BE to the O in olivine, and the BO very close to the BO in SiO_2. Indeed in both diopside and bronzite, it is possible to separate the three structurally inequivalent O atoms in the O 1s spectra: the BO at a BE of about 532.6 eV, a NBO peak from the MgOSi moiety (Mg in the M1 site) at about 531.3 eV, and a NBO peak at 531 eV from the CaOSi or the FeOSi moieties (Ca and Fe in the M2 site). The O 1s BE increases with the increase in the electronegativity Ca < Mg < Fe < Si. Moreover, the linewidths of these peaks increase when Fe and Mg are both present in either M1 (diopside) or M2 (bronzite) sites.The valence band spectra for the two pyroxenes are rather similar, and quite different from the VB spectra of quartz and olivines. The dispersion of the pyroxene VB spectra is intermediate between the VB spectra of quartz and olivines; the valence band spectrum of pyroxenes are more dispersed than in olivines by about 1.5 eV but less dispersed than quartz by about 1.5 eV. These VB spectra can be assigned using the previous olivine VB spectra and high quality pseudopotential density functional theoretical calculations in the generalized gradient (GGA) approximation. As for the olivine VB spectra, the Fe 3d t_2g and e_g orbitals in M1 and M2 sites of the pyroxene are located at the top of the pyroxene valence band, and the BE of the Fe 3d peaks from M1 are about 0.7 eV smaller than the Fe 3d peaks in M2. The theoretical XPS valence band spectra using the theoretical density of states and the Gelius intensity approximation are is in good semi-quantitative agreement with the experimental spectra. This intermediate dispersion of pyroxenes is due to the partial polymerization of the Si-O units in pyroxenes, and the intermediate charge on the Si atoms as indicated by the Si 2p BE.     

Angle-dependent valence photoemission from adsorbed molecular nitrogen a comparison of measurements and model calculations

He I and He II photoelectron spectra for N₂ adsorbed on W (110) have been measured at three different polar emission angles. The behaviour of the adsorbate-induced features with angle is compared to the angle- dependence of dipole matrix elements for oriented N₂, Ni-N₂, and Ni₉-N₂ clusters obtained by SCF-Xa-SW calculations. The agreement is very good for He I and acceptable for He II, if the N₂ axis is assumed normal to the “surface” and the assignment of peaks proposed earlier is used. For He II the use of the big cluster is important to arrive at good agreement.     

On the influence of oxygen on the valence band shape of carbon measured by XPS

XPS-spectra of the valence band of pyrolytic graphite with different amounts of contamination by oxygen have been investigated. The strong influence of oxygen on the XPS-spectra is discussed in terms of adsorbed as well as chemisorbed oxygen. A carbon valence band shape free from distortion by oxygen is presented.     

Application of factor analysis to XPS valence band of superparamagnetic iron oxide nanoparticles

X-Ray photoelectron spectra of nano-sized superparamagnetic iron oxide nanoparticles were examined with the aim to discriminate the different degree of iron oxidation. Careful analysis of the valence band regions reveals the presence of both Fe₃O₄ and Fe₂O₃. The application of factor analysis enabled us to extract the relative molar concentrations of these oxides in the nanoparticles. This is of particular interest in improving the magnetic properties of iron oxide nanoparticles whose superparamagnetic character can be optimized to obtain better contrast in images from nuclear magnetic resonance. As a result, the factor analysis allows tuning the nanoparticle synthesis conditions in order to obtain the optimal magnetic properties for imaging. Results obtained by the XPS valence band analysis were compared to the transmission electron microscopy, X-ray diffraction and Raman measurements.     

XPS valence bands of Ti,Zr, Nb,Mo and Hf

The XPS valence band spectra of polycrystalline Ti, Zr, Nb, Mo and Hf have been measured with a resolution of 0.6 eV. The spectra were compared with theoretical densities of states. Good agreement is achieved if the partial densities of states are considered and a many body line shape function and life time broadening are taken into account. The sigularity index determined at various core lines as well as the experimentally determined relative valence band cross sections are reported.     

On the valence model for radiative capture

We give several parametrizations for the elastic scattering and radiative capture cross sections for low neutron bombarding energy and discuss the relationship between the corresponding resonance parameters. We then perform an extensive investigation of the valence radiative capture model of Lane and Lynn. This model is formulated here in the frame of the shell-model approach. We exhibit the similarities and differences between our results and those derived from the R-matrix approach by Lane and Lynn on the one hand and from the optical-model approach by Lane and Mughabghab on the other hand. Particular attention is paid to the choice of the average potential well in the shell-model approach, in relation to the proper way to identify theoretical quantities and phenomenological parameters. We show that practically equivalent results can be obtained from a complex average potential well and from a suitably chosen real potential well, respectively. The following topics are investigated formally and numerically: dependence of the various theoretical expressions on the choice of the (real or complex) average potential well; relative importance of external and internal capture; dependence of photon widths and background cross section on mass number (for thermal energy and for E = 100 keV); dependence of the resonance parameters and background cross sections on energy, for A = 60; comparison between experimental data and theoretical values for radiative capture on ⁵⁶Fe and ⁶⁰Ni. We discuss the conditions of validity of the valence capture model. In particular, we investigate the role of the giant dipole resonance and of the closed channels. We argue that the success of the valence capture model is intimately related to the importance of external capture. The contribution of the low-lying excited target states is investigated formally and numerically; it increases with mass number and tends to diminish the correlation between neutron and photon widths, which is implied by the valence capture model.     

Photoemission intensity oscillations in the valence bands of C70 film

We have measured and analyzed the photoemission spectra (PES) of a C₇₀ film in the photon energy region from 13.4 eV to 98.4 eV. The photoelectron intensities of two C 2p π-derived features (denoted by A and B) oscillate regularly in the whole energy region with some fine structures below ∼30 eV. To obtain the detailed information of the oscillations, we have developed a sophisticated but practical procedure for intensity calculation. The procedure consists of two core concepts. The first is ascribing the PES features to their corresponding molecular orbitals with the help of density functional calculations. The second is a background subtraction algorithm. With this procedure, we obtained the oscillating behavior for individual features (A and B), which is by and large consistent with the predictions based on the spherical symmetric approximation although C₇₀ has the ellipsoidal shape. Owing to the solid state effect, the oscillating amplitudes of the A/B intensity ratios are smaller than those of gas phase C₇₀, but an orbital shift reported recently was not observed on our sample. The oscillating curve of a deeper feature, which consists of both σ and π states, are also reported.