Band Photoemission

Van Buuren et al. [105] performed photoemission and X-ray absorption experiments on Si nanocrystals to determine the TVB and BCB shifts, respectively, as a function of size. The Si nanocrystals were grown in situ at 1700 °C in an Ar gas buffer of 112 mTorr followed by hydrogen exposure to passivate the surface. The resolution of the photoemission and absorption measurements carried out on a synchrotron radiation source were 0.25 eV and 0.05 eV, respectively. They observed a valence band to conduction band shift ratio of 2 1 for all sizes of Si nanocrystals. This is in agreement with various calculations reported for Si nanocrystals [106]. 

Another method to avoid charging of the semiconductor samples is to mix the samples with a conducting matrix such as graphite. Nanda et al. [108] measured 

ZnS is obtained using the LMTO-ASA method and those for the nanocrystals are from the sp d model with nearest neighbor and anion-anion interactions. Adapted from [108]. 

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Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

Strasser T, Starrost F, Soiterbeck C and Schattke W 1997 Vaience-band photoemission from GaN(OOI) and GaAs ... 

Strasser T, Soiterbeck C, Starrost F and Schattke W 1999 Valence-band photoemission from the GaN(OOOI) surface Phys. Rev. B 60 11 577... 

Figure 6.25. Valence band photoemission spectra of 1 ML Ceo on a Ag(lOO) surface as a function of potassium doping. Also shown are the spectra of the clean Ag(lOO) surface and of a Ceo multilayer (bottom). All binding energies are referred to the L f of polycrystalline silver. Reprinted from Surface Science, Vols. 454-456, C. Cepek, M. Sancrotti, T. Greber and J. Osterwalder, Electronic structure of K doped Ceo monolayers on Ag(OOl), 467 71, Copyright (2000), with permission from Elsevier.

Partial localization of the 5 f states in the light actinides (line III of subsection b) might cause the appearance of satellite structures at energies not very far from Ep in their valence band photoemission spectra. If such structures could be convincingly demonstrated, important information would be added to the theoretical analysis of the locahza-tion vs. itineracy problem of the actinide metal series. 

Important information on this problem has been obtained by Grunze 141). It appears that after CO chemisorption on Pd, the d-band photoemission (UV) is attenuated (differential spectra show a negative band) and two new bands appear due to the chemisorbed CO [(5cr + 7t)-band and (4ff)-band]. A decreasing particle size causes an increase in the apparent BE of all three bands—the shift is almost the same for all three bands. This again indicates that the final photoemission effects could be responsible for the shifts observed. 

Figure 23. Valence band photoemission spectra of D, and D-. Cu(II)Cl = (hv = 200 eV). The relative intensity of Yhe Cu 3d estimated. Reproduced from Ref. 13 Copyright 1982, American Chemical Society.

Valence and core-level photoemission experiments were carried out in the IFW Dresden under the following conditions. Unless otherwise stated, the valence band photoemission data were recorded at room temperature using He Ia radiation (21.22 eV) with a total energy resolution of 100 meV, and the core-level photoemission data, using monochromatised Al Ka radiation (1486.6 eV) with a total energy resolution of 0.4 eV. All such data are angle-integrated in nature ( 4° or more). 

Fig. 2 Valence band photoemission profiles of empty C82 and Gd C82, recorded at room temperature with He Ia (21.22 eV) radiation. The inset shows the region close to the Fermi level on an expanded scale. For this photon energy, the photoionisation cross sections of the C 2s and C 2p levels dominate that of the Gd 4f levels...

Fig. 4a-c Valence band photoemission data from thin films of a Gd C82, b Tm C82 [C82 isomer Cs(6)], and c Tm C82 [C82 isomer C3v(8)]. The inset shows the region near the Fermi level on an expanded scale... 

Fig. 6 Main picture valence band photoemission spectra of Tm C82 recorded at photon energies across the Tm 4d-4f threshold. The inset shows the Tm-N4 5 x-ray absorption spectrum, indicating the choice of photon energies for the resonant photoemission experiment. The photon energies are (1) 169.4 eV, (2) 173.7 eV, (3) 177.8 eV and (4) 183.9 eV...

Figure 7b shows valence band photoemission spectra recorded with He Ia radiation from the Kx(Tm C82) system. On K doping up to x=2, a new low-energy structure grows in intensity this is the former LUMO (lowest unoccupied mole-... 

Fig. 7 aC Is and K 2p core-level spectra of Kx(Tm C82) recorded using Al Ka radiation. The symbols mark the K 2p components signifying K ions in a tetrahedral ( ) and octahedral (O) co-ordination in an fcc-like structure and the tetrahedral site ( ) of a bet or bcc-like structure, b Corresponding valence band photoemission spectra recorded with He Ia radiation. The spectra drawn with a thicker line are adjacent to the onset of occupation of the next previously unoccupied MO of the C82 cage... 

Having determined that it is very likely that six electrons are transferred to the C80 molecule - in agreement with the 13C NMR data which suggest Ih symmetry for the carbon cage [6] - it is now interesting to consider the valence band photoemission spectrum of the endohedral to see if the consequences of this high symmetry are visible in the energy distribution of the n-MO s of the system. 

Fig. 15a-e Valence band photoemission spectra of a C84, b Sc2 C84i c C80, d Sc sN Cso and e C. All data are recorded with He Ia radiation, with the exception of the spectrum of C80) which is taken from Ref. [39] having been recorded with hv= 60 eV... 

Fig. 17 Valence band photoemission spectra of Kx(Sc3N C80) recorded using He Ia radiation at several stages of K intercalation, xy as indicated...

The interpretation of spectral properties in oxides such as NiO, in particular the valence-band photoemission spectra and inverse photoemission data (McKay and Henrich, 1984) has proved controversial. However, recent calculations using a supercell approach have given results for NiO in good agreement with such spectroscopic data (Norman and Freeman, 1986). These ealculations reconcile the band picture of Terakura et al. (1984a,b) and the experimental studies that have indicated large values ( 8 eV) for the intra-atomic Coulomb integral (Hufner et al., 1984 MeKay and Henrich, 1984 Sawatzky and Allen, 1984). 

NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS VALENCE BAND PHOTOEMISSION... 

Valence band photoemission, the Ce 4d photoemission and the O Is x-ray absorption edge are also very sensitive to oxidation state as illustrated in Fig. 9.5. These techniques provide a depth weighted average of the oxidation state making it difficult to precisely distinguish between surface and below surface Ce Nevertheless, they can be used to precisely monitor the effectiveness of reduction/oxidation treatments. 

Mun BS, Watanabe M, Rossi M, Stamenkovic V, Markovic NM, Ross PN (2005) A study of electronic structures of Pt3M (M=U, V, Cr, Fe, Co, Ni) polycrystaUine aUoys with valence-band photoemission spectroscopy. J Chem Phys 123 204717... 

It is essential to have selective experimental and theoretical tools that would allow us to disentangle the different parts of the electronic structure that are important for the formation of the surface chemical bond. The most common way to measure the occupied electronic structure is with valence band photoemission, also denoted as Ultraviolet Photoelectron Spectroscopy (UPS), where the overall electronic structure is probed through ionization of the valence electrons [5]. However, in order to describe the electronic structure around a specific adsorbate, it is necessary to enhance the local information. X-ray Emission Spectroscopy (XES) provides such a method to study the local electronic properties centered around one atomic site [3,6,7]. This is particularly important when investigating complex systems such as molecular adsorbates with many different atomic sites. 

Due to time-reversal symmetry the spin-polarization from a paramagnetic solid using circularly polarized radiation is just reversed if the helicity of the radiation is reversed. This feature is of course removed if the solid is magnetically ordered, giving rise to magnetic circular dichroism in valence-band photoemission (Schneider et al. 1991). In a corresponding experiment we have in general the emission direction. 

These analytical considerations support the analysis and interpretation of corresponding experimental and theoretical dichroic spectra in an appreciable way. As an example of such a combined investigation, valence-band photoemission spectra for a... 

Figure 5.17 Left valence-band photoemission spectra from a seven-monolayer-thick HCP (0001) Co film on W taken with linearly polarized radiation for opposite magnetization directions. Right corresponding asymmetry values for binding energies between —2 eV and Ep-Experiment (Bansmann et al. 2000).

In addition to the valence-band photoemission, useful information can be obtained from 4f core-level spectra, because their line shape is almost exclusively influenced by the various types of screening of a 4f hole by conduction-band states. The presence of the 7 eV satellites is taken as an indication for a decreasing hybridization and consequent increasing localization of the 5f states in the uranium compounds (Schneider and Laubschat 1981a). Various aspects of the core-hole screening are discussed by Naegele et al. (1985). 

Hoechst and Tang [29] investigated the band structure with synchrotron radiation by angular resolved valence-band photoemission spectroscopy, and found good agreement in the band structure mapped along T-X with theoretical calculations by Lubinsky et al [19]. 

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