Photoemission inverse

At a surface, not only can the atomic structure differ from the bulk, but electronic energy levels are present that do not exist in the bulk band structure. These are referred to as surface states . If the states are occupied, they can easily be measured with photoelectron spectroscopy (described in section A 1.7.5.1 and section Bl.25.2). If the states are unoccupied, a teclmique such as inverse photoemission or x-ray absorption is required [22, 23]. Also, note that STM has been used to measure surface states by monitoring the tunnelling current as a fiinction of the bias voltage [24] (see section BT20). This is sometimes called scamiing tuimelling spectroscopy (STS). 

Flimpsei F J 1990 inverse photoemission from semiconductors Surf. Sc/. Rep. 12 1... 

Figure Bl.19.4. (a) Local conductance STS measurements at specific points within the Si(l 11)-(7 x 7) unit cell (symbols) and averaged over whole cell, (b) Equivalent data obtained by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). (Taken from [19], figure 2.)...

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Weaver J H 1992 Eleotronio struotures of Cgg, C g and the fullerides—photoemission and inverse photoemission studies J. Phys. Chem. Solids 53 1433... 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

ESDIAD Electron-stimulated desorption ion angular distribution IPES Inverse photoemission spectroscopy... 

Inverse Photoemission Spectroscopy (IPES) and Bremsstrahlung Isochromat Spectroscopy (BIS)... 

The decreased CO stretch frequencies are easily rationalized as the result of adsorption on sites with a lower work function, as explained in the Appendix. The effect of a lower work function is that all orbitals of CO shift downward with respect to the Fermi level of the substrate. This shift of the occupied CO levels to higher binding energy has been observed in UPS spectra (see Fig. 3.20), while the shift of the unoccupied part of the 2tt -derived chemisorption orbital has been observed in inverse photoemission [33, 34, 45], The overall effect is that the bond between the metal and the CO becomes stronger while at the same time the intramolecular CO bond is weakened. 

Charge-Transfer Reaction Inverse Photoemission at Gold(lll) and Polycrystalline Silver Electrodes... 

McIntyre et AL. Charge-Transfer Reaction Inverse Photoemission 241... 

The unoccupied electronic states of a solid can be experimentally explored by different techniques. The most commonly used are inverse photoemission, where low-energy electrons impinge on the surface of the solid, and the photon-based techniques ellipsometry, NEXAFS and constant-initial-state spectroscopy. Results derived from inverse photoemission spectroscopy might be questionable unless low-energy electrons (c. 10-20 eV) and low beam currents are used as in LEED... 

Gao W, Kahn A (2001) Controlled p-doping of zinc phthalocyanine by coevaporation with tetrafluorotetracyanoquinodimethane a direct and inverse photoemission study. Appl Phys Lett 79 4040... 

Tsutsumi K, Yoshida H, Sato N (2002) Unoccupied electronic states in a hexatriacontane thin film studied by inverse photoemission spectroscopy. Chem Phys Lett 361 367... 

Using inverse photoemission, the unoccupied electronic states of solid surfaces are being studied. Here, instead of injecting an UV light onto the surface and analyzing the emitted electrons, an electron beam is injected onto the surface and the spectrum of the emitted photons is analyzed. Fig. 4.11 shows a summary of the results of photoemission and inverse photoemission of one of the most exhaustively studied surfaces, W(OOl) [Drube et al. (1986)]. As shown, strong surface states immediately below and above the Fermi level are observed. Both are of a character. 

The problem of first-principles calculations of the electronic structure of solid surface is usually formatted as a problem of slabs, that is, consisting of a few layers of atoms. The translational and two-dimensional point group symmetry further reduce the degrees of freedom. Using modern supercomputers, such first-principles calculations for the electronic structure of solid surfaces have produced remarkably reproducible and accurate results as compared with many experimental measurements, especially angle-resolved photoemission and inverse photoemission. 

Binnig, G., Frank, K. H., Fuchs, H., Garcia, N., Reihl, B., Rohrer, H., Salvan, F., and Williams, A. R. (1985a). Tunneling spectroscopy and inverse photoemission Image and field states. Phys. Rev. Lett. 55, 991-994. 

In Chap. E, photoelectron spectroscopic methods, in recent times more and more employed to the study of actinide solids, are reviewed. Results on metals and on oxides, which are representative of two types of bonds, the metallic and ionic, opposite with respect to the problem itineracy vs. localization of 5f states, are discussed. In metals photoemission gives a photographic picture of the Mott transition between Pu and Am. In oxides, the use of photoelectron spectroscopy (direct and inverse photoemission) permits a measurement of the intra-atomic Coulomb interaction energy Uh. 

From the experimental viewpoint 1. the analysis of the variation of photoionization cross sections (affecting the intensities of photoelectron spectroscopy), gives an insight into the orbital composition of the bands of the solid 2. the combination of direct and inverse photoemission provides a powerful tool to assess the structure of occupied and of empty states, and, in the case of localized 5 f states, permits the determination of a fundamental quantity, the Coulomb correlation energy, governing the physical properties of narrow bands. 

In inverse photoemission spectroscopy or Bremsstrahlimg Isochromat Spectroscopy, the sample is bombarded with monoenergetic electrons and the Bremsstrahlung radiation is recorded at constant photon energy for varying kinetic energy of the incident electrons. The electronic process involved is just the inversion of the photoemission process, and therefore, instead of investigating occupied states as in UPS/XPS, empty states are examined in BIS. 

In inverse photoemission (and in case of high electron excitation energy) a final core state containing an additional electron (instead of a hole) may be created (for f-states fN -> f " ). Thus, and in the same hnes as for photoemission, a multiphcity of non... 

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