Electron inverse photoemission

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). 

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. 

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... 

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... 

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. 

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... 

Figure 6 shows the process in a schematic way. Baer describes the two electron excitations of direct and inverse photoemission as two-step processes in which first Ep is reached and then the emission of an electron (direct) or of a photon (inverse photoemission) to vacuum occurs from Ep. A and A+ are the energies associated with the two first steps. They are counted from Ep, and they are to be considered as the minimum energies necessary to create the f and f final state. If the localized level response is a final state multiplet, therefore, Uh = A+ -I- A is given by the sum of the smallest measured energies of the multiplet. 

Figure 4 shows the combined XPS/BIS results for UO2. The main peak in inverse photoemission, centered at approximately 5 eV above Ep is attributed to a 5 f state, in part because of its dominating intensity (high cross section of states at 1500 eV electron excitation energy), in part by a comparison with the measured spectrum of Th (see Fig. 9), in which the 5 f states are well separated from s and d states. Thus, the peak... 

One way of measuring the gap G, in insulators, consists in superposing direct and inverse photoemission spectra (quasi-particle spectra of the compound), and recording the smallest energy difference between them. G is thus related to the ionization potential and electron affinity by ... 

Inverse photoemission involves a time reversed photoemission process in which electrons of varying energy incident upon a sample decay radiatively into empty electronic states. Photons of fixed frequency are counted and the energy dispersion of empty states may be investigated by varying the incident electron angle. 

Both photoemission and inverse photoemission require reasonable sample conductivity and their application to hard insulators such as MgO and AI2O3 is problematic. Both techniques also involve the complication that inelastic electron energy loss processes become convoluted with electron emission or decay. This may give rise to spectral features in regions where none are expected fi om the density of states [24,25] and care must always be taken to exclude these features before considering assignment to surface states. 

The measurement of tunnelling spectra in a scanning tunnelling microscope offers the potential of measuring the local density of states at spatially defined sites whose topography can be established at an atomic scale by STM. This information is only available however at the price of losing the information about the k-dependence of electronic states that is available in photoemission and inverse photoemission. In particular STS offers the prospect of measuring local densities of states at defect sites whose real space atomic structure can be established by STM. 

J. H. Weaver, Electronic Structures of Cso. C70, and the Fullerides Photoemission and Inverse Photoemission Studies, J. Phys. Chem. Solids 53, 1433-1447 (1992). 

Inverse photoemission (IPE, BIS) electrons/photons 0.5-3 electronic stmcture of conduction band... 

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