Photoelectron photoemission spectrum

Fig. 4.28. The experimental valence-band x-ray photoelectron ( photoemission ) spectrum of a-FeO, compared with a calculated spectrum based on a configuration-interaction calculation on an FeO, cluster. The dotted and dashed curves represent O 2p emission and integral background, respectively. The bottom panel shows a decomposition into configuration components for each final-state line (after Fujimori et al., 1986 reproduced with the publisher s permission).

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

Photoelectron spectra have been experimentally measured and calculated for (l,3-di-tert-butyl-l,3,2-diazagermol-2-ylidene). For this germylene, the HOMO is a r-symmetry orbital containing substantial Ge, N, and C=C r-character. The Ge-based lone pair is actually found to be 1 eV lower in energy. The He II gas-phase photoemission spectrum is shown in Figure 1. 

UPS from Adsorbate Core Levels.—As outlined above, an out-going photoelectron in its final state is a super-position of two coherent contributions a direct wave whose amplitude and symmetry are determined by the intra-atomic transition at the emitting site and an indirect wave generated by repeated scattering of the direct wave by the local atomic environment. It was suggested by Liebsch that this final-state scattering should lead to angular variations in the photoemission spectrum and would be examined best in core-level emission, which involves the simplest possible initial... 

Any in-depth analysis of chemical shifts really should take into account a full quantum-chemical description of the emitting atom in the excited state of a partly covalent bonding situation. Then not only are the excited electronic states of the bare atom relevant for the photoemission spectrum, but also the reorganization effects of the hybridized electronic states of the ligand atoms around the photoemitter. The analysis of shake-up satellites, which arise from interactions of the ejected photoelectron with the hgand molecular orbitals, is a prominent... 

Hydrogen Halides.—The X-ray photoemission spectrum of the F Is region of gaseous HF has been measured, using A1 Ka 2 radiation.The spectrum was interpreted in terms of a many-electron theory with configurational interaction. The threshold photoelectron spectra of HF and DF have been reported and a number of vibronic levels of the HF and DF ions have been detected. ... 

An XPS spectrum consists of a plot of N(E)/E, the number of photoelectrons in a fixed small interval of binding energies, versus E. Peaks appear in the spectra at the binding energies of photoelectrons that are ejected from atoms in the solid. Since each photoemission process has a different probability, the peaks characteristic of a particular element can have significantly different intensities. 

Photoelectron spectroscopy (PES) has been applied to determine the structure of 1-aza- and 1,4,7-triazatricy-clo[5.2.1.04,10]decane 37 and 40 <1997JMT(392)21>. The PES spectrum of ATQ shows four composite bands in the region 7-17 eV. A first band peaked at 7.80 eV is attributed to the NLPO (nitrogen lone-pair orbital). A second prominent broad band system, extending from 10.5 to 13.0 eV is associated with photoionizations from the cr-orbital manifold. The third composite band is produced by two photoemissions. The second band may be attributed to emissions arising from a sequence of seven near-lying MOs. 

We have tacitly assumed that the photoemission event occurs sufficiently slowly to ensure that the escaping electron feels the relaxation of the core-ionized atom. This is what we call the adiabatic limit. All relaxation effects on the energetic ground state of the core-ionized atom are accounted for in the kinetic energy of the photoelectron (but not the decay via Auger or fluorescence processes to a ground state ion, which occurs on a slower time scale). At the other extreme, the sudden limit , the photoelectron is emitted immediately after the absorption of the photon before the core-ionized atom relaxes. This is often accompanied by shake-up, shake-off and plasmon loss processes, which give additional peaks in the spectrum. 

In the investigations of molecular adsorption reported here our philosophy has been to first determine the orientation of the adsorbed molecule or molecular fragment using NEXAFS and/or photoelectron diffraction. Using photoemission selection rules we then assign the observed spectral features in the photoelectron spectrum. On the basis of Koopmans theorem a comparison with a quantum chemical cluster calculation is then possible, should this be available. All three types of measurement can be performed with the same angle-resolving photoelectron spectrometer, but on different monochromators. In the next Section we briefly discuss the techniques. The third Section is devoted to three examples of the combined application of NEXAFS and photoemission, whereby the first - C0/Ni(100) - is chosen mainly for didactic reasons. The results for the systems CN/Pd(111) and HCOO/Cu(110) show, however, the power of this approach in situations where no a priori predictions of structure are possible. 

Fig. 9. The full photoelectron spectrum (direct and inverse photoemission) of the conduction band of Th metal (from Ref. 56)...

The basic processes involved in photoemission from a solid are illustrated in Fig. 1. The photon is absorbed by a valence electron (a) or core electron (c) of the Ai-electron initial state, leading to ejection of that electron into the vacuum and an (N— 1) electron final state. By determining the intensity of the emitted electrons as a function of their kinetic energy, one obtains a photoelectron spectrum. The binding energies of the electrons are determined by... 

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