Photoelectron spectroscopy ultraviolet

The technique of ultraviolet photoelectron spectroscopy involves the same principle and the same type of measurement as x-ray photoelectron spectroscopy, but the incident radiation is in the ultraviolet region rather than the x-ray region. The exciting lines are often the helium line at 58.433 nm or the helium ion. Hetransition S i2 

1 Estimate the temperature required for noticeable (say 10%) probability of light absorption by the 25 state in a collection of hydrogen atoms. 

2 How does the Pauli principle couple electron spin angular momenta  

3 Repelling electrons tend to occupy different orbitals (if possible) to increase their average separation. Illustrate this for the carbon 2p electrons. Suggest how Coulomb repulsion between electrons couples their orbital angular momenta. 

4 How do Hund s rules apply to the 2p electrons in the carbon atom  

Acronyms UPES, UVPES, UPS, UVPS ultraviolet photoelectron spectroscopy. The use of the term photoemission spectroscopy (PES) is discouraged. 

Samples are exposed to monochromatic ultraviolet radiation (typically in the range 10—100 eV) and the energies of the emitted photoelectrons are measured to reveal information on the valence band electronic structure of the sample. 

The measurement is carried out in an UHV chamber. Ultraviolet photons from a He lamp (21.2 or 40.8 eV) are shone at the sample to eject photoelectrons. The emitted electrons are collected by an electron energy analyser, such as a hemispherical mirror analyser, to produce a spectrum showing the number of electrons emitted with a given kinetic energy. Such spectra are representative of, although not identical to, the sample s valence band electronic density of states (DOS). 

By using a synchrotron source instead of a He lamp to provide the ultraviolet photons a photon energy can be selected that preferentially favours emission from a particular element by exploiting variations in photoemission cross-section. The partial density of states contribution from each individual element can then be determined. 

Samples are normally metallic or semiconducting UHV compatible materials. 

Fermi ievei (Ep). Ep, and Eh are the kinetic energy of the photoeiectron, work function, and hoie injection barrier, respectiveiy. 

for example, the binding energy ( b) from Ef can be obtained by the energy conservation rule as 

When a photon creates a photoelectron, a photogenerated hole exists during the traveling and detection of the photoelectron. If the photogenerated hole is not eliminated by the electron transfer from the conductive substrate before the second photon ionizes the dielectric sample (many of organics), we observe charging effects upon the second ionization. Thus, one needs to use thin films for the UPS measurement of electrical insulators (most of organic solids). 

In order to obtain the energy band dispersion from UPS experiments, we need to use the momentum conservation role as well as the energy conservation role upon photoelectron emission. A three-step model is generally adopted for the photoelectron spectroscopy process, which consists of an optical dipole excitation in the solid, followed by transport to the surface and emission to the vacuum [37, 38]. General assumptions are as follows (i) both the energy and momentum of the electrons are conserved during the optical transition, (ii) the momentum component parallel to the surface is conserved while the electron escapes through the surface, and (iii) the final continuum state in the solid is a parabolic free-electron-like band in a constant inner potential Vq, 

The use of actual single crystal in UPS measurements requires elimination of the charging effects by using high-quality single crystal with less charge trapping 

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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. 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

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.)...

Briggs D (ed) 1978 Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy (London Fleyden)... 

Ultraviolet photoelectron spectroscopy (UPS) [2, 3 and 4, 6] differs from XPS in that UV light (He I, 21.2 eV He II, 40.8 eV) is used instead of x-rays. At these low excitmg energies, photoemission is limited to valence electrons. 

Cheshnovsky O, Yang S H, Pettiette C L, Craycraft M J, Liu Y and Smalley R E 1987 Ultraviolet photoelectron spectroscopy of semiconductor clusters silicon and germanium Chem. Phys. Lett. 138 119... 

Ultraviolet photoelectron spectroscopy allows the determination of ionization potentials. For thiazole the first experimental measurement using this technique was preformed by Salmona et al. (189) who later studied various alkyl and functional derivatives in the 2-position (190,191). Substitution of an hydrogen atom by an alkyl group destabilizes the first ionization potential, the perturbation being constant for tso-propyl and heavier substituents. Introduction in the 2-position of an amino group strongly destabilizes the first band and only slightly the second. 

Figure 8.1 Processes occurring in (a) ultraviolet photoelectron spectroscopy (UPS), (b) X-ray photoelectron spectroscopy (XPS) and (c) Auger electron spectroscopy (AES)...

Rabalais, J. W. (1977) Principles of Ultraviolet Photoelectron Spectroscopy, John Wiley, New York. Roberts, M. W. and McKee, C. S. (1979) Chemistry of the Metal-Gas Interface, Oxford University Press, Oxford. 

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. 

NIRMS = noble-gas-ion reflection mass spectrometry OSEE = optically stimulated exoelectron emission PES = photoelectron spectroscopy PhD = photoelectron diffraction SIMS = secondary ion mass spectroscopy UPS = ultraviolet photoelectron spectroscopy ... 

Ultraviolet Photoelectron Spectroscopy Ultraviolet Photoemission Spectroscopy Molecular Photoelectron Spectroscopy... 

D. Briggs (ed.) Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Heyden, London 1977. 

Several UHV techniques which have been developed have not found such wide use in corrosion analysis, despite potential applicability. Ultraviolet photoelectron spectroscopy (UPS) is one of these, operating in a similar fashion to XPS (but using an ultraviolet excitation), and probing the valence electrons, rather than the core electrons of the atoms. Because the energies of the valence electrons are so very sensitive to the precise state of the atom, the technique is in principle very informative however exactly this high sensitivity renders the data difficult to interpret, particularly as a routine... 

Vacuum ultraviolet photoelectron spectroscopy of inorganic molecules. R. L. De Kock and D. R. Lloyd, Adv. Inorg. Chem. Radiochem., 1974,16, 66-107 (187). 

Bradshaw AM, Cederbaum LS, Domcke W (1975) Ultraviolet Photoelectron Spectroscopy of Gases Adsorbed on Metal Surfaces. 24 133-170 Braterman PS (1972) Spectra and Bonding in Metal Carbonyls. Part A Bonding. 10 57-86 Braterman PS (1976) Spectra and Bonding in Metal Carbonyls. Part B Spectra and Their Interpretation. 26 1-42... 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

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