Ultraviolet radiation photoelectron spectroscopy

Among the related methods, specific experimental designs for applications are emphasized. As in-system synchrotron radiation photoelectron spectroscopy (SRPES) will be applied below for chemical analysis of electrochemically conditioned surfaces, this method will be presented first, followed by high-resolution electron energy loss spectroscopy (HREELS), photoelectron emission microscopy (PEEM), and X-ray emission spectroscopy (XES). The latter three methods are rather briefly presented due to the more singular results, discussed in Sections 2.4-2.6, that have been obtained with them. Although ultraviolet photoelectron spectroscopy (UPS) is an important method to determine band bendings and surface dipoles of semiconductors, the reader is referred to a rather recent article where all basic features of the method have been elaborated for the analysis of semiconductors [150]. 

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. 

Hepburn J W 1995 Generation of coherent vacuum ultraviolet radiation applications to high-resolution photoionization and photoelectron spectroscopy Laser Techniques in Chemistry vol 23, ed A B Myers and T R Rizzo (New York Wley) pp 149-83... 

One of the most direct methods is photoelectron spectroscopy (PES), an adaptation of the photoelectric effect (Section 1.2). A photoelectron spectrometer (see illustration below) contains a source of high-frequency, short-wavelength radiation. Ultraviolet radiation is used most often for molecules, but x-rays are used to explore orbitals buried deeply inside solids. Photons in both frequency ranges have so much energy that they can eject electrons from the molecular orbitals they occupy. 

The low BE region of XPS spectra (<20 — 30 eV) represents delocalized electronic states involved in bonding interactions [7]. Although UV radiation interacts more strongly (greater cross-section because of the similarity of its energy with the ionization threshold) with these states to produce photoelectrons, the valence band spectra measured by ultraviolet photoelectron spectroscopy (UPS) can be complicated to interpret [1], Moreover, there has always been the concern that valence band spectra obtained from UPS are not representative of the bulk solid because it is believed that low KE photoelectrons have a short IMFP compared to high KE photoelectrons and are therefore more surface-sensitive [1], Despite their weaker intensities, valence band spectra are often obtained by XPS instead of UPS because they provide... 

Torr, K.M., Dawson, B.S.W., Ede, R.M. and Singh, J. (1996). Surface changes on acetylation and exposure to ultraviolet radiation of Pinus radiata using X-ray photoelectron spectroscopy. Holzforschung, 50(5), 449-456. 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

Photoelectron spectroscopy involves detection and analysis of the photoelectrons produced by interaction of radiation with a solid. This radiation may be X-rays (for X-ray photoelectron spectroscopy, XPS or ESCA) or ultraviolet radiation (UPS) it causes the removal of a single core or valence electron, respectively. The kinetic energy, Ek, of these electrons is given by the following equation ... 

In the spectroscopic technique known as photoelectron spectroscopy (PES), ultraviolet radiation... 

The electron, upon excitation, is ejected from an inner shell into vacuum and the energy of the free electron is then measured. This technique is called X-ray photoelectron spectroscopy. If the electron is ejected from the valence band by ultraviolet radiation, the technique is called ultraviolet photoelectron spectroscopy. Excitation energies not greater than those provided by ultraviolet radiation are necessary for electron excitation from the valence band or for electrons from the valence shell of adsorbed molecules. 

Among the methods that have found the most widespread application in the study of radical cations, ultraviolet photoelectron spectroscopy (UV-PES) has a special place, because it provides a wealth of detailed information concerning the orbital energies of organic molecules [57], In this experiment, a substrate is ionized by ultraviolet radiation with photons of known energy (Ehv), e.g. the He(I), line (21.21 eV), and the kinetic energy (Ekin) of the emitted electrons is measured. The vertical ionization potential (Iv) can then be calculated from Ehv and Ekin (Eq. 13). 

XPS spectra were recorded using unmonochromatized Mg K radiation (1253.6 eV), and an unmonochromatized He-resonance lamp was used for ultraviolet photoelectron spectroscopy (UPS). XPS spectra were taken with an analyzer resolution of 0.2 eV, and the net resolution measured as the full width at half-maximum (FWHM) of Au 4f(7/2) was 0.9 eV. The spectrometer is of our own construction and is, e.g., designed to provide optimum angle-dependent XPS or XPS(0) (12,l4). For high 5-values, the photoelectrons leave the sample surface near the grazing angel, and due to the limited escape depth of the electrons, this is a "surface sensitive" mode. In the "bulk sensitive" mode, for low 0-values, the photoelectrons exit near the surface normal, and hence more information from the "bulk" of the sample is obtained (15). 

Figure 7.38. XPS spectrum of an ionic liquid, [EMIM][Tf2N], detailing the C(ls) and N(ls) regions. Since there are no peaks from the Au substrate, the film thickness is hkely >10nm. Also shown (right) is the comparison between XPS, ultraviolet photoelectron spectroscopy (UPS, Hel = 21.2eV, Hell = 40.8 eV radiation), and metastable impact electron spectroscopy (MIES). Whereas XPS and UPS provide information from the first few monolayers of a sample, MIES is used for zero-depth (surface only) analysis, since the probe atoms are excited He atoms that interact with only the topmost layer of sample. Full interpretations for these spectra may be found in the original work Hofft, O. Bahr, S. Himmer-lich, M. Krischok, S. Schaefer, J. A. Kempter, V. Langmuir 2006, 22, 7120. Copyright 2006 American Chemical Society.

By using low-energy photons (energy less than 50 eV) valence shell ionizations occur, and the technique is called ultraviolet photoelectron spectroscopy (UPS). When X-rays are the impinging radiation, core-level ionizations take place providing the basis for X-ray photoelectron spectroscopy (XPS). 

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