Ultraviolet photon spectroscopy

In this context it is instructive to mention the work function 4>. Here, eFermi level of a solid into vacuum far away from the surface [107], The work function can be measured for example by ultraviolet photon spectroscopy (for a discussion see Ref. [108]). 

Figorc 7.22a Electron spectroscopies for surface analysis. Auger electron spectroscopy. X-ray photon spectroscopy, and ultraviolet photon spectroscopy. 

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. 

Colorimetry, in which a sample absorbs visible light, is one example of a spectroscopic method of analysis. At the end of the nineteenth century, spectroscopy was limited to the absorption, emission, and scattering of visible, ultraviolet, and infrared electromagnetic radiation. During the twentieth century, spectroscopy has been extended to include other forms of electromagnetic radiation (photon spectroscopy), such as X-rays, microwaves, and radio waves, as well as energetic particles (particle spectroscopy), such as electrons and ions. ... 

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

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. 

Much has been learned in recent years about the 00 dimer , O2O2, produced in reaction 17. It is actually dichlorine peroxide, OOOCl its geometry is now well established from submillimeter wave spectroscopy (15). Photolysis of OOOO around 310 nm the atmospherically important wavelengths -- yields chlorine atoms and ClOO radicals (16), as given in reaction 18, rather than two OO radicals, even though QO-OQ is the weakest bond (it has a strength of about 17 Kcal/mol (17)). Thermal decomposition of QOOQ (the reverse of reaction 17) occurs very fast at room temperature, but more slowly at polar stratospheric temperatures. Hence, photolysis is the predominant destruction path for CIOOQ in the polar stratosphere and two Q atoms are produced for each ultraviolet photon absorbed. 

An additional advantage of ESA and ESE is the possibility of localizing high energy levels, which could not be accessed using conventional spectrophotometers. This is the case for some bands in the ultraviolet spectral region. Moreover, ESA could allow the observation of optical transitions, which are forbidden by one-photon spectroscopy (Malinowski et al, 1994)... 

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). 

Electrons Auger Electron Spectroscopy, Extended X-Ray Absorption Fine Structure, Low-Energy Electron Diffraction, Scanning Electron Microscopy, Surface Extended X-Ray Absorption Fine Structure, Ultraviolet Photoelectron Spectroscopy, X-Ray Absorption Near Edge Fine Structure, and X-Ray Photon Spectroscopy. 

When UV photons are used, the available energy provides only the possibility of studying the outer electron shells. Therefore UPS (Ultraviolet Photoelectron Spectroscopy) studies the valence band structures of materials. 

We have demonstrated this effect in two experiments [13,14]. One is in single frequency ultraviolet generation by sum frequency mixing and the other is in Doppler-free two-photon spectroscopy, with the FM laser. 

Fig 6 Two experimental arrangements for demonstrating the quality of an FM spectrum a) sum frequency generation in a nonlinear crystal followed by mode analysis of the generated ultraviolet b) Doppler-free two-photon spectroscopy. 

Ultraviolet photoemission spectroscopy( UPS) measurements of a Pd/ SiCVa-Si H structure indicate (Fortunato et al., 1984) that the mechanism responsible for the transport property variations is a change in the contact potential. In these experiments a few angstroms of a-Si have been deposited by an in situ evaporation onto a Pd/SiO substrate. Figure 5 shows the photoemission spectra obtained by synchrotron radiation at a photon energy of 30 eV and for three different conditions (1) after the a-Si deposition, (2) after the H2 exposure at 10-2 Torr for 2 min, and (3) after 02 exposure at 5 X 10-5 Torr and 110°C for 7 min. 

In addition to using X-rays to irradiate a surface, ultraviolet light may be used as the source for photoelectron spectroscopy (PES). This technique, known as ultraviolet photoelectron spectroscopy (UPS, Figure 7.38), is usually carried out using two He lines (Hel at 21.2 eV and Hell at 40.8 eV), or a synchrotron source. This technique is often referred to as soft PES, since the low photon energy is not sufficient to excite the inner-shell electrons, but rather results in photoelectron emission from valence band electrons - useful to characterize surface species based on their bonding motifs. It should be noted that both UPS and XPS are often performed in tandem with an Ar" " source, allowing for chemical analysis of the sample at depths of < 1 J,m below the surface. 

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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