UV photoemission spectroscopy

Tanaka S, Kanai K, Kawahe E, Iwahashi T, Nishi T, Ouchi Y, Seki K (2005) Doping effect of tetrathianaphthacene molecule in organic semiconductors on their interfacial electronic structures studied by UV photoemission spectroscopy. Jpn J Appl Phys 44 3760... 

An important technique is UV photoemission spectroscopy (UPS) which is based on the outer photoelectric effect (in contrast to XPS, where we use the inner photoelectric effect). Photons with energies of 10-100 eV are used to ionize atoms and molecules at the surface. The energy of emitted electrons is detected. To study adsorption of molecules to surfaces, often difference spectra are analyzed which are measured before and after the adsorption. These difference spectra are compared to the spectrum of the molecules in the gaseous phase. 

The X-ray and UV-photoemission spectroscopy study has revealed that ruthenium (II) phthalocyanine 20 has similar HOMO and LUMO energy with that of cis-bis(4,4-dicarboxy-2,2-bipyridine)-bis-(isothiocyanato)-ruthenium(II), [Ru(dcbpy)2 (NCS)2], the most efficient sensitizer so far. The oxidation potentials for both... 

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. 

Figure 5. UV photoemission spectroscopy data for the series of materials pyridine (v) (2), 2-ethyl pyridine (s), and poly(2-vinyl pyridine) (s).

The structures and registries of chemisorbed benzene on Rh(lll) have been thoroughly scrutinized by surface-sensitive techniques such as LEED, EELS, and angle-resolved UV photoemission spectroscopy (ARUPS) in UHV [43]. These previous studies revealed various structures for benzene, including well-known structures such as c 2 i x 4)rect and (3 x 3), depending on whether CO was present unintentionally or intentionally in the UHV chambers. Although it has been repeatedly demonstrated that the adlayer structures of benzene on Rh and Pt were greatly affected by the presence of CO in the adlayer, the structure of the pure benzene adlayer has not yet been fully understood. Neuber et al. reported that a completely new structure with a /19 x yi9)i 23.4° symmetry appeared for the pure benzene adsorption on Rh(lll) under cleaner UHV conditions in the absence of CO, and the previously known structures of 0(2 3 x 4)rect and (3 x 3) were found to appear upon admission of CO [48]. 

Thanks to the extensive literature on Aujj and the related smaller gold cluster compounds, plus some new results and reanalysis of older results to be presented here, it is now possible to paint a fairly consistent physical picture of the AU55 cluster system. To this end, the results of several microscopic techniques, such as Extended X-ray Absorption Fine Structure (EXAFS) [39,40,41], Mossbauer Effect Spectroscopy (MES) [24, 25, 42,43,44,45,46], Secondary Ion Mass Spectrometry (SIMS) [35, 36], Photoemission Spectroscopy (XPS and UPS) [47,48,49], nuclear magnetic resonance (NMR) [29, 50, 51], and electron spin resonance (ESR) [17, 52, 53, 54] will be combined with the results of several macroscopic techniques, such as Specific Heat (Cv) [25, 54, 55, 56,49], Differential Scanning Calorimetry (DSC) [57], Thermo-gravimetric Analysis (TGA) [58], UV-visible absorption spectroscopy [40, 57,17, 59, 60], AC and DC Electrical Conductivity [29,61,62, 63,30] and Magnetic Susceptibility [64, 53]. This is the first metal cluster system that has been subjected to such a comprehensive examination. 

Photoelectron Spectroscopy. As a subdivision of electron spectroscopy, photoelectron or photoemission spectroscopy (PES) includes those instruments that use a photon source to eject electrons from surface atoms. The techniques of x-ray photoelectron spectroscopy (XPS) and uv photoelectron spectroscopy (UPS) are the principles in this group. Auger electrons are emitted also because of x-ray bombardment, but this combination is used infrequent-... 

As with UV-visible spectroscopy in the bulk, such techniques do not yield chemical identification, so that combining other local spectroscopies with STM is typically necessary to identify the atoms and molecules present. Specialized approaches have been developed for this, such as STM photoemission spectroscopy (PESTM) and inelastic electron tunneling spectroscopy to yield vibrational and other information. This area is extremely promising for further work in combining any number of spectroscopies with the exquisite spatial resolution of STM. 

In the field of not only traditional metallurgy but also recently developed nano-technology, it is very interesting and important what change is introduced when it is surrounded by other atoms. Such a change in electronic states has been investigated as chemical shift detected by X-ray (XPS) and UV (UPS) photoemission spectroscopy [1] as well as X-ray emission and absorption spectroscopy [2,3]. Also, such a chemical shift has been simulated by theoretical calculation [4]. However, many problems have been unsolved. In the case of XPS and UPS, since the most outer layers of substances are analyzed, the spectra are easily affected by absorbed gaseous molecules. Also, with the X-ray emission and absorption spectroscopy it is difficult to analyze the complicated X-ray transition states for substances composed of heavy metal elements. Therefore, a complementary method has been demanded for the spectroscopy such as XPS, UPS and X-ray emission and absorption spectroscopy. The coefficient y of the electronic contribution to heat capacity, Cp, near absolute zero Kelvin reflects the density of states (DOS) in the vicinity of Fermi level (EF) [5]. Therefore, the measurement of y is expected to be one of the useful methods to clarify the electronic states of substances composed of heavy metal elements. 

In fact, though the surface strains were completely removed by the treatment of the Ti02 surface in a warm concentrated NaOH solution, this leads to a water contact angle of around 10-20°. At this point, a contaminant Cls peak (approximately at 285 eV) was not detected in the X-ray photoemission spectroscopy (XPS) measurement, i.e. no strains remained at the surface. On the contrary, it was confirmed that even if the surface strains remained, the highly hydrophilic conversion still occurred under UV-light irradiation [34],... 

Photoemission spectroscopy involves measurement of the energy distribution of electrons emitted from a solid under irradiation with mono-energetic photons. In-house experiments are usually performed with He gas discharge lamps which generate vacuum UV photons at 21.2 eV (He la radiation) or 40.8 eV (He Ila radiation ) or with Mg Ka (hv=1284.6 eV) or A1 Ka (hv=1486.6eV) soft X-ray sources. UV photoemission is restricted to the study of valence and conduction band states, but XPS allows in addition the study of core levels. Alternatively photoemission experiments may be performed at national synchrotron radiation facilities. With suitable choice of monochromators it is possible to cover the complete photon energy range from about 5 eV upward to in excess of 1000 eV. The surface sensitivity of photoemission derives from the relatively short inelastic mean free path of electrons in solids, which reaches a minimum of about 5A for electron energies of the order 50-100 eV. 

Figure 1. Ultrahigh vacuum apparatus for studying single-crystal catalysts b ore and after operation at high pressure in catalytic reactor. Position 1 crystal is in position for Auger-electron-spectroscopy study of surface composition, or for UV photoemission spectrum of surface species. Position 2 crystal is in position for deposition of a known coverage of poisons or promoters for a study of their influence on the rate of a catalytic reaction. Position 3 crystal is in position for a study of catalytic reaction rate at elevated pressures, up to 1 atm. Gas at high pressure may he circulated using pump mass spectrometric/gas chromatographic analysis of the reactants/products is carried out by sampling the catalytic chamber.

Fig. 22 Phototautomeric reactions observed for tripeptide molecule, IGF. Reprinted from E. Mateo-Marti and C. M. Pradier, UV irradiation study of a tripeptide isolated in an argon matrix a tautomerism process evidenced by infrared and X-ray photoemission spectroscopies, Spectrochim. Acta, Part A, 2013, 109, 247-252. Copyright (2013), with permissioh from Elsevier.

Multilayers of glycine, adsorbed on Ti02 (110), were studied with synchrotron radiation-based UV light [428]. Ultraviolet photoemission spectroscopy (UPS) and XPS data showed the multilayers as formed by glycine molecules in polar zwit-terionic form (NHs CH2COO ). Photon induced damage of multilayers occurs fast (produces a first order desorption of zwitte-rionic molecules with total cross section). 

Auger electron spectroscopy has become one of the most important tools for the investigation of interfaces in the field of materials science. In fact, it is probably safe to say that it has found its most widespread application in this field. Because it does not give detailed chemical bonding information about the species on a surface, other techniques such as XPS and UV photoemission are more commonly used for detailed chemical investigations. However, in materials science applications the need is more often to detect the presence of elements on the surface of the solid, and for this application Auger electron spectroscopy is the preferred technique. 

Fig. 6.16. Probing the transient electronic structure of TbTes in the course of its ultrafast insulator-to-metal transition induced by femtosecond-laser excitation (a) Time- and angle-resolved photoemission spectroscopy. A TbTcs sample was excited by an IR pulse (hi Pump = I.SeV, about 50 fs duration) and probed after a time delay At with a UV pulse (hi/pump = 6 eV, about 90 fs duration). The photoelectron intensity and kinetic energy E ,i were measured as a function of the emission angles (a, 9). (b) Insulator-to-metal transition Above the critical temperature Tc (or 100fsafterlaserexcitation)the band gap of the CDW phase closes, (c) "Snapshots" of the electronic band structure E(k)in TbTej fordifferenttimedelaysAt.Afterlaserexcitation, the gap has closed and the band dispersion near the Eermi level, Ep, changed after a time delay of 100 fs. Such a delayed collapse of the band gap is characteristic of the "Peierls type" mechanism (see text).

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