Photoemission spectroscopy techniques

All analytical methods that use some part of the electromagnetic spectrum have evolved into many highly specialized ways of extracting information. The interaction of X-rays with matter represents an excellent example of this diversity. In addition to straightforward X-ray absorption, diffraction, and fluorescence, there is a whole host of other techniques that are either directly X-ray-related or come about as a secondary result of X-ray interaction with matter, such as X-ray photoemission spectroscopy (XPS), surface-extended X-ray absorption fine structure (SEXAFS) spectroscopy, Auger electron spectroscopy (AES), and time-resolved X-ray diffraction techniques, to name only a few [1,2]. 

Suitable characterization techniques for surface functional groups are temperature-programmed desorption (TPD), acid/base titration [29], infrared spectroscopy, or X-ray photoemission spectroscopy, whereas structural properties are typically monitored by nitrogen physisorption, electron microscopy, or Raman spectroscopy. The application of these methods in the field of nanocarbon research is reviewed elsewhere [5,32]. 

We next discuss x-ray absorption studies. To put matters in context, it is useful to understand that conventional studies using Auger electron spectroscopy (AES) and x-ray photoemission spectroscopy (XPS) can be carried out only ex situ in high vacuum after electrochemical treatment since the techniques involve electron detection. X-ray absorption spectroscopy can, in contrast, be used for valence and structural environment studies. As x-rays only are involved, they can be carried out in situ in an electrochemical or similar cell. 

The unoccupied electronic states of a solid can be experimentally explored by different techniques. The most commonly used are inverse photoemission, where low-energy electrons impinge on the surface of the solid, and the photon-based techniques ellipsometry, NEXAFS and constant-initial-state spectroscopy. Results derived from inverse photoemission spectroscopy might be questionable unless low-energy electrons (c. 10-20 eV) and low beam currents are used as in LEED... 

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-... 

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. 

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. 

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. 

Angle-Resolved Photoemission. The best experimental technique to resolve the electronic structure of crystals in the momentum-energy space, and, consequently, the Fermi surface, is angle resolved photoemission spectroscopy (ARPES). 

In the mid-50 s it was observed that the energy of a photoelectron, ejected from the core of an atom by an X-ray photon, is a rather sensitive probe of the chemical environment of the atom. From this observation has evolved a major research technique named electron spectroscopy for chemical analysis (ESCA) by the Uppsala group 1,2) which pioneered the subject and called X-ray photoemission spectroscopy (XPS) by many others. The field has developed rapidly a third generation of spectrometers is in use at many laboratories and the understanding of the spectra observed is improving apace. A view of the current status of X-ray photoelectron spectroscopy in application to metals and alloys is presented in this article. We have not been encyclopedic in describing what has been done we have instead attempted to cover the classes of results obtained and the kinds of problems encountered in interpretation of these results. 

In most cases the experimental techniques used to study surface phenomena do not seem to yield consistent values for the surface segregation energies. One important exception is the special case of an atom of atomic number Z+1 in a host of atoms of atomic number Z, where the surface segregation energy may in fact be extracted with a high degree of accuracy from X-ray photoemission spectroscopy (XPS) measurements of surface core-level shifts (SCLS) [39]. In contrast they may be calculated quite accurately by modern first-principles methods [18,25,40]. 

The hexapole technique has been extensively exploited for the study of oriented open shell molecules such as OH (see Ref [46] and references therein) and NO (see Ref [47] and references therein), the latter also for scattering on surfaces [48]. This is a very important topic, because the basic tool for enhancement of chemical reactivity is catalysis at surfaces. In Ref [49], for examples, the oxidation of Si (001) induced by incident energy of O2 molecules is studied by synchrotron radiation photoemission spectroscopy and mass spectrometry, a process of a kind which may show propensities regarding molecular orientation of O2 as it impinges on the surface, possibly controlled by techniques of the kind described in previous sections. 

A few other techniques have been effectively used in the chemical study of OLEDs, including infrared spectroscopy [for the generation of carbonyl impurities in poly(p-phenylenevinylene)],20 photoemission spectroscopy (capable of measuring valence and core electronic states at the surface of the material during deposition),13,30,32 39 and gas chromatography (which is well suited for Alq3 because some of the reactants and products are volatile).16,35... 

The surface dissociation of COj into CO(ads) and O(ads) has been proposed in the case of Na-modified Pd(lLl) and K-modified Pt(lll) 5 at submonolayer coverage on the basis of HREELS , and angle resolved ultraviolet photoemission spectroscopy (ARUPS). 59 pyjg presence of adsorbed atomic oxygen (a species capable of migrating into the bulk metal) for Na/Pd(lll) however, could not be detected with either of these techniques. 

I will illustrate the application of two techniques to the study of problems involving latent-image silver. These techniques are molecular orbital calculations and ultraviolet photoemission spectroscopy (UPS). The calculations are used to model the processes of formation of silver particles through photolysis. The spectroscopic measurements are used to determine properties of the silver particle as a function of its size. 

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