Photon spectroscopies, surface information

Photoelectron spectroscopy, which is based on kinetic energy analysis of electrons ejected from an atom or molecule by an enegetic photon and provides information on the binding energies or ionization potentials of the ejected electrons. Recent developments include X-ray photoelectron spectroscopy (XPS) of surfaces, and the use of lasers as radiation sources. 

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. 

Another largely unexplored area is the change of dynamics due to the influence of the surface. The dynamic behavior of a latex suspension as a model system for Brownian particles is determined by photon correlation spectroscopy in evanescent wave geometry [130] and reported to differ strongly from the bulk. Little information is available on surface motion and relaxation phenomena of polymers [10, 131]. The softening at the surface of polymer thin films is measured by a mechanical nano-indentation technique [132], where the applied force and the path during the penetration of a thin needle into the surface is carefully determined. Thus the structure, conformation and dynamics of polymer molecules at the free surface is still very much unexplored and only few specific examples have been reported in the literature. 

In addition to the surface/interface selectivity, IR-Visible SFG spectroscopy provides a number of attractive features since it is a coherent process (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region, (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29-32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33-37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 

The nature of the final state depends upon the energy, hv, of the exciting photons. In X-ray photoelectron spectroscopy (XPS) the exciting photons are provided by sources such as A1 Ka (1,486 eV) or Mg Ka (1,253 eV) and excitation of the core electrons of the molecules is observed. In UV photoelectron spectroscopy (UPS), Hel (21.2eV) or Hell (40.8 eV) radiation is used and excitation from the valence region of the neutral molecule is observed. XPS and UPS are surface-sensitive techniques, which are capable of providing extremely useful information on the chemical nature of a surface or interface and, in the case of the XPS, the conformational state of the molecules at the surface [64]. 

In this chapter, we introduce some of the most common spectroscopies and methods available for the characterization of heterogeneous catalysts [3-13], These techniques can be broadly grouped according to the nature of the probes employed for excitation, including photons, electrons, ions, and neutrons, or, alternatively, according to the type of information they provide. Here we have chosen to group the main catalyst characterization techniques by using a combination of both criteria into structural, thermal, optical, and surface-sensitive techniques. We also focus on the characterization of real catalysts, and toward the end make brief reference to studies with model systems. Only the basics of each technique and a few examples of applications to catalyst characterization are provided, but more specialized references are included for those interested in a more in-depth discussion. 

X-ray photoelectron spectroscopy also provides information on the chemical composition of a surface. An incoming photon causes electrons to be emitted from atomic core levels, which are then analyzed as a function of kinetic energy. The shifts of these core-level energies provide information about the chemical environment surrounding the excited atom. This information also includes changes in the oxidation state of the sample. 

Particles of a size of less than 2 turn are of particular interest in Process Engineering because of their large specific surface and colloidal properties, as discussed in Section 5.2. The diffusive velocities of such particles are significant in comparison with their settling velocities. Provided that the particles scatter light, dynamic light scattering techniques, such as photon correlation spectroscopy (PCS), may be used to provide information about particle diffusion. 

In UHV surface spectroscopies, the electrode under investigation is bombarded by electrons, photons, or ions, and an analysis of the electrons, ions, molecules, or atoms scattered or released from the surface provides information related to the electronic and structural parameters of the atoms and ions in the interfacial region. As mentioned before, the transfer of the electrode from the electrochemical cell to the UHV chamber is a crucial step in the use of these techniques. This has motivated a few groups to build specially designed transfer systems. Pioneering work in this area was done by Hubbard s group, followed by Yeager. 

None of the techniques described here has to be performed at UHV from an instrumental or fundamental physics point of view. It is necessary that interactions of the probing or signal species (photons, ions, or electrons) with residual gases do not interfere with the surface measurements and there may be some subsidiary instrument factor to be considered (e.g., What pressure will your x-ray source operate at Will your electron gun filament burn out Will your electron multiplier degrade ), but it is quite possible to work at pressures up to 10 3 Torr and with some instrumental ingenuity up to 10 1 Torr. Of course, in the practical world, one is often interested in reactions at these pressures and much higher (atmospheric and above or aqueous environments), and then all the techniques described here become unusuable, and one must rely on the photon-in, photon-out spectroscopies described in later chapters. RBS, in its usual form, is not usually considered a true surface technique. It is typically one for situations where surface layers of 100 s to 1,000 s of A are involved and therefore it is usually quite unnecessary to go to UHV conditions (7). However, in the last few years, work has been published which demonstrates that with refinements, the technique is one of the most powerful for quantitative and structural information at the surface monolayer level. Obviously, if used for this type of work, UHV requirements become important. 

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