Photoemission techniques, applications

Although applications of photoemission techniques in surface chemistry and catalysis are but a few, their sensitivity, which is orders of magnitude higher than that of adsorption measurements, may lead to future investigations of very small surfaces or adsorbed complexes in very small concentrations. The technique is not universal, however, because relatively few surface species will display photoluminescence. 

The book has been written as an introductory text, not as an exhaustive review. It is meant for students at the start of their Ph.D. projects and for anyone else who needs a concise introduction to catalyst characterization. Each chapter describes the physical background and principles of a technique, a few recent applications to illustrate the type of information that can be obtained, and an evaluation of possibilities and limitations. A chapter on case studies highlights a few important catalyst systems and illustrates how powerful combinations of techniques are. The appendix on the surface theory of metals and on chemical bonding at surfaces is included to provide better insight in the results of photoemission, vibrational spectroscopy and thermal desorption. 

In the investigations of molecular adsorption reported here our philosophy has been to first determine the orientation of the adsorbed molecule or molecular fragment using NEXAFS and/or photoelectron diffraction. Using photoemission selection rules we then assign the observed spectral features in the photoelectron spectrum. On the basis of Koopmans theorem a comparison with a quantum chemical cluster calculation is then possible, should this be available. All three types of measurement can be performed with the same angle-resolving photoelectron spectrometer, but on different monochromators. In the next Section we briefly discuss the techniques. The third Section is devoted to three examples of the combined application of NEXAFS and photoemission, whereby the first - C0/Ni(100) - is chosen mainly for didactic reasons. The results for the systems CN/Pd(111) and HCOO/Cu(110) show, however, the power of this approach in situations where no a priori predictions of structure are possible. 

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]. 

NEXAFS experiments on NOM can be conducted in several modes that differ in the type of detected particle and objectives of the experiment transmission (X rays transmitted through the sample), fluorescence (fluorescent X rays due to absorption of the X-ray beam), or electron yield (photo-emitted electron) (Sparks, 2003). Alternatively, the techniques can be divided into full-field applications such as transmission X-ray microscopy (TXM) and X-ray photoemission electron microscopy (PEEM), in comparison to scanning techniques such as scanning transmission X-ray microscopy (STXM) and scanning photoemission microscopy (SPEM) that provide spatial information of elemental forms. 

Capacitance transient spectroscopy encompasses a powerful set of techniques to detect and characterise deep levels in semiconductors. The list of techniques applied for III-V nitrides includes deep level transient spectroscopy (DLTS) [1,2], double correlation DLTS (DDLTS) [3], isothermal capacitance transient spectroscopy (ICTS) [2], photoemission capacitance transient spectroscopy (ODLTS) [4] and optical ICTS (OICTS) [5], This Datareview presents the current status of deep level studies by capacitance transient techniques for III-V nitrides. A brief introduction to the techniques is given, followed by an example that demonstrates the application of DLTS and DDLTS for Si-doped... 

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. 

Both photoemission and inverse photoemission require reasonable sample conductivity and their application to hard insulators such as MgO and AI2O3 is problematic. Both techniques also involve the complication that inelastic electron energy loss processes become convoluted with electron emission or decay. This may give rise to spectral features in regions where none are expected fi om the density of states [24,25] and care must always be taken to exclude these features before considering assignment to surface states. 

A constraint of the DP technique is that it is not applicable to activated carbon [7,28,59] or zeolites [60] due to their high isoelectric point. It is interesting that graphite substrates have often been used in photoemission experiments designed to explore the electronic properties of gold nanoparticles and that such particles appear to have similar electronic character to gold particles on oxides [41]. 

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. 

R. L. Martin and D. A. Shirley, in Electron Spectroscopy Theory, Techniques and Applications, C. R. Brundle and A. D. Baker, Eds., Academic Press, New York, 1977, p. 75. Many-Electron Theory of Photoemission. 

We report on the use of spin polarized electron beams in the study of electronic states in solids, referring in particular to the Inverse Photoemission spectroscopy. In this technique the empty electron states are investigated, and the spin resolution allows to study their spin character, yielding valuable information in magnetic systems. Examples of application to layered magnetic nanostructures are given in particular we present data on Fe/Cr/Fe(001) multilayers, ultrathin Fe films grown on ZnSe(OOl), and LaSrMnO/SnTiO junctions. 

The back-reflection XSW technique has been used mostly on metal and oxide crystals, whose rocking curves often have angular mosaicity up to -0.1°. In application to mineral surfaces, the technique has also been used by Kendelewicz et al. (1998b) to study the exchange between Na and Pb in a Na overlayer on the PbS(100) surface. A review of the BRXSW technique was given by Woodruff (1998). Since the BRXSW method is employed at soft X-ray energies (typically from Er= 1 keV to 4 keV), it is not useful for in situ studies of the mineral-fluid interface. However, the BRXSW is a very useful probe for UHV surface science measurements of adsorbed molecules on metal single crystal surfaces (Jones et al. 2002) and for site-specific valence-band photoemission studies developed by Woicik and coworkers (Woicik et al. 2001 Kim et al. 2002). 

---