Photoemission techniques

The solution chemistry that can be studied by photoemission techniques is largely that instigated by solvated electrons. It therefore has much in common... 

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. 

A general term for structure - sensitive photoemission techniques, including ARPEFS, ARXPS, ARUPS, and ARXPD. 

Various photoemission techniques are a powerful tool for the investigation of especially large-scale features of the occupied electronic states. Numerous photoemission data are available mainly on the uranium intermetallics. These were obtained by all possible kinds of excitations such as X-ray photoemission (XPS), ultraviolet high-resolution photoemission (UPS), or synchrotron-radiation-excited photoemission with the possibility of tuning the energy of the incident radiation. The Bremsstrahlung isochromat spectroscopy (BIS) is a probe of the empty electronic states above EF. 

The distinct advantage of scanning tunneling spectroscopy (STS), compared to conventional experimental techniques like photoelectron spectroscopy and inverse photoemission, is the high lateral resolution. Additionally, electronic states can be investigated in a single measurement on both sides of the Fermi level which cannot be carried out in photoemission techniques photoemission allows to determine occupied states only, inverse photoemission empty states only. 

Information on the spin resolved band structure of ferromagnetic materials can directly be obtained from spin resolving photoelectron spectroscopy. Using polarized radiation spin integrating photoemission techniques already enable to have access to magnetic properties. An enhancement of the surface sensitivity can be achieved using neutral excited spin polarized atoms which move towards the sample and are de-excited by tunneling electrons from the surface with a subsequent emission of electrons. 

The apphcation of photoemission techniques to zeolites is almost exclusively limited to XPS. It has recently been demonstrated that UP spectra maybe measured with zeolites under special circumstances [1], and these have been used as complementary evidence to ISS and XPS data in a study of partially exchanged NaH faujasites [2]. UPS with zeolites provides characteristic shapes of the valence-... 

Photoemission techniques offer a variety of tools for the discrimination between extra- and intra-crystalline location of components introduced (e.g. metal ions, MI ). Nevertheless, due to the specific limitations of these tools, detailed studies, preferably combining several of them, are often required to derive sound conclusions, in particular if both extra- and intra-zeolite species are present. Apart from quality assessment after ion-exchange steps, photoemission has been increasingly applied to describe mobility phenomena, e.g., the preparation of zeolite catalysts by solid-state reactions (soHd-state ion exchange [86-89],reductive dispersion (Ga203 into H-ZSM-5 [90-93]),chemical transport [94-97]), the penetration of metal poisons (Ni,V) into FCC catalysts [98-101] and the redistribution of active catalytic components in zeoHte crystals under reaction conditions [102-105]. Much of the earlier work in this field has been reviewed by Shpiro et al. [33,35]. 

The influence of the zeohte on the redox chemistry of components dispersed in them is another topic studied by photoemission techniques [33,35]. Numerous differences between the redox chemistry of transition elements in the bulk phase and in zeolites have been estabUshed, e.g., the stabilization of low (nonzero) valence states as Rh(I) [122-124], Pd(I) [125-127], Ni(I) [128] a different stabilization of intermediate Cu(I) in Y and ZSM-5 or beta [108,121,129] the stabilization of metal atoms in zeolites (Pd in H-Y [130], Pt, Ir in H-ZSM-5 [131]) the acceleration of the valence-state interchange Cu + Cu+ in overexchanged ZSM-5 and beta [89,108,121]. 

Table 4. Zeolite-encaged alloys studied by photoemission techniques. For multitechnique studies, the major techniques have been underlined ...

Self-consistent calculation using the pseudopotential method enables one to determine the electronic structure of a transition metal. The data on the band structure, the density of states, and the charge distribution were obtained for bulk niobium [54]. The results were compared with an experiment that was carried out by the photoemission technique. 

The photoemission techniques (PE fig. 3a,b,c,) are based on the observation of the energy and intensity distribution of the electrons emitted by a sample irradiated by a monochromatic photon source. The spectra are then interpreted in terms of electronic transitions resulting from the annihilation of these photons of known energy. The photoemission methods can be devided into two broad categories defined by the photon energy range ultraviolet photoemission (UPS) for hv < lOOeV and X-ray photoemission spectroscopy (XPS) for hv 1200-1500eV. 

The two photoemission techniques which were used to demonstrate the doublepeak character of the 4f emission in the EDCs of y-Ce were also brought to bear in studies of a-Ce. Temperature-dependent studies showed the very intriguing result... 

Finally, we note without proof that the wave vector k, which we have formally introduced as a quantum number, also has another physical interpretation as it is related to the crystal momentum phyp = hk. This has the important implication that k is experimentally accessible. So the band structure E (k) can directly be mapped by experiment (e.g., by photoemission techniques). 

The dispersion of surface electron states, measured by angle-resolved photoemission techniques [68], is shown in Figure 9.34. With the help of the STM data shown in Figure 9.33 it was possible to identify the states. The filled surface state Si and the lowest empty surface state Ui are localized at the adatoms. The almost dispersionless band S2 originates from the six rest atoms. The weak overlap of the orbitals is the reason for the flat states. The band S3 originates from the backbonds of the adatoms. 

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