Synchrotron radiation, surface electronic

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

The availability of high-intensity, tunable X-rays produced by synchrotron radiation has resulted in the development of new techniques to study both bulk and surface materials properties. XAS methods have been applied both in situ and ex situ to determine electronic and structural characteristics of electrodes and electrode materials [58, 59], XAS combined with electron-yield techniques can be used to distinguish between surface and bulk properties, In the latter procedure X-rays are used to produce high energy Auger electrons [60] which, because of their limited escape depth ( 150-200 A), can provide information regarding near surface composition. 

As an example let us consider the pentacene/samarium interface (Koch et al, 2002). Samarium has a low work function ((/>m — 2.7 eV), which is comparable to E a from pentacene (— 2.7 eV). Thus, if A 0, the condition Ep, should provide efficient electron injection because in this case fp and LUMO are nearly aligned. In order to avoid contamination that may alter the instrinsic m, h and homo values, such heterostructures have to he prepared in ideally clean conditions, imposing the use of UHV. The UPS experiments performed with synchrotron radiation are shown in Fig. 4.24. After measuring (pM of the clean samarium surface (2.7 eV) as described above, increasing amounts of pentacene are controllably deposited onto the samarium surface. The survey spectra of the valence states and a close-up view of the energy region near E are shown in Figs. 4.24(a) and (b). 

Pi TW, Liu Ch, Hwang J (2006) Surface electronic structure of Ca-deposited tris (8-hydroxyquinolato) aluminum studied by synchrotron radiation photoemission. J Appl Phys 99 123712... 

We attempted to use this increase in refractive index in fabricating polyimide optical waveguides. The fabrication of a fluorinated polyimide waveguide by the direct electron beam writing method is described in Section 4.3.2. We also investigated the changes in the refractive index of fluorinated polyimide films by synchrotron radiation. 7 The refractive index at a wavelength of 589.6 nm increased by 1.3% and the thickness decreased by 0.69% for fluorinated polyimide film after 30 min of synchrotron irradiation. From the XPS data the synchrotron radiation leads to production of a fluorine-poor surface. 

From the perspective of this symposium, analysis of the atomic dynamics and electronic structure of surfaces constitutes an even more exotic topic than surface atomic geometry. In both cases attention has been focused on a small number of model systems, e.g., single crystal transition metal and semiconductor surfaces, using rather specialized experimental facilities. General reviews have appeared for both atomic surface dynamics (21) and spectroscopic measurements of the electronic structure of single-crystal surfaces (, 22). An important emerging trend in the latter area is the use of synchrotron radiation for studying surface electronic structure via photoemission spectroscopy ( 23) Moreover, the use of the very intense synchrotron radiation sources also will enable major improvements in the application of core-level photoemission for surface chemical analysis (13). 

In this symposium emphasis is placed on the first class of methods. Isolated examples of the third class are discussed. Consideration of the second class is omitted entirely. Indeed, the scope of the symposium is perhaps best described as encompassing the more widespread techniques of surface compositional analysis as applied to materials science and electrochemistry oriented problems. Many modern surface analysis methods, e.g., those embodying tip sample geometries (21 22, 3, 34), those based on synchrotron radiation (23, 25), and those dealing with surface structure (15-20) and dynamics ( 3, 21-23), as opposed to surface composition, are not represented in the symposium program even though many of them enjoy "industrial applications" in the areas of electronics, metallurgy and catalytic chemistry. 

Rare-gas samples exist only at cryogenic temperatures and most of the optical spectroscopy of electronic processes should be done in the vacuum ultraviolet. Making experiments requires an indispensable combination of liquid-helium equipment with windowless VUV-spectroscopic devices and synchrotron radiation as a photon source. To study the electronic excitation energy pathways and a variety of subthreshold inelastic processes, we used the complimentary advantages of cathodoluminescence (possibility to vary the excitation depth beneath the sample surface), photoluminescence (selective-state excitation by synchrotron radiation at high-flux SUPERLUMI-station at HASYLAB, DESY, Hamburg) and... 

Secondly, the use of UHV/EHV systems is obviously desirable because of the extreme sensitivity of many materials and processes to residual gas. Work in this pressure range has allowed significant advances to be made in, for example, surface science and the exploitation in many areas of research of synchrotron radiation sources based on electron or positron storage rings. Relevant factors in the operation of these systems, such as adsorption and desorption (including diffusive outgassing), are examined in detail. 

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