Atomic structure, synchrotron-based

Studies by Teplyakov et al. provided the experimental evidence for the formation of the Diels-Alder reaction product at the Si(100)-2 x 1 surface [239,240]. A combination of surface-sensitive techniques was applied to make the assignment, including surface infrared (vibrational) spectroscopy, thermal desorption studies, and synchrotron-based X-ray absorption spectroscopy. Vibrational spectroscopy in particular provides a molecular fingerprint and is useful in identifying bonding and structure in the adsorbed molecules. An analysis of the vibrational spectra of adsorbed butadiene on Si(100)-2 x 1 in which several isotopic forms of butadiene (i.e., some of the H atoms were substituted with D atoms) were compared showed that the majority of butadiene molecules formed the Diels-Alder reaction product at the surface. Very good agreement was also found between the experimental vibrational spectra obtained by Teplyakov et al. [239,240] and frequencies calculated for the Diels-Alder surface adduct by Konecny and Doren [237,238]. 

Mineral-liquid or mineral-gas interfaces under reactive conditions cannot be studied easily using standard UHV surface science methods. To overcome the pressure gap between ex situ UHV measurements and the in situ reactivity of surfaces under atmospheric pressure or in contact with a liquid, new approaches are required, some of which have only been introduced in the last 20 years, including scanning tunneling microscopy [28,29], atomic force microscopy [30,31], non-linear optical methods [32,33], synchrotron-based surface scattering [34—38], synchrotron-based X-ray absorption fine structure spectroscopy [39,40], X-ray standing wave... 

Recently, the structure of the solid/liquid interface has been studied with a wide range of in-situ structural techniques. In particular, scanned probe microscopes [1-5] and synchrotron-based methods [6-9] have yielded a wealth of structural information. The ultimate goal of this work is an understanding of the structure and reactivity of the electrode surface at the atomic level. One of the most extensively studied processes is metal underpotential deposition (UPD) [10], which involves the formation of one or more metal monolayers at a potential positive of the reversible Nemst potential for bulk deposition. 

Among the various spectroscopic methods, the most notable one is synchrotron-based X-ray techniques that are revolutionizing soil and environmental chemistry research (Manceau et al, 1992). The most utilized synchrotron-based X-ray technique in soil and environmental chemistry to date has been X-ray absorption spectroscopy. The extended X-ray absorption fine structure spectroscopy (EXAFS) provides specific information on the local environment of the absorber, including coordination number, identity, and distances to nearest and sometimes next nearest neighboring atoms (Fendorff et al, 1994 Schulze and Bertsch, 1995 Fendorff and Sparks, 1996 Bertsch and Hunter, 1998 Sparks,... 

Since the early days of modern surface science, the main goal in the electrochemical community has been to find correlations between the microscopic structures formed by surface atoms and adsorbates and the macroscopic kinetic rates of a particular electrochemical reaction. The establishment of such relationships, previously only developed for catalysts under ultrahigh vacuum (UHV) conditions, has been broadened to embrace electrochemical interfaces. In early work, determination of the surface structures in an electrochemical environment was derived from ex-situ UHV analysis of emersed surfaces. Although such ex-situ tactics remain important, the relationship between the structure of the interface in the electrolyte and that observed in UHV was always problematic and had to be carefully examined on a case-by-case basis. The application of in-situ surface-sensitive probes, most notably synchrotron-based surface X-ray scattering (SXS) [1-6] and scanning tunneling microscopy (STM) [7, 8], has overcome this emersion gap and provided information on potential-dependent surface structures at a level of sophistication that is on a par with (or even in advance of) that obtained for surfaces in UHV. 

Bulk X-ray diffiraction, thus averaging the X-ray difiiraction signal over a large amount of sample ( mm ), ex situ or under operando conditions can be considered a standard technique using laboratory based or synchrotron based sources. Countless findings rely on the precise description of the atomic order obtained from Laue diffraction patterns to describe structure, phases, reaction pathways, strain, disorder etc. 

While this has long been the most popular method to study crystal structures, it was not until the introduction of ADXRD on synchrotron sources in the 1990s that it became routine to determine atomic position parameters using this technique at high pressure. While such studies have been performed principally at synchrotron sources, laboratory-based studies have also been performed see for example [155-158]. 

A more recent addition to the diverse array of x-ray based methods is x-ray absorption spectroscopy. In contrast to x-ray diffraction methods which derive their utility from the properties of well defined crystallites, x-ray absorption methods are atomic probes, capable of obtaining both electronic and structural information about a specific type of atom. The growing use of x-ray absorption methods is a result of the greater availability of synchrotron radiation sources which provide the intense broad band x-radiation required. In some instances laboratory based spectrometers utilizing either sealed tubes or rotating anode x-ray generators can also be used. 

FIGURE 14 Schematic molecular structure of the (Sc2C2 C84) carbide metallofullerene based on the synchrotron X-ray powder diffraction and NMR experiments. The two (top and bottom) spheres in the fullerene correspond to Sc atoms, whereas the C2 molecules are depicted between the Sc atoms. 

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