Electronic states sample preparation

This technique can be applied to samples prepared for study by scanning electron microscopy (SEM). When subject to impact by electrons, atoms emit characteristic X-ray line spectra, which are almost completely independent of the physical or chemical state of the specimen (Reed, 1973). To analyse samples, they are prepared as required for SEM, that is they are mounted on an appropriate holder, sputter coated to provide an electrically conductive surface, generally using gold, and then examined under high vacuum. The electron beam is focussed to impinge upon a selected spot on the surface of the specimen and the resulting X-ray spectrum is analysed. 

Day-to-day drifts of instrumental characteristics, differences in temperature and sample preparation may affect the recorded lifetimes. For instance, differences in temperature can affect both the excited state lifetime of the fluorophore and instrument properties like noise or delays of wirings and electronics. 

XPS has typically been regarded primarily as a surface characterization technique. Indeed, angle-resolved XPS studies can be very informative in revealing the surface structure of solids, as demonstrated for the oxidation of Hf(Sio.sAso.5)As. However, with proper sample preparation, the electronic structure of the bulk solid can be obtained. A useful adjunct to XPS is X-ray absorption spectroscopy, which probes the bulk of the solid. If trends in the XPS BEs parallel those in absorption energies, then we can be reasonably confident that they represent the intrinsic properties of the solid. Features in XANES spectra such as pre-edge and absorption edge intensities can also provide qualitative information about the occupation of electronic states. 

Transmission electron microscopy micrographs (Fig. 13.24) also indicate an important characteristic of the supported particles. As in the case of suspensions, they are either aggregated or isolated. Support surface properties may he an important factor governing this aggregation. After deposition on the support, we observed that samples prepared from acidic hydrosols are characterized by the presence of aggregated particles constituting flocculates ranging from 10 to 200 nm, whereas samples prepared via basic hydrosols contain only isolated particles. The opposite was observed when hydrosols were concerned. These final states of the supported particles may be controlled... 

To understand the mechanism of water oxidation, it is necessary to characterize each of the S states. A variety of spectroscopic methods have been brought to bear on this problem. Electron paramagnetic resonance (EPR) and X-ray spectroscopies have been especially useful because these techniques allow the Mn complex to be probed directly. EPR spectroscopy has the restriction that the Mn complex must be paramagnetic to be studied. The S2 state is an odd-electron state, and EPR spectroscopy has been used extensively to study the Mn complex in the S2 state. X-ray spectroscopy has the advantage that any state of the Mn complex is observable. However, the successful application of EPR and X-ray spectroscopies requires that a specific S state be prepared in high yield in highly concentrated samples. 

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