Scanning photoemission spectroscopy

Scanning LEED, oscillatory reactions, 39 69 Scanning photoemission microscopy, kinetic oscillations, Pt(lOO), 37 250-253 Scanning photoemission spectroscopy oscillatory reactions, 39 69... 

The other two continuous spatially resolving methods, scanning LEED and scanning photoemission spectroscopy, were developed by Ertl et al. and can be applied only under HV conditions. In a scanning LEED setup, the system is configured similar to a normal LEED experiment. Two Helmholtz coils, however, are added to allow scanning of the primary electron beam over a certain fraction of the surface (50,53). One hundred data points are recorded within 10 s over a scanning area of 4 x 7 mm. The spatial resolution obtained in such an experiment is limited by the diameter of the incident electron beam. 

Thanks to the extensive literature on Aujj and the related smaller gold cluster compounds, plus some new results and reanalysis of older results to be presented here, it is now possible to paint a fairly consistent physical picture of the AU55 cluster system. To this end, the results of several microscopic techniques, such as Extended X-ray Absorption Fine Structure (EXAFS) [39,40,41], Mossbauer Effect Spectroscopy (MES) [24, 25, 42,43,44,45,46], Secondary Ion Mass Spectrometry (SIMS) [35, 36], Photoemission Spectroscopy (XPS and UPS) [47,48,49], nuclear magnetic resonance (NMR) [29, 50, 51], and electron spin resonance (ESR) [17, 52, 53, 54] will be combined with the results of several macroscopic techniques, such as Specific Heat (Cv) [25, 54, 55, 56,49], Differential Scanning Calorimetry (DSC) [57], Thermo-gravimetric Analysis (TGA) [58], UV-visible absorption spectroscopy [40, 57,17, 59, 60], AC and DC Electrical Conductivity [29,61,62, 63,30] and Magnetic Susceptibility [64, 53]. This is the first metal cluster system that has been subjected to such a comprehensive examination. 

Keywords Surface reconstructions surface states angular resolved photoemission scanning tunneling spectroscopy spin polarized spectroscopy self-organized nucleation. 

At a surface, not only can the atomic structure differ from the bulk, but electronic energy levels are present that do not exist in the bulk band structure. These are referred to as surface states . If the states are occupied, they can easily be measured with photoelectron spectroscopy (described in section A 1.7.5.1 and section Bl.25.2). If the states are unoccupied, a technique such as inverse photoemission or x-ray absorption is required [22, 23]. Also, note that STM has been used to measure surface states by monitoring the tunnelling current as a function of the bias voltage [24] (see section B 1.201. This is sometimes called scanning tunnelling spectroscopy (STS). 

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. 

DAPS Disappearance potential spectroscopy CLS Cathodoluminescence spectroscopy IPES Inverse photoemission spectroscopy BIS Bremsstrahlung isochromat spectroscopy AES Auger electron spectroscopy SAM Scanning Auger microscopy ELS (electron) Energy loss spectroscopy HREELS High-resolution electron energy loss spec-... 

Figure 77-S shows the X-ray photoemission spectroscopy (XPS) wide scans of typical GeS films prepared by the two routes. Oxide contamination was low in the GeS films, as shown by the intensity of the O Is peak (which also included HjO adsorbed at the sample surface), in qualitative agreement with the infrared results (Xu, 2000a, 2000b). For the GeS films via Route C, there was 5 at% O in the film and the ratio of S/Ge was 1.90, within the experimental error (10%) (Xu, 2000a). For the GeS films via route D, most of the O came from water adsorbed at the sample surface, judging from the O Is chemical shift, and the ratio of S/Ge was near 2.0, corresponding to GeS2 (Xu, 2000b). No Cl peaks appeared in either of the GeS films.

In this section, we will present and discuss results from Sc2 C84, which is the most widely studied dimetallofullerene to date. Early scanning tunnelling microscopy [26] and transmission electron microscopic [27] investigations provided evidence in favour of the endohedral structure of this system, which was later confirmed by x-ray diffraction experiments utilising maximum entropy methods [28]. Before experimental data from this system were available, the Sc ions were predicted to be divalent from quantum chemical calculations [29]. Subsequent data from vibrational spectroscopy [30,31], core-level photoemission [32] and further theory [33] on this system were indeed interpreted in terms of divalent Sc ions. 

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