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A new adsorption site for adsorbed acetylene on Si(100) is observed by photoelectron imaging based on the holographic principle. The diffraction effects in the carbon 1s angle-resolved photoemission are inverted (including the small-cone method) to obtain an image of the atom's neighboring carbon. The chemisorbed acetylene molecule is bonded to four silicon surface atoms. In contrast to the C2H2 case, the image for adsorbed C2H4 shows it bonded to two Si surface atoms.

The adsorption and reaction of acetylene with the Si͑100͒-2ϫ1 surface has been studied using highresolution photoemission by monitoring the Si 2p, C 1s, and valence-band ͑VB͒ spectra as a function of both acetylene coverage and post-adsorption annealing temperature. After the clean Si͑100͒ surface is exposed to 0.5 monolayer ͑ML͒ acetylene, the surface state in the VB is absent. Meanwhile, the curve-fitting results show that there is only one interface component in the Si 2p core level. These results indicate that the asymmetric Si dimers may become symmetric dimers after acetylene adsorption, which can be explained well by the tetramodel determined from our previous photoelectron holographic results. Significant changes in the electronic structure (Si 2p, C 1s, and VB͒ are found after subsequent annealing of the saturation overlayer. Annealing at lower temperature can induce some acetylene molecule desorption while most of the molecules decompose into C 2 H x (xϭ1,0) and H species. After annealing above 660°C, both of the reacted components of the Si 2p and C 1s lines show that the SiC species form clusterlike features. At the same time, the VB and Si 2p spectra indicate a restoration of a Si͑100͒-2ϫ1 structure, and the asymmetric Si dimers reappear on the surface.

The heat transfer capabilities of beam stops in CHESS wiggler and undulator beamlines is described. The thermal analysis for the design of these crucial in-vacuum beamline components is based on the use of a finite element analysis computer calculation and experimental heat loading tests.

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