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The electronic excitations of potassium clusters embedded in a MgO matrix have been investigated as a function of cluster size and momentum transfer by means of electron-energy-loss spectroscopy. The experimental results show deviations from predictions of classical electrodynamics but they are in qualitative agreement with a calculation for excitations of a jellium sphere made with use of the timedependent local-density approximation.PACS numbers 79.20.Kz, 71.45.GmThe electronic structure of small metal particles has been the subject of a large number of recent theoretical and experimental studies. 1 Deviations from the bulk electronic structure are expected from the influence of the surface and from quantum size effects, i.e., the transition from a continuous conduction band into discrete levels. Experimentally, the electronic structure of small particles can be studied by measuring excitations such as single-particle and collective excitations. Classically, in the framework of an electrodynamical theory, 2 there exist for metallic spheres besides the volume plasmon with energy ho) p , also a series of multipolar surface plasmons with energies ho>i=hco p {l + [U + \)/l]€}-l/ \

We correlate photoconductivity with surface characterization in order to probe the influence of different surface properties of Si ( 111) on electron-hole dynamics. Photoconductivity data show that the carrier recombination is influenced strongly by surface structure and chemistry as well as by trace imperfections which are undetectable by conventional surface characterization techniques. Results on the Si( 111) :As ( 1 x 1) surface indicate carrier recombination at this surface is negligible, in contrast to the (7X 7) reconstructed surface, where surface recombination is fast, with a surface recombination velocity measured to be >2 x lo6 cm/s. We also investigate the effect of sputter induced disorder and compare our finding with results from other techniques. Combining our results and the present understanding of the (7 x 7) electronic structure, we estimate the carrier capture cross section for dangling bonds to be -10 -l5 *