The consistency and accuracy of inelastic electron mean free paths (IMFPs) determined from comparison of the intensity of elastically reflected electrons with theoretical calculations were studied. The variation with experimental geometry, spectrometer energy resolution and the procedure for background subtraction was studied. Four different types of spectrometers -double pass cylindrical mirror analyser (CMA) preretarded CMA, hemispherical analyser (HSA) and retarding field analyser (RFA) -with widely different geometries and energy resolutions, placed in four different laboratories in three countries, were used. INTRODUCTIONThe inelastic electron mean free path (IMFP) is a crucial parameter for quantitative interpretation of surface electron spectra.1,2 Because of the high importance of the IMFP, much effort has been spent in the past to find new improved methods for its determination.The most common theoretical procedure used to calculate the IMFP describes the electron-solid interaction by a dielectric response model, 3 -5 where the dielectric function ε ω is taken from optical experiments. These results consists of comparing theoretical and experimental differential inelastic scattering cross-sections determined from analysis of reflected electron energy-loss spectroscopy (REELS) at a single or preferably at several energies or emission angles. Depending on the electron energy and angle of emission, the cross-section will have different contributions from surface excitations. By a detailed and fairly complicated calculation, ε ω is determined in an iterative process by comparing theory with experiment. Finally, the determined ε ω is used to calculate the IMFP. This method was applied to find the IMFP for Al, Fe, Si, SiO 2 and ZrO 2 7,8 over a wide range of energies. These values deviate by <10 % from the values found by Tanuma et al.

Abstract:We used STM to study the conductivity of 32 nucleotide long DNA molecules chemically attached to a gold surface. Two oligonucleotides containing all four base types namely G, A, C, T, one single stranded and one double helical, all showed conductance data significantly higher than DNA containing only T and A that were either single stranded d(T32) or double helical d(T32).d(A32) in confirmation. Within each sequence group, the conductivity of the double helical form was always higher than that of the single strand. We discuss the impact of structure, particular base stacking and affinity to the phase transition.PACS (2008) DNA is an important and promising molecule, not only due to its genetic function, but also as a molecular scaffold for nanotechnology [1]. The double helical DNA architecture, is well stacked consisting of near parallel bases stacked with their π-electron systems overlapping. Such π-electron systems may be good candidates for long distance and one-dimensional (linear) charge transport. In- * E-mail: krat@fzu.cz vestigations of DNA conductivity and its physical origin have significant implications towards the study of DNA damage and repair in biological systems, application of DNA in electronic nano-devices, and DNA-based electrochemical biosensors. The electronic transport of various types of organic molecules operating through π-conjugated systems, including DNA, has been the subject of several recent studies, both theoretical and experimental [2][3][4][5]. Despite significant achievements, results on DNA conductivity published by different research groups present often conflicting and controversial explanations of 422

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The Rh/Al system has drawn significant attention as a model heterogeneous catalyst due to its unique adsorption properties. In this study, a set of samples consisting of Al substrates made of polished and etched pure metal sheets and deposited in situ with ultrathin Rh overlayers of different thickness was prepared. During the preparation of the samples, in-depth diffusion of Rh into the surface region of Al substrates obviously took place. To quantify this process, i.e. by non-destructively ascertaining the resulting depth profile of the samples, elastic peak electron spectroscopy (EPES) was employed. An EPES measurement was performed for energies of 500, 750 and 1000 eV with the initial Rh overlayer thickness in the range 3-20Å and using pure Rh and Al standards. The measurement was carried out on a precision spherical retarding-field analyser (RFA). The resulting experimental EPES intensities, as a function of the primary energy and the initial thickness of the Rh overlayer, were compared and fitted with the intensities simulated by the Monte Carlo (MC) method. An MC model of the sample surface consisting of three regions of different inelastic mean free path (IMFP) and Rh concentration were assumed: the first -Rh on the substrate top, the second -Rh/Al with exponential decay of Rh in Al and the third -pure Al. The best fit and best consistency for different primary energies were achieved under the assumption that no Rh was left on Al substrate top, but an exponentially decaying profile of Rh in Al was created at the surface.