Bayesian inference in surface physics

Bayesian data analysis provides a consistent method for the extraction of information from physics experiments. The approach provides a unified rationale for data analysis, which both justifies many of the commonly used analysis procedures and reveals some of the implicit underlying assumptions. This paper introduces the general ideas of the Bayesian probability theory with emphasis on the application to the evaluation of experimental data, namely the deconvolution of the apparatus function for improving the energy resolution, the reconstruction of depth profiles from Rutherford backscattering measurements, handling of discordant data sets and mixture modelling for background estimation of Auger data. PACS 82.80.Pv; 02.50Tt; 82.80Yc     

VUV isochromat spectroscopy

A highly sensitive isochromat spectrometer employing a frequency selective helium iodine Geiger Müller photoncounter is described. The overall resolution of ±0.36 eV at an average quantum energy of 9.6 eV compares favourably with conventional set-ups. Sample data for a polycrystalline tantal target show satisfactory agreement with earlier experiments.     

Das elektronische Bremsvermögen von Stickstoff und Sauerstoff für niederenergetische Protonen

The electronic stopping power of molecular oxygen and nitrogen for protons with energies between 1 keV and 30 keV has been measured using a differentially pumped stopping cell. Our results give a surprisingly good confirmation of the Lindhard-Scharff statistical theory which predicts a linear velocity dependence of the electronic stopping power at low projectile energies. Moreover our data are in fair agreement with earlier high energy (E ≧ 20 keV) measurements in other laboratories. The combination of the present measurements with theoretically calculated nuclear stopping powers yields an estimate of the atomic stopping power. This estimate leads to substantially lower atomic stopping powers at low energies compared to values derived from range measurements. This result may have interesting implications on auroral hydrogen emissions.     

A VUV isochromat spectrometer for surface analysis

A vacuum ultraviolet isochromat spectrometer employing an energy selective Geiger-Müller photon counter is described. The mean energy of the spectrometer is 9.7 eV with a standard deviation of ±0.23 eV. Together with the thermal energy distribution of the electrons emitted from a directly heated tungsten cathode, an overall resolution of ±0.33 eV is obtained. Pulsed operation of the x-ray tube avoids additional broadening due to the voltage drop across the filament. Further, pulsed operation of the accelerating voltage is employed to avoid counting losses due to the relatively large dead time of the radiation detector. Operating frequencies of up to 2 kHz are possible. With sensitivities of typically 10⁷ pulses per Coulomb, the anode currents are in the range of 100–200 μA. These attractive properties suggest to employ the method to study surface electronic properties of solids.     

Atomic adsorption of oxygen on Cu(111) and Cu(110)

We have studied angle-resolved inverse photoemission ( = 9.7 eV) after room temperature adsorption of oxygen on Cu(111) and Cu(110). On Cu(111) exposure to 500 L induces a band (3.0 eV aboveE _F at) which shows clear dispersion (1.0 eV) to higher energies for off normal incidence. Since no LEED superstructure is seen for that system, our results present strong evidence for the presence of short-range surface order. Two adsorbate bands are identified (2.8 eV and 6.3 eV at) on Cu(110)p(2×1)-O. Our results are in good agreement with a long-bridge adsorption site.     

The adsorption of Xenon on both low and high work-function metals

The structure of unoccupied electronic states in Xe condensed on differently prepared metal surfaces has been observed by inverse photoemission spectroscopy. The Xe/metal interface is interpreted in the framework of Schottky barrier theory. On low workfunction surfaces a wetting to non-wetting phase transition is inferred from dramatic changes in the unoccupied level structure. Existing data in the literature indicate non-wetting behaviour of Xe on low work-function materials to be a quite general phenomenon.