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Notes: Using synchrotron radiation, Auger electron, and H+/D+-ion yields have been studied at and above the O 1s excitation energies for condensed H2O/D2O layers of varying thickness, and for two reproducible adsorbate layers (so-called bilayers and monolayers) on Ru(001). Decay electron spectra as well as polarization dependences, angular distributions, and energy distributions of desorbing ions have been investigated. For polarizations with sufficient E component perpendicular to the surface, a sharp peak in the H+ NEXAFS spectrum is seen for all layers which has no direct counterpart in the Auger NEXAFS spectra, and whose intensity maximizes for E oriented in the detection direction. This observation is interpreted as due to the 1a1→4a1 core-to-bound transition of the surface molecules whose final state decays electronically and dissociates on comparable time scales. This appears to have the consequence that the symmetry of the coupled excitation is different from that expected for the primary photoabsorption process. There appears also to be an influence of hydrogen bonding on these effects. Similarities and differences between the various layers investigated are also analyzed.

Notes: For condensed benzene ice layers, core photoabsorption near edge structure (x-ray absorption; recorded by Auger electron yield measurements), decay electron spectra for resonant and nonresonant excitation, and fragmentation as evident in yields of hydrogen and other ions, have been measured in the C1s region. The absorption spectrum is better resolved than most previously published spectra, exhibits some new features, and shows a high degree of parallelity to the spectrum of isolated molecules. Interestingly, the hydrogen ion yield indicates a particular dissociativeness of a certain core excitation resonance, X, which in the molecule has previously been assigned to a Rydberg state. This selective dissociation suggests that the responsible excitation is strongly antibonding for the carbon–hydrogen bond, while the degenerate Rydberg states broaden into a conduction band in the solid; and that the bond breaking probably occurs or at least starts in the core-excited state, thus proceeding on an extremely short time scale, similarly to observations for other hydrogen-containing molecules. The decay spectra are analyzed in terms of autoionization vs normal Auger decay and indicate that, apart from the first strong π resonance (which leads to pure autoionization) and the X resonance, the core resonances partly or fully ionize before core decay takes place. For the X resonance, the decay spectrum contains a contribution which cannot be assigned to intact benzene; this is taken as additional evidence for ultrafast dissociation, i.e., competitive with core decay. We use these results for a discussion of the influence of condensation on excitation, decay, and fragmentation.

Notes: Absolute values of the initial sticking coefficients of rare gas atoms (Ne, Ar, Kr, Xe) on a flat, clean Ru(001) surface have been determined with thermal beams and a highly sensitive thermal desorption method. The sticking coefficients increase with increasing mass of the atoms. Their decrease with increasing gas temperature is stronger the lighter the atom; different surface temperatures within the accessible range do not measurably affect the sticking efficiency. At a gas temperature of 300 K and a surface temperature of 6.5 K the initial sticking coefficients are 0.004 for Ne, 0.13 for Ar, 0.25 for Kr, and 0.71 for Xe. Forced oscillator calculations treating the substrate phonons quantum mechanically have been performed. With the well depths derived from experiment, and other reasonable input parameters, absolute values and functional forms of the sticking coefficients can be reproduced. The low values are due to the high elastic reflection probability which is a consequence of the inefficient energy transfer and the phonon quantization. The calculated Debye–Waller factors at zero gas and surface temperature are 0.92 for Ne, 0.36 for Ar, 0.14 for Kr, and 0.01 for Xe. A classical interpretation of the sticking data is impossible at least for Ne and Ar.

Notes: We have developed a video low-energy electron diffraction (LEED) system on the basis of a slow scan charge coupled device (CCD) camera which is capable of collecting LEED IV data at very low electron doses quickly and therefore enables us to study extremely beam sensitive surface structures which have not been accessible to LEED IV analysis before. The slow scan CCD camera allows separating the relatively short data acquisition process from the more lengthy digitizing, storage, and data analysis processes. Typical total effective exposure times can therefore be reduced to about 200–300 s (1 s per energy point) at a primary beam current of 100 nA which corresponds to a total dose of about 12 e per adsorbate particle; further decrease is possible. The total measurement time for collecting a complete set of LEED images is of the order of 30–40 min which assures the exclusion of contamination effects, even for sensitive layers. The IV curves are then extracted from the digitally stored images off-line which allows collecting the intensities of all visible spots simultaneously with a high reliability in tracing beams, even for very dense LEED patterns. © 1996 American Institute of Physics.

Notes: On the basis of detailed considerations of the relevant effects and parameters, we describe techniques which have been developed to accurately set, maintain, and measure the temperature of a small sample (typically a single-crystal disk of about 1 cm in diameter and 1 mm thick) in a strongly nonisothermal surrounding in ultrahigh vacuum, such as required by surface science experiments, in the temperature range from at least 6.5 K up. The resettability and resolution are 30–70 mK and the absolute error is estimated as about 0.7 K at 10 K and about 1.5 K at 100 K. Controlled linear heating with rates from 10−2 to 50 K/s and high constancy (deviations below 0.1 K up to 5 K/s and below 0.5 K up to 50 K/s), and stepwise heating ((approximately-greater-than)100 K/s) without measurable overshoot can be carried out in this whole cryogenic range.

Notes: MnBi films are prepared on quartz and on (001) orientated GaAs substrates by molecular beam epitaxy in ultrahigh vacuum environment. Both kinds of substrates are used simultaneously. The influence of the substrate material is investigated with respect to the structural, the magnetic, and magneto-optical properties of the MnBi films. By evaporating a 100 nm thick SiOx buffer layer the homogeneity of the composition is improved and a grain size of about 100 nm is achieved without adding other elements. In contrast to previous investigations, our measured magneto-optical Kerr rotation spectra show no Kerr rotation peak near 3.35 eV. These results confirm theoretical predictions whereupon this peak is attributed to oxygen which occupies interstitial sites in the regular MnBi lattice. © 1998 American Institute of Physics.

Notes: Angular and velocity distributions of CO2 desorbing as reaction product of CO oxidation on Pt(111) were measured during heating of layers of initially molecular oxygen and CO adsorbed at a surface temperature of 100 K. In the velocity integrated desorption spectra of the reaction product CO2 four different peaks (α, β3, β2, β1) can be discriminated which, for linear heating rates of 5 K/s, appear at 145, 210, 250, and 330 K, respectively. They can be attributed to different reaction mechanisms which depend on the binding conditions of oxygen and the geometric arrangement and coverages of both species. Whereas α-CO2 coincides with the O2 desorption from and the dissociation of pure chemisorbed molecular oxygen, and thus indicates a reaction channel coupled with desorption and dissociation of O2, β1-CO2 corresponds to the reaction path investigated before by many researchers and is most likely due to the reaction at the boundaries of ordered CO and oxygen islands. The structural conditions for β3 and β2 are less clear, but we believe them to stem from reactions in mixed and/or partly mixed layers at high coverages of O and CO. The α-CO2 species is most likely due to reaction of CO with O atoms stemming from O2 dissociation which react before becoming accommodated.The velocity distributions of α, β2, and β3 are far from thermal equilibrium with the surface as indicated by average kinetic energies between 220 and 360 meV, corresponding to (approximately-equal-to)10 (for β3 and β2) and (approximately-equal-to)30 kTs (for α), normalized speed ratios between 0.6 and 0.8, and strongly peaked angular distributions (∼cosn cursive-theta, n=8 for α, n(approximately-greater-than)10 for β3 and β2). For β1 both the angular and velocity distributions show bimodal behavior with one channel fully accommodated to the surface whereas the other contains again an appreciable amount of reaction energy as kinetic energy (〈E〉(approximately-equal-to)330 meV) resulting in a strongly peaked angular distribution with n(approximately-equal-to)9. Some TOF results for steady state reaction at high temperatures (420–800 K) obtained in the same apparatus are given for comparison. The fraction of reaction energy channelled into the translational degree of freedom for the nonequilibrated part of reaction peak β1 is estimated to about 40%. A discussion of the various possible mechanisms is given.

Notes: The initial stages of the multilayer growth of a model system for molecular solids, namely physisorbed benzene on Ru(001), have been studied in detail by infrared reflection absorption spectroscopy and thermal desorption spectroscopy. A variety of different phases have been discriminated spectroscopically and characterized in situ: the parallel oriented first physisorbed layer which is found to rearrange into a more crowded layer with a high tilt angle at slightly higher coverages; an amorphous layer which grows at low temperatures (T ≤55 K), and a crystalline layer to which the former converts at elevated temperatures. Clear evidence for structural disorder of the uppermost layer of the crystalline phase is found. The amorphous–crystalline phase transformation is irreversible and the required temperatures vary considerably with the layer thickness. This is attributed to two different processes: at high coverages (aitch-theta ≥10 ML) crystallization is possible at low T without mass transport and requires only a reorientation and minor rearrangement of the benzene molecules. Low initial coverages (aitch-theta=2.5–5 ML) require nucleation and diffusion of benzene molecules to form stable 3D crystallites with the former process acting as the kinetically limiting factor. Particular attention has been devoted to the unravelling of the nature of the metastable state observed in thermal desorption spectroscopy and its transformation into the more stable crystalline phase. © 1996 American Institute of Physics.