Surface induced broadening of the 2p core levels of Al

We observe a drastic broadening of the photoemission lines of the 2p core levels of Al when the surface sensitivity of the measurement is enhanced by an appropriate choice of the photon energy using synchrotron radiation. This surface broadening appears to be quite universal and also occurs for other substances like, e.g. Au. Its possible origin is discussed. There is no surface induced shift of the Al 2p binding energy within ± 40 meV.     

Einige Anwendungen des Cartier-Operators

Ohne ZusammenfassungHerrnReinhold Baer zum 60. Geburtstag gewidmet     

AB initio calculations of the electronic properties of polyethylene

Previous studies of simple polymers have produced energy bands and densities of states which are not in good agreement with each other. We present here an accurate non-parametric method for determining the electronic energy levels of simple polymers, and apply this method to several simple polymers, both with and without impurity side groups.     

Observation of resonance recombination lines in electron excited auger spectra of Gd

Combined measurements of electron excited N_4,5 Auger spectra and photoelectron emission on clean and oxidized Gd lead to a distinction between Auger lines originating from 4d → continuum and 4d → 4? resonance excitations. Several Auger structures are identified as due to the direct recombination of 4d⁹4?⁸ states with the 4f and valence electrons. The shape of the most prominent Auger line for oxidized Gd agrees perfectly with the Fano profile of the 4? photoemission intensity.     

Multilayer interference mirrors for the XUV range around 100 eV photon energy

For the first time successful results have been obtained demonstrating interference effects in the reflectance of multilayer films at high photon energies where n = 1 and k is large. Films with 3.5 to 4.5 periods of Au/C and Cu/C were investigated for angles of incidence α = 15° to α = 60° in the wavelength interval 80 to 300A?. The highest near-normal incidence reflectance found was R = 2.7% for Au/C and R = 1.2% for Cu/C at λ ～ 190A?and α = 15°. These values were by a factor of 7 higher than those for a single layered opaque Au film (respectively a factor of 6 for Cu) evaporated under the same conditions. Further improvements appear to be possible. Calculations were performed to guide the investigations and to help the interpretation of the results.     

Diffusion von Dotierungsmaterial in Silizium

Ohne Zusammenfassung     

Sampling of rare events using hidden restraints

A method to enhance sampling of rare events is presented. It makes use of distance or dihedral-angle restraints to overcome an energy barrier separating two metastable states or to stabilize a transition state between the two metastable states. In order not to perturb these metastable end states themselves, a prefactor is introduced into the restraining energy function, which smoothly increases the weight of this function from zero to one at the transition state or on top of the separating energy barrier and then decreases the weight again to zero at the final state. The method is combined with multi-configurational thermodynamic integration and applied to two biomolecular systems, which were difficult to treat using standard thermodynamic integration. As first example the free energy difference of a cyclic alpha-aminoxy-hexapeptide-ion complex upon changing the ion from Cl- to Na+ was calculated. A large conformational rearrangement of the peptide was necessary to accommodate this change. Stabilizing the transition state by (hidden) restraints facilitates that. As a second example, the free energy difference between the 4C1 and the 1C4 conformation of beta-D-glucopyranoside was calculated. In unrestrained simulations the change from the 4C1 into the 1C4 conformation was never observed because of the high energy barrier separating the two states. Using (hidden) restraints, the transition from the 4C1 into the 1C4 state and back could be enforced without perturbing the end states. As comparison, for the same transitions the potential of mean force as obtained by using dihedral-angle constraints is provided.     

Propensity for the air/water interface and ion pairing in magnesium acetate vs magnesium nitrate solutions: molecular dynamics simulations and surface tension measurements

Molecular dynamics simulations in slab geometry and surface tension measurements were performed for aqueous solutions of magnesium acetate and magnesium nitrate at various concentrations. The simulations reveal a strong affinity of acetate anions for the surface, while nitrate exhibits only a very weak surface propensity, and magnesium is per se strongly repelled from the air/water interface. CH3COO- also exhibits a much stronger tendency than NO3- for ion pairing with Mg2+ in the bulk and particularly in the interfacial layer. The different interfacial behavior of the two anions is reflected by the opposite concentration dependence (beyond 0.5 M) of surface tension of the corresponding magnesium salts. Measurements, supported by simulations, show that the surface tension of Mg(NO3)2(aq) increases with concentration as for other inorganic salts. However, in the case of Mg(OAc)2(aq) the surface tension isotherm exhibits a turnover around 0.5 M, after which it starts to decrease, indicating a positive net solute excess in the interfacial layer at higher concentrations.