Photoelectron studies of neutral Ag3 in helium droplets

Photoelectron spectra of neutral silver trimers, grown in ultracold helium nanodroplets, are recorded after ionization with laser pulses via a strong optical resonance of this species. Varying the photon energy reveals that direct vertical two-photon ionization is hindered by a rapid relaxation into the lower edge of a long-living excited state manifold. An analysis of the ionization threshold of the embedded trimer yields an ionization potential of 5.74+/-0.09 eV consistent with the value found in the gas phase. The asymmetrical form of the electron energy spectrum, which is broadened toward lower kinetic energies, is attributed to the influence of the matrix on the photoionization process. The lifetime of the excited state was measured in a two-color pump-probe experiment to be 5.7+/-0.6 ns.     

Correlations in the sequence of residence times

Sequences of residence times (RTs) associated with the escape from metastable states are observed in many fields. Here we study analytically and numerically the correlations among RTs for a bistable stochastic system driven by dichotomous noise. Our theory predicts an oscillatory behavior of the correlations with respect to the lag between RTs. Correlations vanish at all lags if the switching rate matches the hopping rate of the unperturbed system. It is also shown that RT correlations may reveal features of the driving which are not present in the single-RT statistics.     

Hydrogen in N-methylacetamide: positions and dynamics of the hydrogen atoms using neutron scattering

This work reports neutron diffraction and incoherent neutron scattering experiments on N-methylacetamide (NMA), which can be considered the model building block for the peptide linkage of polypeptides and proteins. Using the neutron data, we have been able to associate the onset of a striking negative thermal expansion (NTE) along the a-axis with a dynamical transition around 230 K, consistent with our calorimetric experiments. Observation of the NTE raises the question of possible proton transfer in NMA, which, from our data alone, still cannot be settled. We can only speculate that intermolecular repulsive forces increase as the O...H distance decreases upon cooling, and that around 230 K the lattice relaxes without observation of an actual proton transfer. However, the existence of a nonharmonic potential, reflected by the behavior of the phonon vibrations together with the observation of NTE, could be justified by the "vibrational" polaron theory in which a dynamic localization of the vibrational energy is created by coupling an internal molecular mode to a lattice phonon. More generally, this work shows that neutron powder diffraction techniques can be very powerful for investigating structural deformations in small peptide systems.     

Single‐crystal sapphire microstructure for high‐resolution synchrotron X‐ray monochromators

We report on the growth and characterization of sapphire single crystals for X‐ray optics applications. Structural defects were studied by means of laboratory double‐crystal X‐ray diffractometry and white‐beam synchrotron‐radiation topography. The investigations confirmed that the main defect types are dislocations. The best quality crystal was grown using the Kyropoulos technique. Therein the dislocation density was 10²–10³ cm⁻² and a small area with approximately 2*2 mm² did not show dislocation contrast in many reflections. This crystal has suitable quality for application as a backscattering monochromator. A clear correlation between growth rate and dislocation density is observed, though growth rate is not the only parameter impacting the quality.     

A study of gas phase and surface formaldehyde polymerisation from first principles

First principles computational studies have been carried out on the gas phase and surface polymerisation of formaldehyde. Any gas phase polymerisation in dark conditions is shown to occur by way of neutral species, as opposed to earlier suggestions that a charged species may be involved. The resulting paraformaldehyde chains are shown to be stable only when fully bonded at both chain ends, as opposed to some of the earlier suggested mechanisms. Polymerisation is shown to be thermodynamically favourable in the gas phase, but kinetically limited. Derived rate constants are insufficient to account for experimental results, in agreement with prior studies suggesting that gas phase polymerisation actually occurs on surfaces rather than directly in the gas phase. The TiO(2) (110) surface is employed as a model surface to study formaldehyde adsorption and possible polymerisation mechanisms. Formaldehyde monomers are shown to weakly bind by way of coordinate bonding through the carbonyl group to surface Ti. A particularly strong dimerisation configuration is also found where the two ends of paraformaldehyde are terminated by a surface Ti and surface O, consistent with the earlier observation that paraformaldehyde chains are only stable when saturated at both ends.     

Optical properties of polybutadiene in the bulk and near a gold interface

We have investigated thick films from polybutadiene on gold-coated glass using surface plasmon resonance / leaky optical waveguide spectroscopy with the aim of investigating differences in bulk and interphase properties in a single measurement. A broad range of molar masses was studied. Drying under ambient conditions leads to an exponential decay of the film thickness. Subsequent vacuum drying does not result in any further changes in the bulk part of the film but at the polymer-solid interface, indicating the absence of residual solvent. For all molar masses studied, the surface plasmon resonance is observed at angles which are incompatible with the properties of the bulk part of the film. A polymer interphase is thus present next to the gold layer which has a refractive index lower than in the bulk. Using transversal magnetic- and transversal electric polarized light, an optical anisotropy is found in the interphase which is attributed to segment alignment along the interface with gold.     

Synthesis of Boc protected block copolymers based on para-hydroxystyrene via NMRP

Nitroxide mediated free radical polymerization (NMRP) was used for the preparation of orthogonally protected block copolymers based on para-hydroxystyrene. The polymers have a low polydispersity and an active chain end. By a series of polymer analogous reactions, a partly deprotected block copolymer was synthesized consisting of a block with unprotected phenolic OH groups and a further block which is protected by the thermolabile Boc group.     

Quantitative analysis of immobilized proteins and protein mixtures by amino acid analysis

Biomolecular surface engineering of materials often requires precise, versatile and efficient quantification of immobilized proteins at solid surfaces. Acidic hydrolysis of surface-bound proteins and subsequent HPLC analysis of fluorescence-derivatized amino acids were adapted and critically evaluated for that purpose. Contaminations and concentration-dependent amino acid retrieval during HPLC were found to influence the accuracy of the method. In addition to the choice of adequate conditions for hydrolysis, derivatization and chromatographic separation extensions of the data evaluation were suggested to improve the accuracy of the approach when applied to single protein systems: comparing the experimentally obtained amino acid ratio to the protein constitution enabled to identify the properly separated and detected amino acids. Those amino acids were selected for a more precise calculation of the amount of immobilized protein. To further increase the accuracy of the method, the retrieval of amino acids corresponding to protein amounts in the range between 0.5 and 4.0 microg was analyzed for a variety of proteins of interest to derive protein-specific correction factors. The evaluation of amino acid data was furthermore applied to quantify binary protein mixtures at similar settings. This method was proven useful to detect the composition of protein mixtures throughout a wide range of absolute and relative concentrations.     

The quadrupole moment of the 3/2+ nuclear ground state of 197Au from electric field gradient relativistic coupled cluster and density-functional theory of small molecules and the solid state

An attempt is made to improve the currently accepted muonic value for the 197Au nuclear quadrupole moment [+0.547(16)x10(-28) m2] for the 3/2+ nuclear ground state obtained by Powers et al. [Nucl. Phys. A230, 413 (1974)]. From both measured Mossbauer electric quadrupole splittings and solid-state density-functional calculations for a large number of gold compounds a nuclear quadrupole moment of +0.60x10(-28) m2 is obtained. Recent Fourier transform microwave measurements for gas-phase AuF, AuCl, AuBr, and AuI give accurate bond distances and nuclear quadrupole coupling constants for the 197Au isotope. However, four-component relativistic density-functional calculations for these molecules yield unreliable results for the 197Au nuclear quadrupole moment. Relativistic singles-doubles coupled cluster calculations including perturbative triples [CCSD(T) level of theory] for these diatomic systems are also inaccurate because of large cancellation effects between different field gradient contributions subsequently leading to very small field gradients. Here one needs very large basis sets and has to go beyond the standard CCSD(T) procedure to obtain any reliable field gradients for gold. From recent microwave experiments by Gerry and co-workers [Inorg. Chem. 40, 6123 (2001)] a significantly enhanced (197)Au nuclear quadrupole coupling constant in (CO)AuF compared to free AuF is observed. Here, these cancellation effects are less important, and relativistic CCSD(T) calculations finally give a nuclear quadrupole moment of +0.64x10(-28) m2 for 197Au. It is argued that it is currently very difficult to improve on the already published muonic value for the 197Au nuclear quadrupole moment.