Muon Spin Rotation and Relaxation Experiments on Thin Films

The recent development at the Paul Scherrer Institute of a beam of low energy muons allows depth dependent muon spin rotation and relaxation investigations in thin samples, multilayers and near surface regions (low energy SR, LE-SR). After a brief overview of the LE-SR method, some representative experiments performed with this technique will be presented. The first direct determination of the field profile just below the surface of a high-temperature superconductor in the Meissner phase illustrates the power and sensitivity of low energy muons as near-surface probe and is an example of general application to depth profiling of magnetic fields. The evolution of the flux line lattice distribution across the surface of a YBa₂Cu₃O₇ film in the vortex phase has been investigated by implanting muons on both sides of a normal-superconducting boundary. A determination of the relaxation time and energy barrier to thermal activation in iron nanoclusters, embedded in a silver thin film matrix (500nm), demonstrates the use of slow muons to measure the properties of samples that cannot be made thick enough for the use of conventional SR. Other experiments investigated the magnetic properties of thin Cr(001) layers at thicknesses above and below the collapse of the spin density wave.     

Evolution of hexagonal patterns from controlled initial conditions in a Bénard-Marangoni convection experiment

We report quantitative measurements of both wave number selection and defect motion in nonequilibrium hexagonal patterns. A novel optical technique ("thermal laser writing") is used to imprint initial patterns with selected characteristics in a Bénard-Marangoni convection experiment. Initial patterns of ideal hexagons are imposed to determine the band of stable pattern wave numbers while initial patterns containing an isolated penta-hepta defect are imprinted to study defect propagation directions and velocities. The experimental results are compared to recent theoretical predictions.     

CO2 hydrogenation to formic acid on Ni(110)

Hydrogen (H) in the subsurface of transition-metal surfaces exhibits unique reactivity for heterogeneously catalyzed hydrogenation reactions. Here, we explore the potential of subsurface H for hydrogenating carbon dioxide (CO₂) on Ni(110). The energetics of surface and subsurface H reacting with surface CO₂ to form formate, carboxyl, and formic acid on Ni(110) is systematically studied using self-consistent, spin-polarized, periodic density functional theory (DFT-GGA-PW91) calculations. We show that on Ni(110), CO₂ can be hydrogenated to formate by surface H. However, further hydrogenation of formate to formic acid by surface H is hindered by a larger activation energy barrier. The relative energetics of hydrogenation barriers is reversed for the carboxyl-mediated route to formic acid. We suggest that the energetics of subsurface H emerging to the surface is suitable for providing the extra energy needed to overcome the barrier to formate hydrogenation. CO₂ hydrogenation to formic acid could take place on Ni(110) when subsurface H is available to react with CO₂. Additional electronic-structure based dynamic calculations would be needed to elucidate the detailed reaction paths for these transformations.     

Steered molecular dynamics studies of the potential of mean force of a Na+ or K+ ion in a cyclic peptide nanotube

Potential of mean force (PMF) profiles of a single Na+ or K+ ion passing through a cyclic peptide nanotube, cyclo[-(D-Ala-Glu-D-Ala-Gln)2-], in water are calculated to provide insight into ion transport and to understand the conductance difference between these two ions. The PMF profiles are obtained by performing steered molecular dynamics (SMD) simulations that are based on the Jarzynski equality. The computed PMF profiles for both ions show barriers of around 2.4 kcal/mol at the channel entrances and exits and energy wells in the middle of the tube. The energy barriers, so-called dielectric energy barriers, arise due to the desolvation of water molecules when ions move across the nanotube, and the energy wells appear as a result of attractive interactions between the cations and negatively charged carbonyl oxygens on the backbone of the tube. We find more and deeper energy wells in the PMF profile for Na+ than for K+, which suggests that Na+ ions have a longer residence time inside the nanotube and that permeation of Na+ ions is reduced compared to K+ ions. Calculations of the radial distribution functions (RDF) between the ions and oxygens in the water molecules and in carbonyl groups on the tube and an investigation of the orientations of the carbonyl groups show that, in contrast with the dynamic carbonyl groups observed in the selectivity filter of the KcsA ion channel, the carbonyl groups in the cyclic peptide nanotube are relatively rigid, with only slight reorientation of the carbonyl groups as the cations pass through. The rigidity of the carbonyl groups in the cyclic peptide nanotube can be attributed to their role in hydrogen bonding, which is responsible for the tube structure. Comparison of the PMF profiles with the electrostatic energy profiles calculated from the Poisson-Boltzmann (PB) equation, a dielectric continuum model, reveals that the dielectric continuum model breaks down in the confined region within the tube that governs ion transport.     

Multipolar excitation in triangular nanoprisms

Theoretical studies on the optical properties of gold triangular prisms in solution are presented to determine how structural modifications affect the extinction spectrum. Well-defined trends in the particle extinction are found to depend on the triangular edge length and the prism thickness. Calculations performed on large, thin triangular prisms indicate multipolar excitation and display numerous peaks in the extinction spectrum. The dominant peaks are assigned to different in-plane modes corresponding to the lowest three orders of a multipole expansion. Vector polarization plots are presented to support the peak assignments. Altering the prisms by snipping off the points of the triangular cross section significantly blueshifts the dipole peak, but the higher-order modes are only slightly affected. Snipping off large volumes can lead to the suppression of high-order multipoles in the extinction spectrum.     

A reinterpretation of the mechanism of the simplest reaction at an sp3-hybridized carbon atom: H + CD4 --> CD3 + HD

A comparison between theory and experiment for the benchmark H + CD4 --> HD + CD3 abstraction reaction yields a reinterpretation of the reaction mechanism and highlights the unexpected contribution of a stripping mechanism. Whereas the best analytic surface fails to reproduce experiment, a first-principles direct-dynamics (on the fly) treatment is in good agreement, showing that the H + CD4 reaction exhibits extreme sensitivity to modest differences in the potential energy surface. We find that bent H-D-C transition state geometries play an important role in the dynamics. A simple model that relates the scattering angle impact parameter and cone of acceptance accounts well for the overall reaction dynamics.     

A coupled states quantum reactive scattering study of H + D2 → HD + D ATErel(υ = j = 0) = 0.55 eV

In this paper we present the results of accurate quantum coupled states calculations for the H + D₂ → HD + D reaction at 0.55 eV translational energy (relative to υ = 0, j = 0) using the accurate LSTH potential surface.     

Assignment of π → 3d rydberg transitions in ethylene around 9 eV using MCD and synchrotron radiation

Magnetic circular dichroism measurements on ethylene are reported over the region of the 3R₀₀ and 4R₀₀^? origins around 9 eV. The results show conclusively that each origin is a composite of two electronic transitions. The 3R₀₀ origin is assigned to the Rydberg transitions, ^tA₁ → ¹B₂(3dδ) + ¹B₃(3dδ). 4R₀₀^? is assigned as ¹A₁ → 3R₀₀ + ν₃(u).