Surface chemistry of (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)Ru on polycrystalline Ta

The adsorption and desorption of (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)Ru [DER] on polycrystalline Ta have been studied by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). DER exposures to Ta at 140 K result in primarily molecular adsorption and desorption, while a minor surface reaction occurs at defect sites. Monolayer DER desorbs between 278 and 297 K with increasing coverage, exhibiting a first order, zero coverage desorption energy of 2.3 eV. Multilayer DER desorbs between 272 and 263 K, most likely with fractional order kinetics, and exhibits a zero coverage desorption energy of 0.9 eV. XPS Ru 3d binding energies increase with increasing coverage due to core hole screening in the monolayer regime and increasing sample charging as the DER overlayer becomes thicker in the multilayer regime. DER exhibits a three-dimensional (3D) “hit and stick” growth mode in which random 3D structures form due to the lack of adsorbate mobility at 140 K. DER exposures to Ta between 298 and 773 K result in minor decomposition resulting primarily in adsorbed hydrocarbon species on the surface. When the Ta is pre-covered with atomic iodine, DER dissociation is significantly decreased while adsorption is increased.     

Asymmetric Copper‐Catalyzed Vinylogous Mukaiyama Michael Addition of Cyclic Dienol Silanes to Unsaturated α‐Keto Phosphonates

A highly stereoselective vinylogous Mukaiyama Michael reaction (VMMR) leading to α‐keto phosphonate‐containing γ‐butenolides with two stereogenic centers is described. The presented transformation is catalyzed by a combination of a commercially available C₂‐symmetric bisoxazoline (BOX) ligand and a copper salt and tolerates a variety of nucleophiles and electrophiles. The stereoselectivities of the reactions are good to excellent and the products are obtained in moderate to high yields.     

Tracing the Photoaddition of Pharmaceutical Psoralens to DNA

The psoralens 8-methoxypsoralen (8-MOP), 4,5′,8-trimethylpsoralen (TMP) and 5-methoxypsoralen (5-MOP) find clinical application in PUVA (psoralen + UVA) therapy. PUVA treats skin diseases like psoriasis and atopic eczema. Psoralens target the DNA of cells. Upon photo-excitation psoralens bind to the DNA base thymine. This photo-binding was studied using steady-state UV/Vis and IR spectroscopy as well as nanosecond transient UV/Vis absorption. The experiments show that the photo-addition of 8-MOP and TMP involve the psoralen triplet state and a biradical intermediate. 5-MOP forms a structurally different photo-product. Its formation could not be traced by the present spectroscopic technique.     

Radiation defects induced by helium implantation in gold-based alloys investigated by positron annihilation spectroscopy

The formation of gas bubbles in metallic materials may result in drastic degradation of in-service properties. In order to investigate this effect in high density and medium-low melting temperature (T _M) alloys, positron annihilation spectroscopy measurements were performed on helium-implanted gold–silver solid solutions after isochronal annealing treatments. Three recovery stages are observed, attributed to the migration and elimination of defects not stabilized by helium atoms, helium bubble nucleation and bubble growth. Similarities with other metals are found for the recovery stages involving bubble nucleation and growth processes. Lifetime measurements indicate that He implantation leads to the formation of small and over-pressurized bubbles that generate internal stresses in the material. A comprehensive picture is drawn for possible mechanisms of helium bubble evolution. Two values of activation energy (0.26 and 0.53 eV) are determined below and above 0.7T _M, respectively, from the variation of the helium bubble radius during the bubble growth stage. The migration and coalescence mechanism, which accounts for these very low activation energies, controls the helium bubble growth.     

Parabolic finite element equations in nonconvex polygonal domains

Let Ω be a bounded nonconvex polygonal domain in the plane. Consider the initial boundary value problem for the heat equation with homogeneous Dirichlet boundary conditions and semidiscrete and fully discrete approximations of its solution by piecewise linear finite elements in space. The purpose of this paper is to show that known results for the stationary, elliptic, case may be carried over to the time dependent parabolic case. A special feature in a polygonal domain is the presence of singularities in the solutions generated by the corners even when the forcing term is smooth. These cause a reduction of the convergence rate in the finite element method unless refinements are employed.     

Far-out surface science: radiation-induced surface processes in the solar system

Interplanetary space is a cosmic laboratory for surface scientists. Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids. Many of these bodies exist in ultrahigh vacuum environments, so that direct particle–surface collisions dominate the interactions. In this article, we discuss the origins of the very tenuous planetary atmospheres observed on a number of bodies, space weathering of the surface of asteroids and comets, and magnetospheric processing of the surfaces of Jupiter's icy satellites. We emphasize non-thermal processes and the important relationships between surface composition and the gas phase species observed. We also discuss what laboratory and computational modeling should be done to support the current and future space missions––e.g. the Genesis mission to recover solar wind particles, the Cassini mission to probe Saturn, the Europa Lander mission to explore the subsurface ocean hypothesis, and the Pluto/Kuiper Express to sample the outer reaches of the solar system.