Thermal diffusivity measurements of natural and isotopically enriched diamond by picosecond infrared transient grating experiments

The photopyroelectric (PPE) technique is used to measure the specific heat of YBa₂Cu₃O_7–X thin films. Hysteresis in the PPE amplitude is observed in the temperature range 90 K to 250 K. Peaks in the inverse PPE amplitude at T=110 K in the cooling path and at T230 K in the heating path are observed. The magnitude of hysteresis depends upon the thermal history of the sample.     

A model for the plane to plane variations in catalytic activity seen during nitric oxide decomposition on platinum

Previous work has shown that the reactivity of platinum for nitric oxide decomposition varies significantly with the geometry of the exposed surface of the platinum. Here, the available data are compared to the predictions of a model, which is based on conservation of orbital symmetry and a simplified picture of the band structure of the surface. The model predicts that the (100) face of platinum should be more active than either the (111) or the (110). The (410) should be more active still. These predictions agree with recent experiment. The model makes several other predictions which need to be tested experimentally. These results suggest that symmetry conservation methods, similar to the ones developed by Woodward and Hoffmann for organic reactions, could yield many valuable insights into the relative activity of various catalyst structures.     

An Electrochemical Approach to and the Physical Consequences of Preparing Nanostructures from Gold Nanorods with Smooth Ends

We have developed a method to smooth the end sections of nanowires and nanograps generated via the On-Wire Lithography process and studied these rods with optical spectroscopies and theoretical modeling (Discrete Dipole Approximation). The first step of the smoothing process is a reductive one aimed at controlling the diffusion and migration of metal ions to the growing nanorod surface by adjusting the applied potential and concentration of the metal ions in the growth solution. A second oxidative smoothing step, based in part on the energetic differences between topologically rough and smooth surfaces, is used to further smooth the nanorod. The RMS roughness can be reduced over five fold to approximately 5 nm. The properties of these smoothed rods were investigated by empirical and theoretical methods, where it was found the smoothed rods have sharper plasmon resonances and decreased SERS intensity.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Directed desorption as a probe of the structure of the desorption site

This paper attempts to assess the effects of surface structure on the angular variation of the desorption flux. Quantum calculations are done which show that if the gas-surface interaction potential is sufficiently corrugated in the neighborhood of the desorption site, and the adsorbate molecules desorb without going through a mobile precursor state, then they desorb preferentially in the direction of the surface normal. An activation barrier for adsorption need not be invoked. The predictions of the model agree with recent experiments. More theoretical and experimental verification of the model is needed, but it appears that it should be possible to use a directed desorption experiment to probe the forces on a molecule when it is about to desorb from a surface.     

A reflection-absorption infrared study of carbon monoxide and nitric oxide adsorption on platinum (100)

The adsorption of NO, CO, and NO/CO mixtures, onto Pt(100), is studied by RAIRS. CO and NO are found to adsorb into islands at 300 K, but the islands breakup upon heating to 400 K. Dosing with a mixture of NO and CO at temperatures below 325 K is found to produce a mixed NO/CO island. There is a shift in the CO peak and NO peak during mixed island formation which is attributed to a strong chemical interaction between the adsorbed NO and CO. This interaction is found to produce an increase in the desorption temperature of CO. Autocatalytic behavior is found to arise because of an enhanced reactivity when CO enters a mobile state. The autocatalytic behavior could be responsible for the “surface explosion” reported by Lesly and Schmidt.