Convective roll dynamics in liquid 4He near the onset of convection

We present results of experiments on Rayleigh-Bénard convection in liquid 4He at several temperatures. We show visually that with carefully defined boundary conditions the basic convection state consists of parallel rolls which are aligned in one of two directions, the angle thus defined as being temperature dependent and we attempt to explain this behavior. We also show directly the skew-varicose instability acting on the basic state and correlate it with fluctuations in the temperature difference across the fluid layer.     

Bundles of identical double-walled carbon nanotubes

In a sample produced by catalytic chemical vapor deposition (CCVD), the structure of the carbon nanotubes (diameter and helicity) which governs their electronic properties, is determined by electron diffraction. We found that most of the smallest bundles are constituted of identical double-walled carbon nanotubes.     

Optimisation of the vibrational response of ultrasonic cutting systems

^** Email: m.cartmell{at}mech.gla.ac.uk This paper provides an account of an investigation into possibledynamic interactions between two coupled non-linear sub-systems,each possessing opposing non-linear overhang characteristicsin the frequency domain in terms of positive and negative cubicstiffnesses. This system is a two-degree-of-freedom Duffingoscillator in which certain non-linear effects can be advantageouslyneutralised under specific conditions. This theoretical vehiclehas been used as a preliminary methodology for understandingthe interactive behaviour within typical industrial ultrasoniccutting components. Ultrasonic energy is generated within apiezoelectric exciter, which is inherently non-linear, and whichis coupled to a bar- or block-horn, and to one or more materialcutting blades, for example. The horn/blade configurations arealso non-linear, and within the whole system there are responsefeatures which are strongly reminiscent of positive and negativecubic stiffness effects. The two-degree-of-freedom model isanalysed and it is shown that a practically useful mitigatingeffect on the overall non-linear response of the system canbe created under certain conditions when one of the cubic stiffnessesis varied. It has also been shown experimentally that couplingof ultrasonic components with different non-linear characteristicscan strongly influence the performance of the system and thatthe general behaviour of the hypothetical theoretical modelis indeed borne out in practice. Further experiments have shownthat a multiple horn/blade configuration can, under certaincircumstances, display autoparametric responses based on theforced response of the desired longitudinal mode parametricallyexciting an undesired lateral mode. Typical autoparametric responsephenomena have been observed and are presented at the end ofthe paper.     

Isomers in neutron-rich A ≈ 190 nuclides from 208Pb fragmentation

Relativistic projectile fragmentation of ²⁰⁸Pb has been used to produce isomers in neutron-rich, A 190 nuclides. A forward-focusing spectrometer provided ion-by-ion mass and charge identification. The detection of -rays emitted by stopped ions has led to the assignment of isomers in ¹⁸⁸Ta, ¹⁹⁰W, ¹⁹²Re, ¹⁹³Re, ¹⁹⁵Os, ¹⁹⁷Ir, ¹⁹⁸Ir, ²⁰⁰Pt, ²⁰¹Pt, ²⁰²Pt and ²⁰³Au, with half-lives ranging from approximately 10 ns to 1 ms. Tentative isomer information has been found also for ¹⁷⁴Er, ¹⁷⁵Er, ¹⁸⁵Hf, ¹⁹¹Re, ¹⁹⁴Re and ¹⁹⁹Ir. In most cases, time-correlated, singles -ray events provided the first spectroscopic data on excited states for each nuclide. In ²⁰⁰Pt and ²⁰¹Pt, the assignments are supported by - coincidences. Isomeric ratios provide additional information, such as half-life and transition energy constraints in particular cases. The level structures of the platinum isotopes are discussed, and comparisons are made with isomer systematics.     

High-spin structures of 51 121, 123, 125, 127Sb nuclei: Single proton and core-coupled states

The ^{121,123,125,127}Sb nuclei have been produced as fission fragments in three reactions induced by heavy ions: ¹²C + ²³⁸U at 90 MeV bombarding energy, ¹⁸O + ²⁰⁸Pb at 85 MeV, and ³¹P + ¹⁷⁶Yb at 152 MeV. Their level schemes have been built from gamma rays detected using the EUROBALL III and IV arrays. High-spin states of ^123,125,127Sb nuclei have been identified for the first time. Moreover isomeric states lying around 2.3 MeV have been established in ^123,125,127Sb from the delayed coincidences between the fission fragment detector SAPhIR and the gamma array. All the observed states can be described by coupling a d_5/2 or g_7/2 proton to an excited Sn core involving either vibrational states or broken neutron pairs.     

Electron attachment to strongly polar clusters

Electron localization is studied in formamide cluster anions. The isolated formamide molecule has a large dipole moment and its clusters can give birth to multipole-bound anions as well as valence anions. The vertical valence electron affinity of the isolated molecule is determined by electron transmission spectroscopy. The anion formation process is studied as a function of cluster size with Rydberg electron transfer spectroscopy. DFT calculations of the neutral and negatively-charged cluster structures show that the anion excess electron localizes on a single molecule. The adiabatic valence electron affinity of isolated formamide is deduced from the observation of the cluster size threshold for valence attachment.     

Modular nanometer-scale structuring of gel fibres by sequential self-organization

Ag(I) and Cu(II) complexes of a series of simple bis(urea) ligands form soft metallogels. X-ray crystallographic results suggests that the gels' structure is based on hydrogen bonding to counter anions and thus suggests a route to tunable gel rheological properties.     

Influence of the dielectric medium on the carbonyl infrared absorption peak of acetylferrocene

The solvent effect on the position of the carbonyl vibrational stretching of acetylferrocene in aprotic media was studied in this work. The solvent-induced shifts in this organometallic compound were interpreted in terms of the alternative reaction field model(SCRF-MO) proposed by Kolling. In contrast to the established trends for carbonyl groups in organic systems, the results suggest that the continuum models for the reaction field are not adequate and that the influence of dipolarity-polarizability described by an inhomogeneous coupling function theta(epsilon )L(n(2)) that assumes optical dielectric saturation is responsible for the carbonyl band shift and, there is empirical evidence that the effect of field-induced intermolecular interaction on band shift, interpreted in terms of the van der Waals forces from the solvent, have a important contribution to this phenomena.     

Nanocrystalline metal oxides as unique chemical reagents/sorbents

A new family of porous inorganic solids based on nanocrystalline metal oxides is discussed. These materials, made up of 4-7 nm MgO, CaO, Al2O3, ZnO, and others, exhibit unparalleled destructive adsorption properties for acid gases, polar organics, and even chemical/biological warfare agents. These unique sorption properties are due to nanocrystal shape, polar surfaces, and high surface areas. Free-flowing powders or consolidated pellets are effective, and pore structure can be controlled by consolidation pressures. Chemical properties can be adjusted by choice of metal oxide as well as by incorporating other oxides as monolayer films.