The mechanism for pair production in the α-Decay of241Am

The mechanism for pair production in the α-decay of²⁴¹Am is discussed. The pair creation is dominantly involved with the dynamics of the α-decay process and contributions from²³⁷Np levels with energies >2mc ² are relatively small. One of the authors (B.A.L.) would like to thank the Natural Sciences and Engineering Council of Canada for financial support.     

The kinetic equations for isotopic exchange reactions allowing for the presence of an isotope effect

It is shown that the occasionally applied isotopic exchange, kinetic treatment of reactions is fundamentally flawed. This implies, quite wrongly, that the concentration of the products of the exchange reaction should rise linearly with time. Reactions in which there may be an appreciable isotope effect on the rate are carefully examined, resulting in equations appropriate to stable isotopes and to radioisotopes.     

Application of a parallel computer system to polymer calculations

The LCAP (loosely coupled array of processors) parallel processing systems developed at IBM Kingston have been employed in the simulation of Brownian dynamics on ring polymers. Significant speedups over the serial version of the FORTRAN code have been obtained by using bulk shared memories to expedite information passing between the attached array processors. In this manner we have been able to obtain data for chains with 128 and 256 units. Data on these larger chains indicate that the radius of gyration follows the predicted power law.This paper was presented at the International Conference on The Impact of Supercomputers on Chemistry, held at the University of London, London, UK, 13–16 April 1987     

Local quantum critical point in the pseudogap anderson model: finite-T dynamics and omega/T scaling

The pseudogap Anderson impurity model is a paradigm for locally critical quantum phase transitions. Within the framework of the local moment approach we study its finite-T dynamics, as embodied in the single-particle spectrum, in the vicinity of the symmetric quantum critical point (QCP) separating generalized Fermi liquid (Kondo screened) and local moment phases. The scaling spectra in both phases, and at the QCP itself, are obtained analytically. A key result is that pure omega/T scaling obtains at the QCP, where the Kondo resonance has just collapsed. The connection between the scaling spectra in either phase and that at the QCP is explored in detail.     

Residence time, loading force, pH, and ionic strength affect adhesion forces between colloids and biopolymer-coated surfaces

Exopolymers are thought to influence bacterial adhesion to surfaces, but the time-dependent nature of molecular-scale interactions of biopolymers with a surface are poorly understood. In this study, the adhesion forces between two proteins and a polysaccharide [Bovine serum albumin (BSA), lysozyme, or dextran] and colloids (uncoated or BSA-coated carboxylated latex microspheres) were analyzed using colloid probe atomic force microscopy (AFM). Increasing the residence time of an uncoated or BSA-coated microsphere on a surface consistently increased the adhesion force measured during retraction of the colloid from the surface, demonstrating the important contribution of polymer rearrangement to increased adhesion force. Increasing the force applied on the colloid (loading force) also increased the adhesion force. For example, at a lower loading force of approximately 0.6 nN there was little adhesion (less than -0.47 nN) measured between a microsphere and the BSA surface for an exposure time up to 10 s. Increasing the loading force to 5.4 nN increased the adhesion force to -4.1 nN for an uncoated microsphere to a BSA surface and to as much as -7.5 nN for a BSA-coated microsphere to a BSA-coated glass surface for a residence time of 10 s. Adhesion forces between colloids and biopolymer surfaces decreased inversely with pH over a pH range of 4.5-10.6, suggesting that hydrogen bonding and a reduction of electrostatic repulsion were dominant mechanisms of adhesion in lower pH solutions. Larger adhesion forces were observed at low (1 mM) versus high ionic strength (100 mM), consistent with previous AFM findings. These results show the importance of polymers for colloid adhesion to surfaces by demonstrating that adhesion forces increase with applied force and detention time, and that changes in the adhesion forces reflect changes in solution chemistry.     

A complete classification of homogeneous plane continua

We show that the only compact and connected subsets (i.e. continua) X of the plane \({\mathbb{R}^2}\) which contain more than one point and are homogeneous, in the sense that the group of homeomorphisms of X acts transitively on X, are, up to homeomorphism, the circle \({\mathbb{S}^1}\), the pseudo-arc, and the circle of pseudo-arcs. These latter two spaces are fractal-like objects which do not contain any arcs. It follows that any compact and homogeneous space in the plane has the form X × Z, where X is either a point or one of the three homogeneous continua above, and Z is either a finite set or the Cantor set.     

Dithiomethylether as a ligand in the hydrogenase h-cluster

An X-ray crystallographic refinement of the H-cluster of [FeFe]-hydrogenase from Clostridium pasteurianum has been carried out to close-to atomic resolution and is the highest resolution [FeFe]-hydrogenase presented to date. The 1.39 A, anisotropically refined [FeFe]-hydrogenase structure provides a basis for examining the outstanding issue of the composition of the unique nonprotein dithiolate ligand of the H-cluster. In addition to influencing the electronic structure of the H-cluster, the composition of the ligand has mechanistic implications due to the potential of the bridge-head gamma-group participating in proton transfer during catalysis. In this work, sequential density functional theory optimizations of the dithiolate ligand embedded in a 3.5-3.9 A protein environment provide an unbiased approach to examining the most likely composition of the ligand. Structural, conformational, and energetic considerations indicate a preference for dithiomethylether as an H-cluster ligand and strongly disfavor the dithiomethylammonium as a catalytic base for hydrogen production.     

Exciton-phonon coupling in indirect AlxGa1−xAs

Several persistent high-energy peaks in the photo-luminescence spectrum of indirect Al_xGa_1?xAs are reported. These transitions are assigned to zero-phonon and phonon-assisted bound-exciton annihilation processes. Symmetry determined selection rules indicate that the major interband recombination mechanism is due to LO phonon scattering of electrons near X(100) to the energetically adjacent Γ^CB₁ (000) intermediate state. Scattering associated with alloy disorder may also contribute to the spectrum. The change in the phonon-assisted spectra at x = 0.43 ± 0.01 provides an additional means of identification of the direct-indirect conduction band crossover.