833 research outputs found
Coulomb potential from a particle in uniform ultrarelativistic motion
The Coulomb potential produced by an ultrarelativistic particle (such as a
heavy ion) in uniform motion is shown in the appropriate gauge to factorize
into a longitudinal Dirac delta function of (z - t) times the simple two
dimensional potential solution in the transverse direction. This form makes
manifest the source of the energy independence of the interaction.Comment: 5 pages, latex, revtex source, no figure
Cognitive representations of disability behaviours in people with mobility limitations : consistency with theoretical constructs
Disability is conceptualised as behaviour by psychological theory and as a result of bodily impairment by medical models. However, how people with disabilities conceptualise those disabilities is unclear. The purpose of this study was to examine disability representations in people with mobility disabilities. Thirteen people with mobility disabilities completed personal repertory grids (using the method of triads) applied to activities used to measure disabilities. Ten judges with expertise in health psychology then examined the correspondence between the elicited disability constructs and psychological and medical models of disability. Participants with mobility disabilities generated 73 personal constructs ofdisability. These constructs were judged consistent with the content of two psychological models, namely the theory of planned behaviour and social cognitive theory and with the main medical model of disability, the International Classification of Functioning Disability and Health.Individuals with activity limitations conceptualise activities in a manner that is compatible with both psychological and medical models. This ensures adequate communication in contexts where the medical model is relevant, e.g. clinical contexts, as well as in everyday conversation about activities and behaviours. Finally, integrated models of disability may be of value for theory driven interdisciplinary approaches to disability and rehabilitation
Synthesis and biological activity of α-glucosyl C24:0 and C20:2 ceramides
a-Glucosyl ceramides 4 and 5 have been synthesised and evaluated for their ability to stimulate the activation
and expansion of human iNKT cells. The key challenge in the synthesis of both target molecules was the stereoselective synthesis of the a-glycosidic linkage. Of the methods examined, glycosylation using per-TMS-protected glucosyl iodide 16 was completely a-selective and provided gram quantities of amine 11, from which a-glucosyl ceramides 4 and 5 were obtained by N-acylation. a-GlcCer 4, containing a C24 saturated acyl chain, stimulated a marked proliferation and expansion of human circulating iNKT cells in short-term cultures. a-GlcCer 5, which contains a C20 11,14-cis-diene acyl chain (C20:2),induced extremely similar levels of iNKT cell activation and expansion
Attosecond VUV Coherent Control of Molecular Dynamics
High harmonic light sources make it possible to access attosecond
time-scales, thus opening up the prospect of manipulating electronic wave
packets for steering molecular dynamics. However, two decades after the birth
of attosecond physics, the concept of attosecond chemistry has not yet been
realized. This is because excitation and manipulation of molecular orbitals
requires precisely controlled attosecond waveforms in the deep ultraviolet,
which have not yet been synthesized. Here, we present a novel approach using
attosecond vacuum ultraviolet pulse-trains to coherently excite and control the
outcome of a simple chemical reaction in a deuterium molecule in a non-Born
Oppenheimer regime. By controlling the interfering pathways of electron wave
packets in the excited neutral and singly-ionized molecule, we unambiguously
show that we can switch the excited electronic state on attosecond timescales,
coherently guide the nuclear wave packets to dictate the way a neutral molecule
vibrates, and steer and manipulate the ionization and dissociation channels.
Furthermore, through advanced theory, we succeed in rigorously modeling
multi-scale electron and nuclear quantum control in a molecule for the first
time. The observed richness and complexity of the dynamics, even in this very
simplest of molecules, is both remarkable and daunting, and presents intriguing
new possibilities for bridging the gap between attosecond physics and
attochemistry
Asymptotic channels and gauge transformations of the time-dependent Dirac equation for extremely relativistic heavy-ion collisions
We discuss the two-center, time-dependent Dirac equation describing the
dynamics of an electron during a peripheral, relativistic heavy-ion collision
at extreme energies. We derive a factored form, which is exact in the
high-energy limit, for the asymptotic channel solutions of the Dirac equation,
and elucidate their close connection with gauge transformations which transform
the dynamics into a representation in which the interaction between the
electron and a distant ion is of short range. We describe the implications of
this relationship for solving the time-dependent Dirac equation for extremely
relativistic collisions.Comment: 12 pages, RevTeX, 2 figures, submitted to PR
A light-fronts approach to electron-positron pair production in ultrarelativistic heavy-ion collisions
We perform a gauge-transformation on the time-dependent Dirac equation
describing the evolution of an electron in a heavy-ion collision to remove the
explicit dependence on the long-range part of the interaction. We solve, in an
ultra-relativistic limit, the gauged-transformed Dirac equation using
light-front variables and a light-fronts representation, obtaining
non-perturbative results for the free pair-creation amplitudes in the collider
frame. Our result reproduces the result of second-order perturbation theory in
the small charge limit while non-perturbative effects arise for realistic
charges of the ions.Comment: 39 pages, Revtex, 7 figures, submitted to PR
Ionization of hydrogen and hydrogenic ions by antiprotons
Presented here is a description of the ionization of hydrogen and hydrogenic
ions by antiproton-impact, based on very large scale numerical solutions of the
time-dependent Schr\"odinger equation in three spatial dimensions and on
analysis of the topology of the electronic eigenenergy surfaces in the plane of
complex internuclear distance. Comparison is made with other theories and very
recent measurements.Comment: RevTex document, 11 pages, 4 Postscript figures are available from
the authors, in press Phys. Rev. Let
A Self-Consistent Model for Positronium Formation from Helium Atoms
The differential and total cross sections for electron capture by positrons
from helium atoms are calculated using a first-order distorted wave theory
satisfying the Coulomb boundary conditions. In this formalism a parametric
potential is used to describe the electron screening in a consistent and
realistic manner. The present procedure is self consistent because (i) it
satisfies the correct boundary conditions and post-prior symmetry, and (ii) the
potential and the electron binding energies appearing in the transition
amplitude are consistent with the wave functions describing the collision
system. The results are compared with the other theories and with the available
experimental measurements. At the considered range of collision energies, the
results agree reasonably well with recent experiments and theories.
[Note: This paper will be published on volume 42 of the Brazilian Journal of
Physics
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