25,148 research outputs found
Features in the ion emission of Cu, Al, and C plasmas produced by ultrafast laser ablation
© 2015 AIP Publishing LLC. The bi-modal nature of charge integrated ion kinetic energy distributions, which result from ultrafast laser produced plasmas, is discussed in this paper. A negatively biased Faraday cup was used as a charge collector to measure ion distributions from three different solid targets that had been irradiated with an ultrafast laser in the fluence range 0.1-1 J/cm2. A bi-modal time of flight distribution is found for all three targets (C, Al, and Cu). In the case of the metallic targets (Al and Cu), high- and low-kinetic energy peaks exhibit quite different dependencies on laser fluence, whereas for the semi-metallic target (C), both peaks scale similarly with ultrafast laser fluence. The results are discussed within the framework of a one dimensional capacitor model resulting in ion acceleration
Collision of High Frequency Plane Gravitational and Electromagnetic Waves
We study the head-on collision of linearly polarized, high frequency plane
gravitational waves and their electromagnetic counterparts in the
Einstein-Maxwell theory. The post-collision space-times are obtained by solving
the vacuum Einstein-Maxwell field equations in the geometrical optics
approximation. The head-on collisions of all possible pairs of these systems of
waves is described and the results are then generalised to non-linearly
polarized waves which exhibit the maximum two degrees of freedom of
polarization.Comment: Latex file, 17 pages, accepted for publication in International
Journal of Modern Physics
High Resolution CO and H2 Molecular Line Imaging of a Cometary Globule in the Helix Nebula
We report high resolution imaging of a prominent cometary globule in the
Helix nebula in the CO J=1-0 (2.6 mm) and H2 v=1-0 S(1) (2.12 micron) lines.
The observations confirm that globules consist of dense condensations of
molecular gas embedded in the ionized nebula. The head of the globule is seen
as a peak in the CO emission with an extremely narrow line width (0.5 km/s) and
is outlined by a limb-brightened surface of H2 emission facing the central star
and lying within the photo-ionized halo. The emission from both molecular
species extends into the tail region. The presence of this extended molecular
emission provides new constraints on the structure of the tails, and on the
origin and evolution of the globules.Comment: 12 pages, 3 figures. To appear in The Astrophysical Journal Letter
Two-electron processes in multiple ionization under strong soft-x-ray radiation
In a combined experimental and theoretical study we have investigated the ionization of atomic argon upon irradiation with intense soft-x-ray pulses of 105 eV photon energy from the free-electron laser FLASH. The measured ion yields show charge states up to Ar7+. The comparison with the theoretical study of the underlying photoionization dynamics highlights the importance of excited states in general and of processes governed by electron correlation in particular, namely, ionization with excitation and shake-off, processes usually inaccessible by measurements of ionic yields only. The Ar7+ yield shows a clear deviation from the predictions of the commonly used model of sequential ionization via single-electron processes and the observed signal can only be explained by taking into account the full multiplet structure of the involved configurations and by inclusion of two-electron processes. The competing process of two-photon ionization from the ground state of Ar6+ is calculated to be orders of magnitude smaller
Stochastic theory of large-scale enzyme-reaction networks: Finite copy number corrections to rate equation models
Chemical reactions inside cells occur in compartment volumes in the range of
atto- to femtolitres. Physiological concentrations realized in such small
volumes imply low copy numbers of interacting molecules with the consequence of
considerable fluctuations in the concentrations. In contrast, rate equation
models are based on the implicit assumption of infinitely large numbers of
interacting molecules, or equivalently, that reactions occur in infinite
volumes at constant macroscopic concentrations. In this article we compute the
finite-volume corrections (or equivalently the finite copy number corrections)
to the solutions of the rate equations for chemical reaction networks composed
of arbitrarily large numbers of enzyme-catalyzed reactions which are confined
inside a small sub-cellular compartment. This is achieved by applying a
mesoscopic version of the quasi-steady state assumption to the exact
Fokker-Planck equation associated with the Poisson Representation of the
chemical master equation. The procedure yields impressively simple and compact
expressions for the finite-volume corrections. We prove that the predictions of
the rate equations will always underestimate the actual steady-state substrate
concentrations for an enzyme-reaction network confined in a small volume. In
particular we show that the finite-volume corrections increase with decreasing
sub-cellular volume, decreasing Michaelis-Menten constants and increasing
enzyme saturation. The magnitude of the corrections depends sensitively on the
topology of the network. The predictions of the theory are shown to be in
excellent agreement with stochastic simulations for two types of networks
typically associated with protein methylation and metabolism.Comment: 13 pages, 4 figures; published in The Journal of Chemical Physic
A Theoretical Analysis of Two-Stage Recommendation for Cold-Start Collaborative Filtering
In this paper, we present a theoretical framework for tackling the cold-start
collaborative filtering problem, where unknown targets (items or users) keep
coming to the system, and there is a limited number of resources (users or
items) that can be allocated and related to them. The solution requires a
trade-off between exploitation and exploration as with the limited
recommendation opportunities, we need to, on one hand, allocate the most
relevant resources right away, but, on the other hand, it is also necessary to
allocate resources that are useful for learning the target's properties in
order to recommend more relevant ones in the future. In this paper, we study a
simple two-stage recommendation combining a sequential and a batch solution
together. We first model the problem with the partially observable Markov
decision process (POMDP) and provide an exact solution. Then, through an
in-depth analysis over the POMDP value iteration solution, we identify that an
exact solution can be abstracted as selecting resources that are not only
highly relevant to the target according to the initial-stage information, but
also highly correlated, either positively or negatively, with other potential
resources for the next stage. With this finding, we propose an approximate
solution to ease the intractability of the exact solution. Our initial results
on synthetic data and the Movie Lens 100K dataset confirm the performance gains
of our theoretical development and analysis
Sensitivity of nonlinear photoionization to resonance substructure in collective excitation
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources
Career Transitions in Sport
The transition out of sport prompts an identity change for the athlete. When putting an end to their sporting career, an athlete loses an important part of themselves, particularly if they have been training and competing in their sport for much of their life. This may apply to the youth athlete who will not compete in college, the collegiate athlete who will not compete professionally, and the professional athlete who is retiring. Because athletes spend a great deal of time training and devoting themselves to their sport, they may not have time for exploration of activities or careers outside of sport. This leads to identity foreclosure and a strong athletic identity, which has been associated with difficulty adjusting following sport career termination (Murphy, Petitpas, & Brewer, 1996)
Career Transitions in Sport
The transition out of sport prompts an identity change for the athlete. When putting an end to their sporting career, an athlete loses an important part of themselves, particularly if they have been training and competing in their sport for much of their life. This may apply to the youth athlete who will not compete in college, the collegiate athlete who will not compete professionally, and the professional athlete who is retiring. Because athletes spend a great deal of time training and devoting themselves to their sport, they may not have time for exploration of activities or careers outside of sport. This leads to identity foreclosure and a strong athletic identity, which has been associated with difficulty adjusting following sport career termination (Murphy, Petitpas, & Brewer, 1996)
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