High-temporal-resolution electron microscopy for imaging ultrafast electron dynamics

Ultrafast Electron Microscopy (UEM) has been demonstrated to be an effective table-top technique for imaging the temporally-evolving dynamics of matter with subparticle spatial resolution on the time scale of atomic motion. However, imaging the faster motion of electron dynamics in real time has remained beyond reach. Here, we demonstrate more than an order of magnitude (16 times) enhancement in the typical temporal resolution of UEM by generating isolated 30 fs electron pulses, accelerated at 200 keV, via the optical-gating approach, with sufficient intensity for efficiently probing the electronic dynamics of matter. Moreover, we investigate the feasibility of attosecond optical gating to generate isolated subfemtosecond electron pulses, attaining the desired temporal resolution in electron microscopy for establishing the Attomicroscopy to allow the imaging of electron motion in the act.Comment: 19 Pages, 4 Figure

Proton transport in polarizable water

Proton mobility in water determines the conductive properties of water-based proton conductors. We address the problem of proton mobility in pure water using a new, simple, Newtonian molecular dynamics water model which is applicable to proton-rich environments (e.g., polymer electrolyte membranes). This model has degrees of freedom that are "inertial" and "inertialess" relative to the proton. The solvated proton is treated using a local empirical valence bond Hamiltonian, which allows for the efficient simulation of full charge, energy-conserving dynamics in single and multiple-proton systems. The solvated proton displays the Grotthus-type proton transfer mechanism, giving significantly enhanced transport in comparison with the classical diffusion of an H3O+ ion. The model yields an activation energy of 0.11 eV, in excellent agreement with experiment. The results are consistent with the observation that nonpolarizable water models, conditioned to reproduce correct values of the static dielectric constant, are predestined to give too large activation energies of proton mobility due to the overweighted spectrum of the slower nuclear modes. (C) 2001 American Institute of Physics

Using Temporal Session Types to Analyze Time Complexities of Concurrent Programs

Das et al. develop a method for analyzing the time complexity of concurrent, message-passing algorithms. Their method is based on adding timing information to datatypes. Specifically, they use a family of datatypes called session types; these constrain the structure of interactions that may take place over a channel of communication. In Das’s system, the timing properties of an algorithm can be verified by a typechecker: if the timing information in the session types is mismatched, the computer will report a type error. In their paper, Das et al. develop the theory for such a typechecker, but do not provide an implementatio

Mechanisms of proton conductance in polymer electrolyte membranes

We provide a phenomenological description of proton conductance in polymer electrolyte membranes, based on contemporary views of proton transfer processes in condensed media and a model for heterogeneous polymer electrolyte membrane structure. The description combines the proton transfer events in a single pore with the total pore-network performance and, thereby, relates structural and kinetic characteristics of the membrane. The theory addresses specific experimentally studied issues such as the effect of the density of proton localization sites (equivalent weight) of the membrane material and the water content of the pores. The effect of the average distance between the sulfonate groups, which changes during membrane swelling, is analyzed in particular, and the factors which determine the temperature dependence of the macroscopic membrane conductance are disclosed. Numerical estimates of the specific membrane conductivity obtained from the theory agree very well with typical experimental data, thereby confirming the appropriateness of the theoretical concepts. Moreover, the versatility of the models offers a useful and transparent frame for combining the analysis of both experimental data and the results of molecular dynamics simulations

Grayling and how to catch them, and recollections of a sportsman.

Mode of access: Internet

Customized 3D printed ankle-foot orthosis with adaptable carbon fibre composite spring joint

Neuromuscular disorders and injuries such as cerebral palsy and stroke often result in foot-drop which can result in a person having great difficulty walking. Ankle foot orthoses (AFOs) or splints have been prescribed for many years now to limit the range of motion of the ankle, provide the patients with support and assist with rehabilitation. However the majority of AFOs require a long, labour-intensive manufacturing process which results in unacceptable waiting times for children that are rapidly growing and patients with varying conditions. This research proposes a new approach to AFO manufacturing that utilizes digital and additive manufacturing technologies to customise the fit and form to an individual. By implementing an interchangeable carbon fibre spring at the ankle joint the design will result in a stronger, more comfortable, more flexible AFO that can adaptively constrain ankle movement for various different activities. Three iterations of AFO design have been developed and tested to validate their efficacy. A custom machine has been designed and constructed in order to empirically test stiffness values for the AFO and allow for optimal AFO geometry based on input parameters. This machine has proven the structural integrity of the final AFO design. Progress has been made in automating parts of the design process which will significantly reduce labour requirements and hence manufacturing delay times