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

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

Trigger and DAQ issues in low-mass Higgs searches

Results from ongoing trigger and DAQ studies at the Superconducting Super Collider Laboratory are presented. Specific Higgs decay modes are identified, calorimetric triggering algorithms are examined, and implications for data acquisition are discussed