9,263 research outputs found
Differential cross-section for positronium formation in electron-atomic hydrogen collisions
The L=0 and 1 partial wave amplitudes obtained by a two-state coupled static approximation with correlation with the L greater than or equal to 2 Born amplitudes were combined to obtain the differential cross section for positronium formation in electron-atomic hydrogen collisions. For positron energies of 0.64 and 0.75 ryd, minima at the scattering angles of 57 deg and 51 deg are found. Total cross sections for positronium formation for low and intermediate impact energies are given. Measurement of the differential cross section for the process positron + helium yields positronium + helium ion for the detection of possible minima is suggested
Angular distribution in two-photon double ionization of helium by intense attosecond soft X-ray pulses
We investigate two-photon double ionization of helium by intense () ultrashort ( as) soft X-ray pulses (E = 91.6 eV). The
time-dependent two-electron Schr\"odinger equation is solved using a coupled
channel method. We show that for ultrashort pulses the angular distribution of
ejected electrons depends on the pulse duration and provides novel insights
into the role of electron correlations in the two-electron photoemission
process. The angular distribution at energies near the ``independent electron''
peaks is close to dipolar while it acquires in the ``valley'' of correlated
emission a significant quadrupolar component within a few hundred attoseconds.Comment: 17 pages, 6 fig
Time Ordering in Kicked Qubits
We examine time ordering effects in strongly, suddenly perturbed two-state
quantum systems (kicked qubits) by comparing results with time ordering to
results without time ordering. Simple analytic expressions are given for state
occupation amplitudes and probabilities for singly and multiply kicked qubits.
We investigate the limit of no time ordering, which can differ in different
representations.Comment: 26 pages, 5 figure
Assessment of the Fluorescence and Auger Data Base used in Plasma Modeling
We have investigated the accuracy of the 1s-vacancy fluorescence data base of
Kaastra & Mewe (1993, A&AS, 97, 443) resulting from the initial atomic physics
calculations and the subsequent scaling along isoelectronic sequences. In
particular, we have focused on the relatively simple Be-like and F-like
1s-vacancy sequences. We find that the earlier atomic physics calculations for
the oscillator strengths and autoionization rates of singly-charged B II and Ne
II are in sufficient agreement with our present calculations. However, the
substantial charge dependence of these quantities along each isoelectronic
sequence, the incorrect configuration averaging used for B II, and the neglect
of spin-orbit effects (which become important at high-Z) all cast doubt on the
reliability of the Kaastra & Mewe data for application to plasma modeling.Comment: 19 pages with 6 figures, AAS TeX, accepted for publication in Ap
Some characteristics of the solar flare event of February 16, 1984
In the morning of February 16, 1984 a solar cosmic ray event (GLE) was recorded by the world wide network of neutron monitors (NM). The counting rate vs. time profile of the Goose Bay NM (geog. lat. = 53.3 deg. N, deog. long. = 299.6 deg E) where the increase is expressed as percent of the counting rate of an equatorial sea level NM is presented. The Goose Bay NM was observed to have the maximum response to the solar particles. Its counting rate vs. time profile exhibits a rapid increase to maximum, has a large amplitude (approx. 170%) and decays rapidly to background in approx. 90 min. In Fig. 1 we also show the counting rate vs. time profile for the Tixie Bay NM (71.6 deg, 128.9 deg) which recorded an increase of only a few percent. Since the NMs at Goose Bay and Tixie Bay have asymptotic viewing directions approx. 180 deg apart in longitude, the anisotropy of the solar particle flux at Earth from these stations
Multi-Modal Human-Machine Communication for Instructing Robot Grasping Tasks
A major challenge for the realization of intelligent robots is to supply them
with cognitive abilities in order to allow ordinary users to program them
easily and intuitively. One way of such programming is teaching work tasks by
interactive demonstration. To make this effective and convenient for the user,
the machine must be capable to establish a common focus of attention and be
able to use and integrate spoken instructions, visual perceptions, and
non-verbal clues like gestural commands. We report progress in building a
hybrid architecture that combines statistical methods, neural networks, and
finite state machines into an integrated system for instructing grasping tasks
by man-machine interaction. The system combines the GRAVIS-robot for visual
attention and gestural instruction with an intelligent interface for speech
recognition and linguistic interpretation, and an modality fusion module to
allow multi-modal task-oriented man-machine communication with respect to
dextrous robot manipulation of objects.Comment: 7 pages, 8 figure
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