1,922 research outputs found
Failure of the adiabatic criterion, structure and coherence in the low energy excitation of helium atoms by helium ions
Producing optical excitations by low energy ion atom and ion molecule collision
The production of low-energy neutral oxygen beams by grazing-incidence neutralization
The Vanderbilt University neutral oxygen facility produces beams of low-energy neutral oxygen atoms by means of grazing-incidence collisions between ion beams and metal surfaces. Residual ions are reflected by applied electric fields. This method can utilize initial ion beams of either O(+) or O2(+) since a very large percentage of molecular oxygen ions are dissociated when they undergo grazing-incidence neutralization. The method of neutralization is applicable to low-energy beams and to all ions. Particular emphasis is on O and N2 beams for simulation of the low Earth orbit space environment. Since the beam is a pure O-neutral beam and since measurements of the interaction of the beam with solid surfaces are made spectroscopically, absolute reaction rates can be determined. The technique permits the beams to be used in conjunction with electron and photon irradiation for studies of synergistic effects. Comparisons of optical spectra of Kapton excited by 2.5-keV O, O(+), and O2(+) show significant differences. Optical spectra of Kapton excited by neutral oxygen beams of less than 1 keV have been recorded
Optical radiation from the interaction of energetic atoms, ions, electrons, and photons with surfaces
Heavy particle, electron, and UV photon bombardment of solid surfaces has been recently observed to result in the emission of infrared, visible, and ultraviolet radiation. This effect occurs over a wide range of incident projectile energies. Line radiation arising from transitions between discrete atomic or molecular levels may be attributed to the decay of excited particles which have been sputtered or electronically/chemically desorbed from the surface. Broadband continuum radiation, which is also observed, is believed to arise either from fluorescence of the near surface bulk or from the radiative decay of desorbed excited clusters. Spacecraft, in the ambient near Earth environment, are subject to such bombardment. The dynamics of energetic particle and photon beam interactions with surfaces which lead to surface erosion and glow phenomena will be treated. In addition, projected experimental and theoretical studies of oxygen and nitrogen beam surface interactions on materials characteristic of spacecraft surfaces will be discussed
Embedding Simulation Education into the Engineering Management Body of Knowledge
The American Society for Engineering Management (ASEM) established a Body of Knowledge (BoK). As simulation is of growing interest to engineers in general and to engineering managers in particular, simulation is part of this documentation of domains of interest that characterise the profession. The basis for the Body of Knowledge comprises of established and accredited curricula and additional input from practitioners of the field. As it is essential to cover the basic topics and core competences as well as application specific domain knowledge, the simulation education for engineers is categorised into topics on simulation theory and simulation application
Ultrafast Spin Dynamics in GaAs/GaSb/InAs Heterostructures Probed by Second Harmonic Generation
We report the first application of pump-probe second harmonic generation
(SHG) measurements to characterize optically-induced magnetization in
non-magnetic multilayer semiconductors. In the experiment, coherent spins are
selectively excited by a pump beam in the GaAs layer of GaAs/GaSb/InAs
structures. However, the resulting net magnetization manifests itself through
the induced SHG probe signal from the GaSb/InAs interface, thus indicating a
coherent spin transport across the heterostructure. We find that the
magnetization dynamics is governed by an interplay between the spin density
evolution at the interfaces and the spin dephasing.Comment: 4 pages + 4 Fig
Ultrafast Dynamics of Interfacial Electric Fields in Semiconductor Heterostructures Monitored by Pump-Probe Second Harmonic Generation
We report first measurements of the ultrafast dynamics of interfacial
electric fields in semiconductor multilayers using pump-probe second harmonic
generation (SHG). A pump beam was tuned to excite carriers in all layers of
GaAs/GaSb and GaAs/GaSb/InAs heterostructures. Further carrier dynamics
manifests itself via electric fields created by by charge separation at
interfaces. The evolution of interfacial fields is monitored by a probe beam
through the eletric-field-induced SHG signal. We distinguish between several
stages of dynamics originating from redistribution of carriers between the
layers. We also find a strong enhancement of the induced electric field caused
by hybridization of the conduction and valence bands at the GaSb/InAs
interface.Comment: 4 pages + 2 fig
Near-bandgap wavelength-dependent studies of long-lived traveling coherent longitudinal acoustic phonon oscillations in GaSb/GaAs systems
We report first studies of long-lived oscillations in optical pump-probe
measurements on GaSb-GaAs heterostructures. The oscillations arise from a
photogenerated coherent longitudinal acoustic phonon wave, which travels from
the top surface of GaSb across the interface into the GaAs substrate, providing
information on the optical properties of the material as a function of
time/depth. Wavelength-dependent studies of the oscillations near the bandgap
of GaAs indicate strong correlations to the optical properties of GaAs.Comment: 11 pages, 4 figure
Reconfiguration and dissociation of bonded hydrogen in silicon by energetic ions
We report in situ infrared measurements of ion-induced reconfiguration and dissociation of bonded hydrogen associated with various defects in silicon at low temperatures. Defect-associated Si-H complexes were prepared by low-temperature proton implantation in silicon followed by room-temperature annealing. As a result of subsequent low-temperature (3)He ion irradiation, we observed (1) ion-induced dissociation of Si-H complexes, (2) a notable difference in the dissociation rate of interstitial- and vacancy-type defects, and, unexpectedly, (3) the growth of bond-centered hydrogen, which is generally observed in association with low-temperature proton implantation. These findings provide insight into the mechanisms responsible for the dissociation of hydrogen bonds in silicon and thus have important implications for bond-selective nanoscale engineering and the long-term reliability of state-of-the-art silicon semiconductor and photovoltaic devices
Hybrid Models as Transdisciplinary Research Enablers
This is the final version. Available on open access from Elsevier via the DOI in this recordModelling and simulation (M&S) techniques are frequently used in Operations Research (OR) to aid decision-making. With growing complexity of systems to be modelled, an increasing number of studies now apply multiple M&S techniques or hybrid simulation (HS) to represent the underlying system of interest. A parallel but related theme of research is extending the HS approach to include the development of hybrid models (HM). HM extends the M&S discipline by combining theories, methods and tools from across disciplines and applying multidisciplinary, interdisciplinary and transdisciplinary solutions to practice. In the broader OR literature, there are numerous examples of cross-disciplinary approaches in model development. However, within M&S, there is limited evidence of the application of conjoined methods for building HM. Where a stream of such research does exist, the integration of approaches is mostly at a technical level. In this paper, we argue that HM requires cross-disciplinary research engagement and a conceptual framework. The framework will enable the synthesis of discipline-specific methods and techniques, further cross-disciplinary research within the M&S community, and will serve as a transcending framework for the transdisciplinary alignment of M&S research with domain knowledge, hypotheses and theories from diverse disciplines. The framework will support the development of new composable HM methods, tools and applications. Although our framework is built around M&S literature, it is generally applicable to other disciplines, especially those with a computational element. The objective is to motivate a transdisciplinarity-enabling framework that supports the collaboration of research efforts from multiple disciplines, allowing them to grow into transdisciplinary research
Coherent Magnetization Precession in GaMnAs induced by Ultrafast Optical Excitation
We use femtosecond optical pulses to induce, control and monitor
magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures
below ~40 K we observe coherent oscillations of the local Mn spins, triggered
by an ultrafast photoinduced reorientation of the in-plane easy axis. The
amplitude saturation of the oscillations above a certain pump intensity
indicates that the easy axis remains unchanged above ~TC/2. We find that the
observed magnetization precession damping (Gilbert damping) is strongly
dependent on pump laser intensity, but largely independent on ambient
temperature. We provide a physical interpretation of the observed light-induced
collective Mn-spin relaxation and precession.Comment: 7 pages,3 figure
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