1,167 research outputs found
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
Unified Band Theoretic Description of Electronic and Magnetic Properties of Vanadium Dioxide Phases
The debate about whether the insulating phases of vanadium dioxide (VO2) can
be described by band theory or must be described by a theory of strong electron
correlations remains unresolved even after decades of research. Energy-band
calculations using hybrid exchange functionals or including self-energy
corrections account for the insulating or metallic nature of different phases,
but have not yet successfully accounted for the observed magnetic orderings.
Strongly-correlated theories have had limited quantitative success. Here we
report that, by using hard pseudopotentials and an optimized hybrid exchange
functional, the energy gaps and magnetic orderings of both monoclinic VO2
phases and the metallic nature of the high-temperature rutile phase are
consistent with available experimental data, obviating an explicit role for
strong correlations. We also report a potential candidate for the newly-found
metallic monoclinic phase and present a detailed magnetic structure of the M2
monoclinic phase
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
Ultrafast Plasmonic Control of Second Harmonic Generation
Efficient frequency conversion techniques are crucial to the development of
plasmonic metasurfaces for information processing and signal modulation. In
principle, nanoscale electric-field confinement in nonlinear materials enables
higher harmonic conversion efficiencies per unit volume than those attainable
in bulk materials. Here we demonstrate efficient second-harmonic generation
(SHG) in a serrated nanogap plasmonic geometry that generates steep electric
field gradients on a dielectric metasurface. An ultrafast pump is used to
control plasmon-induced electric fields in a thin-film material with inversion
symmetry that, without plasmonic enhancement, does not exhibit an an even-order
nonlinear optical response. The temporal evolution of the plasmonic near-field
is characterized with ~100as resolution using a novel nonlinear interferometric
technique. The ability to manipulate nonlinear signals in a metamaterial
geometry as demonstrated here is indispensable both to understanding the
ultrafast nonlinear response of nanoscale materials, and to producing active,
optically reconfigurable plasmonic device
Ultrafast Insulator-Metal Phase Transition in VO2 Studied by Multiterahertz Spectroscopy
The ultrafast photoinduced insulator-metal transition in VO2 is studied at
different temperatures and excitation fluences using multi-THz probe pulses.
The spectrally resolved mid-infrared response allows us to trace separately the
dynamics of lattice and electronic degrees of freedom with a time resolution of
40 fs. The critical fluence of the optical pump pulse which drives the system
into a long-lived metallic state is found to increase with decreasing
temperature. Under all measurement conditions we observe a modulation of the
eigenfrequencies of the optical phonon modes induced by their anharmonic
coupling to the coherent wave packet motion of V-V dimers at 6.1 THz.
Furthermore, we find a weak quadratic coupling of the electronic response to
the coherent dimer oscillation resulting in a modulation of the electronic
conductivity at twice the frequency of the wave packet motion. The findings are
discussed in the framework of a qualitative model based on an approximation of
local photoexcitation of the vanadium dimers from the insulating state.Comment: 10 pages, 8 figures submitted to Physical Review
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