4,191 research outputs found
Theory of Direct Scattering, Trapping and Desorption in Atom-Surface Collisions
When gas atoms or molecules collide with clean and ordered surfaces, under
many circumstances the energy-resolved scattering spectra exhibit two clearly
distinct features due to direct scattering and to trapping in the physisorption
well with subsequent desorption. James Clerk Maxwell is credited with being the
first to describe this situation by invoking the simple assumption that when an
impinging gas beam is scattered from a surface it can be divided into a part
that exchanges no energy and specularly reflects and another part that
equilibrates or accommodates completely and then desorbs with an equilibrium
distribution. In this paper a scattering theory is developed, using an
iterative algorithm and classical mechanics for the collision process, that
describes both direct scattering and trapping-desorption of the incident beam.
The initially trapped fraction of particles can be followed as they continue to
make further interactions with the surface until they are all eventually
promoted back into the positive energy continuum and leave the surface region.
Consequently, this theory allows a rigorous test of the Maxwell assumption and
determines the conditions under which it is valid. The theory also gives
quantitative explanations of recent experimental measurements which exhibit
both a direct scattering contribution and a trapping-desorption fraction in the
energy-resolved spectra.Comment: 46 pages including 14 figure
Measurement of the horizontal velocity of wind perturbations in the middle atmosphere by spaced MF radar systems
Two remote receiving sites have been set up at a distance of approx 40 km from the main MF radar system. This allows measurement of upper atmosphere winds from 60-120 km (3 km resolution) at the corners of an approximately equilateral triangle of side approx 20 km. Some preliminary data are compared through cross correlation and cross spectral analysis in an attempt to determine the horizontal velocity of wind perturbations and/or the horizontal wavelength and phase velocity of gravity waves
Winds and Waves (4 Min - 11 Yrs) in the Upper Middle Atmosphere (60-110 Km) at Saskatoon, Canada (52 Deg N, 107 Deg W): MF Radar (2.2 Mhz) Soundings 1973 - 1983
Examples of gravity waves (GW), tides, planetary waves (PW), and circulation effects in the upper middle atmosphere are presented. Energy densities of GW, tides, and PW are compared. Fourier and spectral analyses are applied to the data
The nitrogen-vacancy center in diamond re-visited
Symmetry considerations are used in presenting a model of the electronic
structure and the associated dynamics of the nitrogen-vacancy center in
diamond. The model accounts for the occurrence of optically induced spin
polarization, for the change of emission level with spin polarization and for
new measurements of transient emission. The rate constants given are in
variance to those reported previously.Comment: 12 pages 10 figure
Separation of the strain components for use in strainrange partitioning
Two methods are presented for separating the inelastic strain components of a complex hysteresis loop so that strainrange partitioning formulas can be applied to accurately determine cyclic life at elevated temperatures. These methods are required only if lower bounds established by strainrange partitioning concepts have been deemed inadequate in the establishment of expected lifetime. In one method, rapid loading and unloading is applied in the tensile and compressive half to isolate the plastic strain. In the second method, the creep is measured at a discrete number of points along the hysteresis loop by combining load-control tests into the general pattern of strain cycling under arbitrary temperature. Both methods are shown to give good results
Isotope effect in quasi-two-dimensional metal-organic antiferromagnets
Although the isotope effect in superconducting materials is well-documented,
changes in the magnetic properties of antiferromagnets due to isotopic
substitution are seldom discussed and remain poorly understood. This is perhaps
surprising given the possible link between the quasi-two-dimensional (Q2D)
antiferromagnetic and superconducting phases of the layered cuprates. Here we
report the experimental observation of shifts in the N\'{e}el temperature and
critical magnetic fields (; ) in a Q2D organic molecular antiferromagnets on
substitution of hydrogen for deuterium. These compounds are characterized by
strong hydrogen bonds through which the dominant superexchange is mediated. We
evaluate how the in-plane and inter-plane exchange energies evolve as the
hydrogens on different ligands are substituted, and suggest a possible
mechanism for this effect in terms of the relative exchange efficiency of
hydrogen and deuterium bonds
Photon Echoes Produced by Switching Electric Fields
We demonstrate photon echoes in Eu:YSiO by controlling the
inhomogeneous broadening of the Eu
FD optical transition. This transition has
a linear Stark shift and we induce inhomogeneous broadening by applying an
external electric field gradient. After optical excitation, reversing the
polarity of the field rephases the ensemble, resulting in a photon echo. This
is the first demonstration of such a photon echo and its application as a
quantum memory is discussed.Comment: improved introduction, including theoretical outline of the relvant
quantum memory proposa
Hydrodynamic modelling of accretion flows
In the proceedings of this, and of several recent close binary conferences,
there have been several contributions describing smoothed particle
hydrodynamics simulations of accretion disks. It is apposite therefore to
review the numerical scheme itself with emphasis on its advantages for disk
modelling, and the methods used for modelling viscous processes.Comment: 3 pages, to appear in proceedings of IAU Colloquium 194: Compact
binaries in the galaxy and beyon
Assignment of the NV0 575 nm zero-phonon line in diamond to a 2E-2A2 transition
The time-averaged emission spectrum of single nitrogen-vacancy defects in
diamond gives zero-phonon lines of both the negative charge state at 637 nm
(1.945 eV) and the neutral charge state at 575 nm (2.156 eV). This occurs
through photo-conversion between the two charge states. Due to strain in the
diamond the zero-phonon lines are split and it is found that the splitting and
polarization of the two zero-phonon lines are the same. From this observation
and consideration of the electronic structure of the nitrogen-vacancy center it
is concluded that the excited state of the neutral center has A2 orbital
symmetry. The assignment of the 575 nm transition to a 2E - 2A2 transition has
not been established previously.Comment: 5 pages, 5 figure
Empirical wind model for the middle and lower atmosphere. Part 1: Local time average
The HWM90 thermospheric wind model was revised in the lower thermosphere and extended into the mesosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, and longitude (stationary wave 1). The model represents a smoothed compromise between the data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Root mean square differences between data and model are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal wind, and 10 m/s and 4 m/s, respectively, for meridional wind
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