3,589 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
Modifying molecule-surface scattering by ultrashort laser pulses
In recent years it became possible to align molecules in free space using
ultrashort laser pulses. Here we explore two schemes for controlling
molecule-surface scattering process, which are based on the laser-induced
molecular alignment. In the first scheme, a single ultrashort non-resonant
laser pulse is applied to a molecular beam hitting the surface. This pulse
modifies the angular distribution of the incident molecules, and causes the
scattered molecules to rotate with a preferred sense of rotation (clockwise or
counter-clockwise). In the second scheme, two properly delayed laser pulses are
applied to a molecular beam composed of two chemically close molecular species
(isotopes, or nuclear spin isomers). As the result of the double pulse
excitation, these species are selectively scattered to different angles after
the collision with the surface. These effects may provide new means for the
analysis and separation of molecular mixtures
Modifying molecular scattering from rough solid surfaces using ultrashort laser pulses
We consider solid surface scattering of molecules that were subject to strong
non-resonant ultrashort laser pulses just before hitting the surface. The
pulses modify the rotational states of the molecules, causing their field free
alignment, or a rotation with a preferred sense. We show that field-free
laser-induced molecular alignment leads to correlations between the scattering
angle and the sense of rotation of the scattered molecules. Moreover, by
controlling the sense of laser induced unidirectional molecular rotation, one
may affect the scattering angle of the molecules. This provides a new means for
separation of mixtures of molecules (such as isotopes and nuclear-spin isomers)
by laser controlled surface scattering
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
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
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