199 research outputs found
Dynamical bunching and density peaks in expanding Coulomb clouds
Expansion dynamics of single-species, non-neutral clouds, such as electron
bunches used in ultrafast electron microscopy, show novel behavior due to high
acceleration of particles in the cloud interior. This often leads to electron
bunching and dynamical formation of a density shock in the outer regions of the
bunch. We develop analytic fluid models to capture these effects, and the
analytic predictions are validated by PIC and N-particle simulations. In the
space-charge dominated regime, two and three dimensional systems with Gaussian
initial densities show bunching and a strong shock response, while one
dimensional systems do not; moreover these effects can be tuned using the
initial particle density profile and velocity chirp.Comment: 16 pages, 6 figures(spread over 18 png files); No changes to the text
--- however I had mis-spelled Chong-Yu Ruan's first name in the metadata. (It
was originally Chung-Yu). This typo has been addresse
Emission Optics of the Steigerwald Type Electron Gun
The emission optics of a Steigerwald type electron gun is re-examined. The
virtual and real points of divergence, divergence angles and beam-widths of the
electron beams at different telefocusing strength are measured in detail for
first time . Two different Wehnelt cylinders are used to establish a
contrasting viewpoint. The original `focusing' curves measured by Braucks are
reconstructed and will be explained only through a `new' interpretation which
is different from the conventional views. While the image of the emitting
surface in front of the filament is indeed telefocused beyond the anode, the
envelope of the beam does not `focus' as expected. A new model for the emission
mechanism is established based on our results.Comment: 14 pages, 10 figure
Ultrafast imaging of photoelectron packets generated from graphite surface
We present an electron projection imaging method to study the ultrafast
evolution of photoelectron density distribution and transient fields near the
surface. The dynamical profile of the photoelectrons from graphite reveals an
origin of a thermionic emission, followed by an adiabatic process leading to
electron acceleration and cooling before a freely expanding cloud is
established. The hot electron emission is found to couple with a surface charge
dipole layer formation, with a sheet density several orders of magnitude higher
than that of the vacuum emitted cloud.Comment: 9 pages, 4 figures. Applied Physics Letter, in pres
Dynamics of Size-Selected Gold Nanoparticles Studied by Ultrafast Electron Nanocrystallography
We report the studies of ultrafast electron nanocrystallography on
size-selected Au nanoparticles (2-20 nm) supported on a molecular interface.
Reversible surface melting, melting, and recrystallization were investigated
with dynamical full-profile radial distribution functions determined with
sub-picosecond and picometer accuracies. In an ultrafast photoinduced melting,
the nanoparticles are driven to a non-equilibrium transformation, characterized
by the initial lattice deformations, nonequilibrium electron-phonon coupling,
and upon melting, the collective bonding and debonding, transforming
nanocrystals into shelled nanoliquids. The displasive structural excitation at
premelting and the coherent transformation with crystal/liquid coexistence
during photomelting differ from the reciprocal behavior of recrystallization,
where a hot lattice forms from liquid and then thermally contracts. The degree
of structural change and the thermodynamics of melting are found to depend on
the size of nanoparticle.Comment: 16 pages, 4 figure
The development and applications of ultrafast electron nanocrystallography
We review the development of ultrafast electron nanocrystallography as a
method for investigating structural dynamics for nanoscale materials and
interfaces. Its sensitivity and resolution are demonstrated in the studies of
surface melting of gold nanocrystals, nonequilibrium transformation of graphite
into reversible diamond-like intermediates, and molecular scale charge
dynamics, showing a versatility for not only determining the structures, but
also the charge and energy redistribution at interfaces. A quantitative scheme
for three-dimensional retrieval of atomic structures is demonstrated with
few-particle (< 1000) sensitivity, establishing this nanocrystallographic
method as a tool for directly visualizing dynamics within isolated
nanomaterials with atomic scale spatio-temporal resolution.Comment: 33 pages, 17 figures (Review article, 2008 conference of ultrafast
electron microscopy conference and ultrafast sciences
Precision-controlled ultrafast electron microscope platforms. A case study: Multiple-order coherent phonon dynamics in 1T-TaSe probed at 50 femtosecond - 10 femtometer scales
We report on the first detailed beam test attesting the fundamental principle
behind the development of high-current-efficiency ultrafast electron microscope
systems where a radio-frequency cavity is incorporated as a condenser lens in
the beam delivery system. To allow the experiment to be carried out with a
sufficient resolution to probe the performance at the emittance floor, a new
cascade loop RF controller system is developed to reduce the RF noise floor.
Temporal resolution at 50 femtoseconds in full-width-at-half-maximum and
detection sensitivity better than 1% are demonstrated on exfoliated 1T-TaSe
layers where the multi-order edge-mode coherent phonon excitation is employed
as the standard candle to benchmark the performance. The high temporal
resolution and the significant visibility to very low dynamical contrast in
diffraction signals give strong support to the working principle of the
high-brightness beam delivery via phase-space manipulation in the electron
microscope system
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