22 research outputs found
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
Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography
We use ultrafast electron crystallography to study structural changes induced
in graphite by a femtosecond laser pulse. At moderate fluences of ~< 21mJ/cm^2,
lattice vibrations are observed to thermalize on a time scale of ~8ps. At
higher fluences approaching the damage threshold, lattice vibration amplitudes
saturate. Following a marked initial contraction, graphite is driven
nonthermally into a transient state with sp^3-like character, forming
interlayer bonds. Using ab initio density functional calculations, we trace the
governing mechanism back to electronic structure changes following the
photo-excitation.Comment: 5 pages, 4 figures; to appear in Phys. Rev. Let
Photovoltage Dynamics of the Hydroxylated Si(111) Surface Investigated by Ultrafast Electron Diffraction
We present a novel method to measure transient photovoltage at nanointerfaces
using ultrafast electron diffraction. In particular, we report our results on
the photoinduced electronic excitations and their ensuing relaxations in a
hydroxyl-terminated silicon surface, a standard substrate for fabricating
molecular electronics interfaces. The transient surface voltage is determined
by observing Coulomb refraction changes induced by the modified space-charge
barrier within a selectively probed volume by femtosecond electron pulses. The
results are in agreement with ultrafast photoemission studies of surface state
charging, suggesting a charge relaxation mechanism closely coupled to the
carrier dynamics near the surface that can be described by a drift-diffusion
model. This study demonstrates a newly implemented ultrafast diffraction method
for investigating interfacial processes, with both charge and structure
resolution.Comment: 5 pages, 5 figure
Protein expression, characterization and activity comparisons of wild type and mutant DUSP5 proteins
Pattern of cardiac fibrosis in rabbits periodically fed a magnesium-restricted diet and administered rare earth chloride through drinking water
It has been postulated that causation of the tropical cardiomyopathy endomyocardial fibrosis (EMF) is linked to magnesium (Mg) deficiency and cardiac toxicity of the rare earth element cerium (Ce). The aim of the present study was to define the myocardial lesions in rabbits that were fed on Mg-restricted diet (70-80 ppm) periodically and were provided drinking water contaminated with rare earth chloride (1 g/L). Forty New Zealand white rabbits were divided into four groups following a 2 × 2 factorial design. Two groups were periodically fed on Mg-restricted diet with one of them receiving water contaminated with rare earth chloride. The other two groups were continuously fed on Mgsufficient diet (350-400 ppm) with one of them receiving water contaminated with rare earth chloride. All animals were sacrificed at the end of 6 mo. Cardiac tissues were subjected to histology, elemental analysis (calcium [Ca], Mg, and Ce) and estimation of collagen content and collagen phenotypes. Histological lesions were compared with those of EMF in humans and those of acute Mg deficiency in animals. The results suggest that in rabbits, recurrent episodes of Mg deficiency lead to myocardial fibrosis similar to the pattern observed in human EMF