435 research outputs found
Analytical Model of an Isolated Single-atom Electron Source
An analytical model of a single-atom electron source is presented, where
electrons are created by near-threshold photoionization of an isolated atom.
The model considers the classical dynamics of the electron just after the
photon absorption, i.e. its motion in the potential of a singly charged ion and
a uniform electric field used for acceleration. From closed expressions for the
asymptotic transverse electron velocities and trajectories, the effective
source temperature and the effective source size can be calculated. The
influence of the acceleration field strength and the ionization laser energy on
these properties has been studied. With this model, a single-atom electron
source with the optimum electron beam properties can be designed. Furthermore,
we show that the model is also applicable to ionization of rubidium atoms, thus
also describes the ultracold electron source, which is based on photoionization
of laser-cooled alkali atoms
Wireless network control of interacting Rydberg atoms
We identify a relation between the dynamics of ultracold Rydberg gases in
which atoms experience a strong dipole blockade and spontaneous emission, and a
stochastic process that models certain wireless random-access networks. We then
transfer insights and techniques initially developed for these wireless
networks to the realm of Rydberg gases, and explain how the Rydberg gas can be
driven into crystal formations using our understanding of wireless networks.
Finally, we propose a method to determine Rabi frequencies (laser intensities)
such that particles in the Rydberg gas are excited with specified target
excitation probabilities, providing control over mixed-state populations.Comment: 6 pages, 7 figures; includes corrections and improvements from the
peer-review proces
Polarization effects on the effective temperature of an ultracold electron source
The influence has been studied of the ionization laser polarization on the
effective temperature of an ultracold electron source, which is based on
near-threshold photoionization. This source is capable of producing both
high-intensity and high-coherence electron pulses, with applications in for
example electron diffraction experiments. For both nanosecond and femtosecond
photoionization, a sinusoidal dependence of the temperature on polarization
angle has been found. For most experimental conditions, the temperature is
minimal when the polarization coincides with the direction of acceleration.
However, surprisingly, for nanosecond ionization a regime exists when the
temperature is minimal when the polarization is perpendicular to the
acceleration direction. This shows that in order to create electron bunches
with the highest transverse coherence length, it is important to control the
polarization of the ionization laser. The general trends and magnitudes of the
temperature measurements are described by a model, based on the analysis of
classical electron trajectories; this model further deepens our understanding
of the internal mechanisms during the photoionization process. Furthermore, for
nanosecond ionization, charge oscillations as a function of laser polarization
have been observed; for most situations the oscillation amplitude is small
Energy spread of ultracold electron bunches extracted from a laser cooled gas
Ultrashort and ultracold electron bunches created by near-threshold
femtosecond photoionization of a laser-cooled gas hold great promise for
single-shot ultrafast diffraction experiments. In previous publications the
transverse beam quality and the bunch length have been determined. Here the
longitudinal energy spread of the generated bunches is measured for the first
time, using a specially developed Wien filter. The Wien filter has been
calibrated by determining the average deflection of the electron bunch as a
function of magnetic field. The measured relative energy spread
agrees well with the theoretical model
which states that it is governed by the width of the ionization laser and the
acceleration length
Ultrafast electron diffraction using an ultracold source
We present diffraction patterns from micron-sized areas of mono-crystalline
graphite obtained with an ultracold and ultrafast electron source. We show that
high spatial coherence is manifest in the visibility of the patterns even for
picosecond bunches of appreciable charge, enabled by the extremely low source
temperature (~ 10 K). For a larger, ~ 100 um spot size on the sample, spatial
coherence lengths > 10 nm result, sufficient to resolve diffraction patterns of
complex protein crystals. This makes the source ideal for ultrafast electron
diffraction of complex macromolecular structures such as membrane proteins, in
a regime unattainable by conventional photocathode sources. By further reducing
the source size, sub-um spot sizes on the sample become possible with spatial
coherence lengths exceeding 1 nm, enabling ultrafast nano-diffraction for
material science.Comment: 5 pages, 4 figure
Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams
A two-step photoionization strategy of an ultracold rubidium beam for
application in a focused ion beam instrument is analyzed and implemented. In
this strategy the atomic beam is partly selected with an aperture after which
the transmitted atoms are ionized in the overlap of a tightly cylindrically
focused excitation laser beam and an ionization laser beam whose power is
enhanced in a build-up cavity. The advantage of this strategy, as compared to
without the use of a build-up cavity, is that higher ionization degrees can be
reached at higher currents. Optical Bloch equations including the
photoionization process are used to calculate what ionization degree and
ionization position distribution can be reached. Furthermore, the ionization
strategy is tested on an ultracold beam of Rb atoms. The beam current is
measured as a function of the excitation and ionization laser beam intensity
and the selection aperture size. Although details are different, the global
trends of the measurements agree well with the calculation. With a selection
aperture diameter of 52 m, a current of pA is
measured, which according to calculations is 63% of the current equivalent of
the transmitted atomic flux. Taking into account the ionization degree the ion
beam peak reduced brightness is estimated at A/(msreV).Comment: 13 pages, 9 figure
Design and experimental validation of a compact collimated Knudsen source
In this paper we discuss the design and performance of a collimated Knudsen
source which has the benefit of a simple design over recirculating sources.
Measurements of the flux, transverse velocity distribution and brightness at
different temperatures were conducted to evaluate the performance. The scaling
of the flux and brightness with the source temperature follow the theoretical
predictions. The transverse velocity distribution in the transparent operation
regime also agrees with the simulated data. The source was found able to
produce a flux of s at a temperature of 433 K. Furthermore the
transverse reduced brightness of an ion beam with equal properties as the
atomic beam reads A/(m sr eV) which is sufficient for
our goal: the creation of an ultra-cold ion beam by ionization of a
laser-cooled and compressed atomic rubidium beam
Anharmonic mixing in a magnetic trap
We have experimentally observed re-equilibration of a magnetically trapped
cloud of metastable neon atoms after it was put in a non-equilibrium state.
Using numerical simulations we show that anharmonic mixing, equilibration due
to the collisionless dynamics of atoms in a magnetic trap, is the dominant
process in this equilibration. We determine the dependence of its time on trap
parameters and atom temperature. Furthermore we observe in the simulations a
resonant energy exchange between the radial and axial trap dimensions at a
ratio of trap frequencies \omega_r / \omega_z = 3/2. This resonance is
explained by a simple oscillator model.Comment: 9 pages, 6 figure
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