66 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
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
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
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
An intense, slow and cold beam of metastable Ne(3s) ^3P_2 atoms
We employ laser cooling to intensify and cool an atomic beam of metastable
Ne(3s) atoms. Using several collimators, a slower and a compressor we achieve a
^{20}Ne^* flux of 6 10^{10} atoms/s in an 0.7 mm diameter beam traveling at 100
m/s, and having longitudinal and transverse temperatures of 25mK and 300microK,
respectively. This constitutes the highest flux in a concentrated beam achieved
to date with metastable rare gas atoms. We characterize the action of the
various cooling stages in terms of their influence on the flux, diameter and
divergence of the atomic beam. The brightness and brilliance achieved are 2.1
10^{21} s^{-1} m^{-2} sr^{-1} and 5.0 10^{22} s^{-1} m^{-2} sr^{-1},
respectively, comparable to the highest values reported for alkali-metal beams.
Bright beams of the ^{21}Ne and ^{22}Ne isotopes have also been created.Comment: 18 pages, 9 figures, RevTe
Measurement of the temperature of an ultracold ion source using time-dependent electric fields
We report on a measurement of the characteristic temperature of an ultracold
rubidium ion source, in which a cloud of laser-cooled atoms is converted to
ions by photo-ionization. Extracted ion pulses are focused on a detector with a
pulsed-field technique. The resulting experimental spot sizes are compared to
particle-tracking simulations, from which a source temperature
mK and the corresponding transversal reduced emittance m rad are determined. We find that this result is
likely limited by space charge forces even though the average number of ions
per bunch is 0.022.Comment: 8 pages, 11 figure
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