23 research outputs found
Analytical model of an isolated single-atom electron source
No full textAn 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
High-coherence Electron bunches produced by femtosecond photoionization
No full textBest Oral 2013 - awar
Polarization effects on the effective temperature of an ultracold electron source
No full textThe 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 highintensity
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 the 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
High-coherence electron bunches produced by femtosecond photoionization
No full textWith the development of ultrafast electron and X-ray sources it is becoming possible to study structural dynamics with atomic-level spatial and temporal resolution. Because of their short mean free path, electrons are particularly well suited for investigating surfaces and thin films, such as the challenging and important class of membrane proteins. To perform single-shot diffraction experiments on protein crystals, an ultracold electron source was proposed, based on near-threshold photoionization of laser-cooled atoms, which is capable of producing electron pulses of both high intensity and high coherence. Here we show that high coherence electron pulses can be produced by femtosecond photoionization, opening up a new regime of ultrafast structural dynamics experiments. The transverse coherence turns out to be much better than expected on the basis of the large bandwidth of the femtosecond ionization laser pulses. This surprising result can be explained by analysis of classical electron trajectorie
Effective temperature of an ultracold electron source based on near-threshold photoionization
No full textWe present a detailed description of measurements of the effective temperature of a pulsed electron source, based on near-threshold photoionization of laser-cooled atoms. The temperature is determined by electron beam waist scans, source size measurements with ion beams, and analysis with an accurate beam line model. Experimental data is presented for the source temperature as a function of the wavelength of the photoionization laser, for both nanosecond and femtosecond ionization pulses. For the nanosecond laser, temperatures as low as 14±3 K were found; for femtosecond photoionization, 30±5 K is possible. With a typical source size of 25 um , this results in electron bunches with a relative transverse coherence length in the 10-4 range and an emittance of a few nm rad
Ultrafast electron diffraction using an ultracold source
No full textThe study of structural dynamics of complex macromolecular crystals using electrons requires bunches of sufficient coherence and charge. We present diffraction patterns from graphite, obtained with bunches from an ultracold electron source, based on femtosecond near-threshold photoionization of a laser-cooled atomic gas. By varying the photoionization wavelength, we change the effective source temperature from 300¿K to 10¿K, resulting in a concomitant change in the width of the diffraction peaks, which is consistent with independently measured source parameters. This constitutes a direct measurement of the beam coherence of this ultracold source and confirms its suitability for protein crystal diffraction
