Ultrabright Electron Research Articles

Femtosecond Crystallography with Ultrabright Electrons and X-rays: Capturing Chemistry in Action

With the recent advances in ultrabright electron and x-ray sources, it is now possible to extend crystallography to the femtosecond time domain to literally light up atomic motions involved in the primary processes governing structural transitions. This review chronicles the development of brighter and brighter electron and x-ray sources that have enabled atomic resolution to structural dynamics for increasingly complex systems. The primary focus is on achieving sufficient brightness using pump-probe protocols to resolve the far-from-equilibrium motions directing chemical processes that in general lead to irreversible changes in samples. Given the central importance of structural transitions to conceptualizing chemistry, this emerging field has the potential to significantly improve our understanding of chemistry and its connection to driving biological processes.

Mapping Atomic Motions with Ultrabright Electrons: The Chemists' Gedanken Experiment Enters the Lab Frame

This review documents the development of high-bunch charge electron pulses with sufficient combined spatiotemporal resolution and intensity to literally light up atomic motions. This development holds promise in coming to a first-principles understanding of diverse problems, ranging from molecular reaction dynamics and structure-function correlations in biology to cooperativity in strongly correlated electron-lattice systems. It is now possible to directly observe the key modes involved in propagating structural changes and the enormous reduction in dimensionality that occurs in barrier crossing regions, which is central to chemistry and makes reaction mechanisms transferrable concepts. This information will help direct theoretical advances that will undoubtedly lead to generalized principles with respect to scaling relations in structural dynamics that will bridge chemistry to biology. In this quest, the limitations and future directions for further development are discussed to give an overview of the present status of the field.

Ring-Closing Reaction in Diarylethene Captured by Femtosecond Electron Crystallography

The photoinduced ring-closing reaction in diarylethene, which serves as a model system for understanding reactive crossings through conical intersections, was directly observed with atomic resolution using femtosecond electron diffraction. Complementary ab initio calculations were also performed. Immediately following photoexcitation, subpicosecond structural changes associated with the formation of an open-ring excited-state intermediate were resolved. The key motion is the rotation of the thiophene rings, which significantly decreases the distance between the reactive carbon atoms prior to ring closing. Subsequently, on the few picosecond time scale, localized torsional motions of the carbon atoms lead to the formation of the closed-ring photoproduct. These direct observations of the molecular motions driving an organic chemical reaction were only made possible through the development of an ultrabright electron source to capture the atomic motions within the limited number of sampling frames and the low data acquisition rate dictated by the intrinsically poor thermal conductivity and limited photoreversibility of organic materials.

Generating High-Brightness Electron Beams via Ionization Injection by Transverse Colliding Lasers in a Plasma-Wakefield Accelerator

The production of ultrabright electron bunches using ionization injection triggered by two transversely colliding laser pulses inside a beam-driven plasma wake is examined via three-dimensional particle-in-cell simulations. The relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. The result is that the residual momentum of the ionized electrons in the transverse plane of the wake is reduced, and the injection is localized along the propagation axis of the wake. This minimizes both the initial thermal emittance and the emittance growth due to transverse phase mixing. Simulations show that ultrashort (~8 fs) high-current (0.4 kA) electron bunches with a normalized emittance of 8.5 and 6 nm in the two planes, respectively, and a brightness of 1.7×10(19) A rad(-2) m(-2) can be obtained for realistic parameters.

Ultrafast structural dynamics with table top femtosecond hard X-ray and electron diffraction setups

The following tutorial review is directed to graduate students willing to be part of the emerging field of ultrafast structural dynamics. It provides them with an introduction to the field and all the very basic assumptions and experimental tricks involved in femtosecond (fs) diffraction techniques. The concept of stroboscopic photography and its implication in ultrafast science are introduced. Special attention is paid to the generation of ultrashort electron and hard X-ray pulses in table top setups, and a direct comparison in terms of brightness and temporal resolution between current table top and facility-based methodologies is given for proper calibration. This review is focused on ultrafast X-ray and electron diffraction techniques. The progress in the development of fs-structural probes during the last twenty years has been tremendous. Current ultrafast structural probes provide us with the temporal and spatial resolutions required to observe atoms in motion. Different compression approaches have made it possible the generation of ultrashort and ultrabright electron pulses with an effective brightness close to that of fs-hard X-ray pulses produced by free electron lasers. We now have in hand a variety of ultrafast structural cameras ready to be applied for the study of an endless list of dynamical phenomena at the atomic level of inspection.

Mapping molecular motions leading to charge delocalization with ultrabright electrons

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Molecular motion watched

A laser pulse can switch certain crystals from an insulating phase to a highly conducting phase. The ultrafast molecular motions that drive the transition have been directly observed using electron diffraction. See Letter p.343 It is now possible — using ultrafast diffraction methods — to directly observe atomic motions in condensed-phase processes. But organic materials, in which so many important chemical and biological interactions take place, are a different matter. Weak scattering centres, thermal lability and poor heat conduction often degrade the sample before useful data can be obtained. However, a recent technical breakthrough has delivered an ultrabright femtosecond electron source that can operate at a low repetition rate, and this has now been used by Meng Gao et al. to record time-delayed diffraction patterns with sufficient quality to map the molecular motions associated with the photo-induced charge delocalization in an organic salt. These findings illustrate the potential for ultrabright femtosecond electron sources in studies that aim to probe complex dynamic structural process in labile systems relevant to chemistry and biology.

Magnet Design and Testing for the 250 MeV Injector of the SwissFEL at the Paul Scherrer Institute

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The Paul Scherrer Institute (PSI/Switzerland) is planning an X-ray free electron laser facility (SwissFEL) using novel concepts for electron emission, high gradient acceleration and effective bunch compression. Based on a normal-conducting 5.8 GeV linear accelerator, SwissFEL will cover a spectral range of 0.1–7 nm wavelength. An ultra bright electron source was first developed and commissioned in 2008 followed by the construction of a 250 MeV accelerator facility in order to validate the concepts to generate and transport high brightness beams. This facility will be used in the future as the injector for the SwissFEL linac. An amount of typically 100 magnets of 4 different types (solenoids, quadrupoles, dipoles and correctors) is required in order to preserve the high brightness and the low emittance of the femto-second electron pulses. All the magnets are designed and magnetically measured at PSI in order to ensure that the required tolerances are fulfilled. The field gradient, the multipole contents and the magnetic center position are measured with two independent systems, an automated scanner and a uniaxial Hall Probe. We review the magnet design required for the injection phase and discuss the first results of the magnetic measurements. </para>

Vacuum breakdown of carbon-nanotube field emitters on a silicon tip

Findings are given from the experimental observation of the vacuum breakdown of carbon-nanotube (CNT) field emitters on a Si tip. The CNTs were grown on the apex of a Si microtip by microwave plasma-enhanced chemical vapor deposition. The electrical contact of the CNT-Si junction was shown to be of ohmic type. A fine tungsten microprobe in combination with a scanning electron microscopy (SEM) system was employed for both the field emission and the contact conductivity measurements. This arrangement allows to precisely measure the characteristics of individual CNT and to in situ inspect the morphology of the CNT emitters on Si tips before and after vacuum breakdown events. An upper limit in emission current density of ∼103 A/m2 from the CNT emitters was recorded before a vacuum breakdown event is initiated. Clear evidence was found to show that the vacuum breakdown of the CNTs results in melting of the Si tip. These findings enhance the understanding of the failure mechanism of CNT emitters. It also has important technical implication to the development of ultrabright electron source.

Ponderomotive laser acceleration and focusing in vacuum for generation of attosecond electron bunches.

A novel approach for the generation of ultrabright attosecond electron bunches is proposed, based on acceleration in vacuum, by a short laser pulse. The laser-pulse profile is tailored such that the electrons are both focused and accelerated by the ponderomotive force of the light. Using time-averaged equations of motion, analytical criteria for optimal regime of acceleration are found. It is shown that for realistic laser parameters, a beam with up to 10(6) particles and normalized transverse and longitudinal emittances <10(-8) m can be produced.