The long held objective of directly observing atomic motions during the defining moments of chemistry has been achieved based on ultrabright electron sources that have given rise to a new field of atomically resolved structural dynamics. This class of experiments requires not only simultaneous sub-atomic spatial resolution with temporal resolution on the 100 femtosecond time scale but also has brightness requirements approaching single shot atomic resolution conditions. The brightness condition is in recognition that chemistry leads generally to irreversible changes in structure during the experimental conditions and that the nanoscale thin samples needed for electron structural probes pose upper limits to the available sample or "film" for atomic movies. Even in the case of reversible systems, the degree of excitation and thermal effects require the brightest sources possible for a given space-time resolution to observe the structural changes above background. Further progress in the field, particularly to the study of biological systems and solution reaction chemistry, requires increased brightness and spatial coherence, as well as an ability to tune the electron scattering cross-section to meet sample constraints. The electron bunch density or intensity depends directly on the magnitude of the extraction field for photoemitted electron sources and electron energy distribution in the transverse and longitudinal planes of electron propagation. This work examines the fundamental limits to optimizing these parameters based on relativistic electron sources using re-bunching cavity concepts that are now capable of achieving 10 femtosecond time scale resolution to capture the fastest nuclear motions. This analysis is given for both diffraction and real space imaging of structural dynamics in which there are several orders of magnitude higher space-time resolution with diffraction methods. The first experimental results from the Relativistic Electron Gun for Atomic Exploration (REGAE) are given that show the significantly reduced multiple electron scattering problem in this regime, which opens up micron scale systems, notably solution phase chemistry, to atomically resolved structural dynamics.

We report on a general mechanism for photo-induced phase transitions. The process relies on the photo-injection of hot electrons from an adjacent metallic layer to trigger the structural dynamics of the materials of interest. This mechanism is demonstrated for the semiconductor-to-metal phase transition of VO 2 using a 20 nm Au injection layer. The nature of the phase transition is demonstrated by time-resolved optical transmission measurements, as well as a well defined bias dependence that illustrates that the Au film is the source of nonequilibrium electrons driving the phase transition.

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The authors report on the first experimental characterization of a fiber tip-based electron source, where electron emission can be triggered by both electric field and optical excitation. Our approach consists of coating the open aperture of a commercial 100 nm apex size near-field scanning optical microscopy fiber tip with a 10 nm thick tungsten (W) layer, which is back-illuminated by a 405 nm continuous-wave laser beam in the presence of an extraction electric field. Despite the very low optical transmission of the fiber due to the subwavelength aperture size, measurements show a clearly enhanced emission when photoexciting the W layer with respect to pure field emission. The emission response time is slower than the optical trigger time, suggesting that thermal effects are predominant in the studied regime. To back up this hypothesis, the authors fabricated a nanometric thermocouple probe based on a Pt/Au junction and measured the temporal response of the tip temperature. The measured switch-on time for the tip temperature is consistent with the switch-on time of the optically enhanced electron emission

With the relativistic electron gun for atomic exploration (REGAE) we seek to observe structural dynamics both in real space imaging and diffraction [1]. REGAE is based on a RF gun accelerator and operates in the range from 2 to 5 MeV. RF gun technology allows high brightness for electron pulses at high energy. The machine is equipped with an RF cavity system, allowing for picosecond bunches with energy spread compensation to prevent limitations by chromatic aberrations and space charge. A custom made lens system for 3 MeV pulse energy system is set in place. The presentation will include a discussion on space charge induced aberrations in dynamic HVEM, give estimates about the anticipated resolution and discuss the prospects for dynamic transmission electron microscopy of organic samples in environmental cells. Recent results making use of nano‐fluidic cell technology developed in house will be presented as an outlook: DNA – nanoparticle multimers have been studied regarding their dynamic in liquid and stability under electron irradiation during TEM imaging in a conventional 200 keV microscope [2]. First experiments towards imaging dynamics in living cancer cells under environmental conditions in the TEM will be discussed [3].

We study field emission characteristics of an allmetal double-gate single nanotip emitter to explore the feasibility of such emitters for applications that require extremely high beam brightness and coherence. The single-tip device showed an excellent beam collimation characteristic including an order of magnitude reduction of the transverse velocity spread and an order of magnitude enhancement of beam intensity as reported with array devices previously. The evolution of the beam image with the increase of the collimation potential indicated the importance of subnanometer corrugation at the nanotip apex surface

In this paper we report on the first experimental characterization of a fiber tip-based electron source where the electron emission is triggered by both, electric field and optical excitation. Our approach consists of coating a commercial 100 nm apex size NSOM multi-mode fiber tip with a 10 nm thick tungsten layer, which is back-illuminated in the presence of an extraction electric field. The measurements show a clear enhancement of the emission by the incident light, but the emission response time is slower than the optical trigger time, suggesting that thermal effects are predominant. This hypothesis is backed up by the temporal response measurements of the tip temperature