Visualization of atomic-scale structural motion by ultrafast electron diffraction and microscopy requires electron packets of shortest duration and highest coherence. We report on the generation and application of single-electron pulses for this purpose. Photoelectric emission from metal surfaces is studied with tunable ultraviolet pulses in the femtosecond regime. The bandwidth, efficiency, coherence, and electron pulse duration are investigated in dependence on excitation wavelength, intensity, and laser bandwidth. At photon energies close to the cathode's work function, the electron pulse duration shortens significantly and approaches a threshold that is determined by interplay of the optical pulse width and the acceleration field. An optimized choice of laser wavelength and bandwidth results in sub-100-fs electron pulses. We demonstrate single-electron diffraction from polycrystalline diamond films and reveal the favorable influences of matched photon energies on the coherence volume of single-electron wave packets. We discuss the consequences of our findings for the physics of the photoelectric effect and for applications of singleelectron pulses in ultrafast 4D imaging of structural dynamics.I nvestigation of structural dynamics in condensed matter and molecules by four-dimensional imaging calls for resolution of Angstrom-scale distances with femtosecond timing. These resolutions are provided by ultrafast electron diffraction and microscopy techniques, which are based on pump-probe arrangements with a femtosecond laser for excitation and with ultrashort electron packets for measuring sequences of atomic-scale structures during changes (1). Recently reported studies include such of reaction pathways during phase transformations (2), chemical reactions (3), laser ablation (4), molecular alignment (5), changes at interfaces (6), heating and melting processes (7-9), cantilever motion (10), or evanescent fields around nanostructures (11), among many others (12). Possibilities to access the attosecond regime of charge density motion are also discussed (13), taking into account the generation of pulses (14-16) and the quantum dynamics of the scattering process (17).These (and many more) achievements are made possible by the use of ultrashort electron packets at multi-kiloelectron-volt energies for time-resolved diffraction and imaging. Such packets can be generated by photoelectric emission from metal films illuminated with femtosecond lasers, followed by acceleration in static or radio-frequency electric fields. In multielectron packets, space-charge effects dominate the temporal structure and energy distribution; hence electron density has to be traded off against resolution (18,19). In contrast, space charge is absent in packets containing only one single electron at a time (12). In this regime, the longitudinal and transverse emittance, and hence the pulse duration, bandwidth, and coherence, are determined by the spatiotemporal statistics of the photoelectric effect.Some theoretical models consider the femtos...
Time-resolved diffraction and microscopy with femtosecond electron pulses provide four-dimensional recordings of atomic motion in space and time. However, the limited coherence of electron pulses, reported in the range of 2-3 nm, has so far prevented the study of complex organic molecules with relevance to chemistry and biology. Here we characterize the coherence of femtosecond single-electron pulses that are generated by laser photoemission. We show how the absence of space charge and the minimization of the source size allow the transverse coherence to be extended to 20 nm at the sample position while maintaining a useful beam diameter. The extraordinary coherence is experimentally demonstrated by recording singleelectron diffraction snapshots from a complex organic molecular crystal and identifying more than 80 sharp Bragg reflections. Further optimization affords promise for coherences of 100 nm. These advances will allow time-resolved imaging of functional dynamics in biological systems, uniting picometre and femtosecond resolutions in a compact, table-top instrumentation.
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