Single-electron pulses for ultrafast diffraction

Abstract

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...

Femtosecond electrons probing currents and atomic structure in nanomaterials

“…Temporal compression techniques can be used to obtain femtosecond many-electron pulses at a distant sample 15 , but have yet to be demonstrated for low electron energies. Alternatively, space charge broadening can be eliminated by using single electron pulses at high repetition rates 16,17 . Still, achieving femtosecond time resolution with dispersing sub-keV single electron pulses further requires considerable reduction of the propagation distances 18,19 .…”

Imaging population transfer in atoms with ultrafast electron pulses

“…Also, diffraction images have identified transient molecular structures [8] and established evidence of deformation and dissociation of molecules interacting with laser pulses [9]. Nowadays, electron pulses with femtosecond (fs) duration have been reported [10][11][12][13][14]. Recently, single-electron pulses with a full-width at half-maximum (fwhm) duration of 28 fs have been demonstrated [15].…”

Diffraction using laser-driven broadband electron wave packets

“…Combining conventional electron or X-ray beam methods with femtosecond laser pulses provides improved temporal resolutions for molecular dynamics investigation approaching atomic time scales [23][24][25][26] . However, recent experiments [27][28][29][30][31] demonstrated that intense, isolated femtosecond pulses alone are sufficient for imaging simple molecules via a strong-field three-step process 32,33 (see Supplementary Discussion).…”