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Register a new userNon-invasive time-sorting in radio-frequency compressed ultrafast electron diffraction
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We demonstrate a non-invasive time-sorting method for ultrafast electron diffraction (UED) experiments with radio-frequency (rf) compressed electron beams. We show that electron beam energy and arrival time at the sample after rf compression are strongly correlated such that the arrival time jitter may be corrected through measurement of the beam energy. The method requires minimal change to the infrastructure of most of the UED machines and is applicable to both keV and MeV UED. In our experiment with ~3 MeV beam, the timing jitter after rf compression is corrected with 35 fs root-mean-square (rms) accuracy, limited by the 3x10^-4 energy stability. For keV UED with high energy stability, sub-10 fs accuracy in time-sorting should be readily achievable. This time-sorting technique allows us to retrieve the 2.5 THz oscillation related to coherent A1g phonon in laser excited Bismuth film and extends the temporal resolution of UED to a regime far beyond the 100-200 fs rms jitter limitation.
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Knowledge of molecular structure is paramount in understanding, and ultimately influencing, chemical reactivity. For nearly a century, diffractive imaging has been used to identify the structures of many biologically-relevant gas-phase molecules with atomic (i.e. Angstrom, A; 1 A = 10$^{-10}$ m) spatial resolution. Unravelling the mechanisms of chemical reactions requires the capability to record multiple well-resolved snapshots of the molecular structure as it is evolving on the nuclear (i.e. femtosecond, fs; 1 fs = 10$^{-15}$ s) timescale. We present the latest, state-of-the-art ultrafast electron diffraction methods used to retrieve the molecular structure of gas-phase molecules with Angstrom and femtosecond spatio-temporal resolution. We first provide a historical and theoretical background to elastic electron scattering in its application to structural retrieval, followed by details of field-free and field-dressed ultrafast electron diffraction techniques. We discuss the application of these ultrafast methods to time-resolving chemical reactions in real-time, before providing a future outlook of the field and the challenges that exist today and in the future.
Streaking of photoelectrons with optical lasers has been widely used for temporal characterization of attosecond extreme ultraviolet pulses. Recently, this technique has been adapted to characterize femtosecond x-ray pulses in free-electron lasers with the streaking imprinted by farinfrared and Terahertz (THz) pulses. Here, we report successful implementation of THz streaking for time-stamping of an ultrashort relativistic electron beam of which the energy is several orders of magnitude higher than photoelectrons. Such ability is especially important for MeV ultrafast electron diffraction (UED) applications where electron beams with a few femtosecond pulse width may be obtained with longitudinal compression while the arrival time may fluctuate at a much larger time scale. Using this laser-driven THz streaking technique, the arrival time of an ultrashort electron beam with 6 fs (rms) pulse width has been determined with 1.5 fs (rms) accuracy. Furthermore, we have proposed and demonstrated a non-invasive method for correction of the timing jitter with femtosecond accuracy through measurement of the compressed beam energy, which may allow one to advance UED towards sub-10 fs frontier far beyond the ~100 fs (rms) jitter.
One of the frontiers of modern electron scattering instrumentation is improving temporal resolution in order to enable the observation of dynamical phenomena at their fundamental time-scales. We analyze how a radiofrequency cavity can be used as an electron longitudinal lens in order to produce a highly magnified temporal replica of an ultrafast process, and, in combination with a deflecting cavity, enable streaked electron images of optical-frequency phenomena. We present start-to-end simulations of an MeV electron beamline for two variations of this idea (a `magnifying-glass and a `point-projection configuration) showing the feasibility for an electron probe to achieve single shot 1.4 fs(rms) temporal resolution.
This review article discusses advances in the use of time-resolved photoelectron spectroscopy for the study of non-adiabatic processes in molecules. A theoretical treatment of the experiments is presented together with a number of experimental examples.
We introduce ultrafast low-energy electron diffraction (ULEED) in backscattering for the study of structural dynamics at surfaces. Using a tip-based source of ultrashort electron pulses, we investigate the optically-driven transition between charge-density wave phases at the surface of 1T-TaS2. The large transfer width of the instrument allows us to employ spot-profile analysis, resolving the phase-ordering kinetics in the nascent incommensurate charge-density wave phase. We observe a coarsening that follows a power-law scaling of the correlation length, driven by the annihilation of dislocation-type topological defects of the charge-ordered lattice. Our work opens up the study of a wide class of structural transitions and ordering phenomena at surfaces and in low-dimensional systems.
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