Femtosecond time-resolved MeV electron diffraction

Abstract

We report the experimental demonstration of femtosecond electron diffraction using high-brightness MeV electron beams. High-quality, single-shot electron diffraction patterns for both polycrystalline aluminum and single-crystal 1T-TaS 2 are obtained utilizing a 5 fC (∼3 × 10 4 electrons) pulse of electrons at 2.8 MeV. The high quality of the electron diffraction patterns confirms that electron beam has a normalized emittance of ∼50 nm rad. The transverse and longitudinal coherence length is ∼11 and ∼2.5 nm, respectively. The timing jitter between the pump laser and probe electron beam was found to be ∼100 fs (rms). The temporal resolution is demonstrated by observing the evolution of Bragg and superlattice peaks of 1T-TaS 2 following an 800 nm optical pump and was found to be 130 fs. Our results demonstrate the advantages of MeV electrons, including large elastic differential scattering cross-section and access to high-order reflections, and the feasibility of ultimately realizing below 10 fs time-resolved electron diffraction. X-ray free electron laser (FEL) sources and ultrafast electron diffraction (UED) are two of the most powerful tools for exploring the ultrasmall and ultrafast world [1,2]. The large electron scattering cross section and compactness of electron facilities make UED a particularly attractive option for exploring ultrafast processes and the technique has been used for studying strongly correlated electron systems and revealing transient intermediates in gas phase chemical reactions [1][2][3][4]. However, to date, the time resolution of such experiments has been limited by pulse broadening from repulsive space-charge effects (SCE) and the limited acceleration field [5].To reduce such effects, MeV electron beams generated by a photocathode RF gun have been proposed for UED applications [6][7][8][9]. In such schemes, a laser is used to illuminate a photocathode, producing a highbrightness electron beam, and to control the initial spatial and temporal distributions of that beam. An RF cavity then rapidly accelerates the electrons to a few MeV. The RF field also compresses the electron beam as it is accelerated in the time-dependent acceleration field [6]. For electron pulses at MeV energies, the magnetic field induced by the moving electron beam, together with relativistic effects, greatly reduce the SCE. Specifically, the transverse and longitudinal SCE scale as 1/β 2 γ 3 and 1/β 2 γ 5 , respectively [10], where β and γ are the relativistic velocity and energy, respectively. Thus, increasing the electron energy has the potential to compress more electrons into a shorter electron bunch. Another important advantage of relativistic beams is that they eliminate the velocity mismatch between the pump laser and the electron beam. This mismatch often limits the time resolution of ultrafast dynamics study for gas samples [11,12]. In addition to the higher temporal resolution, MeV electrons can penetrate thicker samples and have much less dynamic scattering effects. Finally, the higher electron beam energ...

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