title: Femtosecond gas phase electron diffraction with MeV electrons.
abstract: We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.
authors.affiliation: University of Nebraska-Lincoln, 855 N 16th Street, Lincoln, Nebraska 68588, USA. martin.centurion@unl.edu.
authors.forename: Jie
authors.initials: J
authors.lastname: Yang
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA and Physics and Astronomy, Potsdam University, Potsdam, 14476, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Guehr
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Theodore
authors.initials: T
authors.lastname: Vecchione
authors.affiliation: University of Nebraska-Lincoln, 855 N 16th Street, Lincoln, Nebraska 68588, USA. martin.centurion@unl.edu.
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Robinson
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Renkai
authors.initials: R
authors.lastname: Li
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Nick
authors.initials: N
authors.lastname: Hartmann
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Xiaozhe
authors.initials: X
authors.lastname: Shen
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Ryan
authors.initials: R
authors.lastname: Coffee
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Jeff
authors.initials: J
authors.lastname: Corbett
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Alan
authors.initials: A
authors.lastname: Fry
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Kelly
authors.initials: K
authors.lastname: Gaffney
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Tais
authors.initials: T
authors.lastname: Gorkhover
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Carsten
authors.initials: C
authors.lastname: Hast
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Keith
authors.initials: K
authors.lastname: Jobe
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Igor
authors.initials: I
authors.lastname: Makasyuk
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Alexander
authors.initials: A
authors.lastname: Reid
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Joseph
authors.initials: J
authors.lastname: Robinson
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Sharon
authors.initials: S
authors.lastname: Vetter
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Fenglin
authors.initials: F
authors.lastname: Wang
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Stephen
authors.initials: S
authors.lastname: Weathersby
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Charles
authors.initials: C
authors.lastname: Yoneda
authors.affiliation: SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
authors.forename: Xijie
authors.initials: X
authors.lastname: Wang
authors.affiliation: University of Nebraska-Lincoln, 855 N 16th Street, Lincoln, Nebraska 68588, USA. martin.centurion@unl.edu.
authors.forename: Martin
authors.initials: M
authors.lastname: Centurion