Abstract
The development of ultrafast gas electron diffraction with nonrelativistic electrons has enabled the determination of molecular structures with atomic spatial resolution. It has, however, been challenging to break the picosecond temporal resolution barrier and achieve the goal that has long been envisioned—making space- and-time resolved molecular movies of chemical reaction in the gas-phase. Recently, an ultrafast electron diffraction (UED) apparatus using mega-electron-volt (MeV) electrons was developed at the SLAC National Accelerator Laboratory for imaging ultrafast structural dynamics of molecules in the gas phase. The SLAC gas-phase MeV UED has achieved 65 fs root mean square temporal resolution, 0.63 Å spatial resolution, and 0.22 Å−1 reciprocal-space resolution. Such high spatial-temporal resolution has enabled the capturing of real-time molecular movies of fundamental photochemical mechanisms, such as chemical bond breaking, ring opening, and a nuclear wave packet crossing a conical intersection. In this paper, the design that enables the high spatial-temporal resolution of the SLAC gas phase MeV UED is presented. The compact design of the differential pump section of the SLAC gas phase MeV UED realized five orders-of-magnitude vacuum isolation between the electron source and gas sample chamber. The spatial resolution, temporal resolution, and long-term stability of the apparatus are systematically characterized.
Highlights
The study of the photoinduced dynamics of small isolated molecules is of crucial importance to the understanding of structurefunction relationships in nature.[1,2] Given adequate temporal and spatial resolutions, the imaging of structural changes following photoexcitation can provide a glimpse into the mechanisms governing the conversion of light into chemical and mechanical energy
We report on the experimental demonstration of the SLAC gas-phase MeV ultrafast electron diffraction (UED) apparatus capable of 70 fs root mean square, or 150 fs full-width-at-halfmaximum (FWHM), temporal resolution, 0.63 Aspatial resolution, and 0.22 A À1 reciprocal-space resolution, which has enabled molecular movies, capturing the rotational dynamics in N2,49 vibrational dynamics in I2,50 a nuclear wave packet crossing a conical intersection in CF3I,51 and ring-opening in 1,3Cyclohexadiene (CHD).[52]
We present the SLAC gas-phase MeV ultrafast electron diffraction (MeV UED) apparatus, its design layout, resolution, and stability characterization
Summary
The study of the photoinduced dynamics of small isolated molecules is of crucial importance to the understanding of structurefunction relationships in nature.[1,2] Given adequate temporal and spatial resolutions, the imaging of structural changes following photoexcitation can provide a glimpse into the mechanisms governing the conversion of light into chemical and mechanical energy. The temporal resolution, or instrument response function, of a UGED experiment, in which a gas sample is excited (pumped) by a laser pulse and probed by an electron bunch, is given by qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s 1⁄4 s2L þ s2e þ s2VM þ s2VOA;. The temporal resolution of UGED experiments using keV (subrelativistic) electrons is dominated by the velocity mismatch between the pump laser and the probe electron, sVM, associated with a typical gas target thickness much larger than a few micrometers. In UGED apparatus using MeV electrons, the sample chamber can be moved away from the electron gun without severely compromising the electron bunch length The high resolution and sensitivity of the SLAC gas-phase MeV UED apparatus has enabled the identification and study of ultrafast events with onsets separated by 70 fs.[51,52] In this paper, we present the SLAC gas-phase MeV UED apparatus, its design layout, resolution, and stability characterization
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