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Appl. Sci. 2017, 7(6), 534; doi:10.3390/app7060534

Fundamental Limits on Spatial Resolution in Ultrafast X-ray Diffraction

EaStCHEM, School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh EH9 3FJ, UK
Department of Chemistry, Brown University, Providence, RI 02912, USA
Author to whom correspondence should be addressed.
Academic Editor: Kiyoshi Ueda
Received: 7 April 2017 / Revised: 16 May 2017 / Accepted: 17 May 2017 / Published: 23 May 2017
(This article belongs to the Special Issue X-Ray Free-Electron Laser)
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X-ray Free-Electron Lasers have made it possible to record time-sequences of diffraction images to determine changes in molecular geometry during ultrafast photochemical processes. Using state-of-the-art simulations in three molecules (deuterium, ethylene, and 1,3-cyclohexadiene), we demonstrate that the nature of the nuclear wavepacket initially prepared by the pump laser, and its subsequent dispersion as it propagates along the reaction path, limits the spatial resolution attainable in a structural dynamics experiment. The delocalization of the wavepacket leads to a pronounced damping of the diffraction signal at large values of the momentum transfer vector q, an observation supported by a simple analytical model. This suggests that high-q measurements, beyond 10–15 Å 1 , provide scant experimental payback, and that it may be advantageous to prioritize the signal-to-noise ratio and the time-resolution of the experiment as determined by parameters such as the repetition-rate, the photon flux, and the pulse durations. We expect these considerations to influence future experimental designs, including source development and detection schemes. View Full-Text
Keywords: X-ray free-electron lasers; ultrafast dynamics; diffraction; spatial resolution; pump-probe; quantum dynamics; wavepackets; photochemistry X-ray free-electron lasers; ultrafast dynamics; diffraction; spatial resolution; pump-probe; quantum dynamics; wavepackets; photochemistry

Figure 1

This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Kirrander, A.; Weber, P.M. Fundamental Limits on Spatial Resolution in Ultrafast X-ray Diffraction. Appl. Sci. 2017, 7, 534.

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