Journal
ULTRAMICROSCOPY
Volume 113, Issue -, Pages 145-151Publisher
ELSEVIER
DOI: 10.1016/j.ultramic.2011.11.018
Keywords
Ultrafast 4D microscopy; Time-resolved diffraction; Solenoid lens; Femtosecond pulses
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Funding
- Rudolf-Kaiser-Stiftung
- Munich-Centre for Advanced Photonics
- European Research Council
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We consider the temporal aberrations of femtosecond electron pulses in magnetic lens systems. We identify two mechanisms for distortions: Within the lens, rotational motion around the propagation axis results in outer parts of the pulse being delayed. After the lens, conversion from radial to longitudinal velocity causes a dynamical reshaping of the electron pulse during propagation towards the image plane. In combination, the pulses refocus back closely to the original pulse duration, but at a position far away from the image plane. Only for a magnification of two there is an approximate coincidence. Calculations of simplified trajectories provide a conceptual picture of signs and magnitudes; details are studied by numerical simulations of realistic lenses. For typical geometries in ultrafast diffraction and microscopy, distortions can amount to tens of femtoseconds and thus limit the temporal resolution even when using single-electron sources without space charge. Compensation of temporal aberrations can be achieved by combining lenses in sequence, resulting in what we call an 'isochronic' lens system for charged particles. We discuss the implications for ultrafast atomic-scale imaging by 4D electron microscopy and diffraction. (C) 2011 Elsevier B.V. All rights reserved.
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