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In femtosecond ultrafast electron microscopy (UEM) experiments, the initial excitation period is composed of spatiotemporal overlap of the temporally commensurate pump photon pulse and probe photoelectron packet. Generation of evanescent near-fields at the nanostructure specimens produces a dispersion relation that enables coupling of the photons (= 2.4 eV, for example) and freely propagating electrons (200 keV, for example) in the near-field. Typically, this manifests as discrete peaks occurring at integer multiples (n) of the photon energy in the low-loss/gain region of electron-energy spectra (i.e., at 200 keV ± neV). Here, we examine the UEM imaging resolution implications of the strong inelastic near-field interactions between the photons employed in optical excitation and the probe photoelectrons. We find that the additional photoinduced energy dispersion occurring when swift electrons pass through intense evanescent near-fields results in a discrete chromatic aberration that limits the spatial resolving power to several angstroms during the excitation period.
Bibliographical noteFunding Information:
This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013, in part by a 3M Nontenured Faculty Award under Award Number 13673369, and in part by the Arnold and Mabel Beckman Foundation through a Beckman Young Investigator Award. Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research under Award Number 53116-DNI7. D.A.P. acknowledges support in the form of a Doctoral Dissertation Fellowship from the University of Minnesota Graduate College.
© 2016 American Chemical Society.
Copyright 2016 Elsevier B.V., All rights reserved.
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