Coherence of femtosecond single electrons exceeds biomolecular dimensions

P Baum,F O Kirchner,F Krausz,S Lahme

doi:10.1088/1367-2630/15/6/063021

Abstract

Time-resolved diffraction and microscopy with femtosecond electron pulses provide four-dimensional recordings of atomic motion in space and time. However, the limited coherence of electron pulses, reported in the range of 2–3 nm, has so far prevented the study of complex organic molecules with relevance to chemistry and biology. Here we characterize the coherence of femtosecond single-electron pulses that are generated by laser photoemission. We show how the absence of space charge and the minimization of the source size allow the transverse coherence to be extended to 20 nm at the sample position while maintaining a useful beam diameter. The extraordinary coherence is experimentally demonstrated by recording single-electron diffraction snapshots from a complex organic molecular crystal and identifying more than 80 sharp Bragg reflections. Further optimization affords promise for coherences of 100 nm. These advances will allow time-resolved imaging of functional dynamics in biological systems, uniting picometre and femtosecond resolutions in a compact, table-top instrumentation.

Highlights

The transverse velocity spread of laser-emitted single electrons is remarkably low in the absence of space charge
In dense pulses, where many electrons are concentrated to femtosecond duration [3], Coulomb forces cause a significant increase of radial spread, i.e. divergence [18], which reduces the transverse coherence. Such effects are absent in the single-electron regime employed here, where the transverse velocity spread is solely determined by the physics of the photoemission process [17]
The transverse coherence length can be defined by analogy with optics, using a criterion of 88% for the visibility of interference [23, 24]

Summary

Introduction

Such effects are absent in the single-electron regime employed here, where the transverse velocity spread is solely determined by the physics of the photoemission process [17]. In order to determine the coherence properties of our electron beam, we measured its radius on the screen as a function of the focal length of the magnetic lens; the source parameters were fitted using transfer matrices for the beam propagation (see appendix B).

Results

Conclusion