Relative Time-of-Flight Measurement in an Ultrafast Electron Microscope

TL;DR

This paper measures the relative time-of-flight of photoelectron packets in an ultrafast electron microscope using in-situ phonon excitation and simulations, revealing how electron-gun geometry affects temporal resolution.

Contribution

It introduces a method to quantify relative electron packet timing shifts caused by electron-gun geometry in ultrafast electron microscopes, supported by experimental and simulation data.

Findings

Discovered tens of picoseconds shifts in electron packet TOF due to aperture geometry.

Linked electron-gun off-axis orientation to changes in electron momentum and timing.

Demonstrated the impact of aperture geometry on temporal resolution in UEM imaging.

Abstract

Efforts to push the spatiotemporal imaging-resolution limits of femtosecond (fs) laser-driven ultrafast electron microscopes (UEMs) to the combined angstrom-fs range will benefit from stable sources capable of generating high bunch charges. Recent demonstration of unconventional off-axis photoemitting geometries are promising, but connections to the observed onset of structural dynamics are yet to be established. Here we use the in-situ photoexcitation of coherent phonons to quantify the relative time-of-flight (r-TOF) of photoelectron packets generated from the Ni Wehnelt aperture and from a Ta cathode set-back from the aperture plane. We further support the UEM experiments with particle-tracing simulations of the precise electron-gun architecture and photoemitting geometries. In this way, we measure discernable shifts in electron-packet TOF of tens of picoseconds for the two photoemitting surfaces. These shifts arise from the impact the Wehnelt-aperture off-axis orientation has on the electron-momentum distribution, which modifies both the collection efficiency and the temporal-packet distribution relative to on-axis emission. Future needs are identified; we expect this and other developments in UEM electron-gun configuration to expand the range of materials phenomena that can be directly imaged on scales commensurate with fundamental structural dynamics.