  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.
