  A dc electron gun, generating picosecond pulses with up to $8\times10^{6}$
electrons per pulse, was developed. Its applicability for future
time-resolved-diffraction experiments on state- and conformer-selected
laser-aligned or oriented gaseous samples was characterized. The focusing
electrodes were arranged in a velocity-map imaging spectrometer configuration.
This allowed to directly measure the spatial and velocity distributions of the
electron pulses emitted from the cathode. The coherence length and pulse
duration of the electron beam were characterized by these measurements combined
with electron trajectory simulations. Electron diffraction data off a thin
aluminum foil illustrated the coherence and resolution of the electron-gun
setup.
