  Similar to the optical diffraction of light passing through a material
grating, the Kapitza-Dirac effect occurs when an electron is diffracted by a
standing light wave. In its original description the effect is
time-independent. In the present work, we extend the Kapitza-Dirac concept to
the time domain. By tracking the spatiotemporal evolution of a pulsed electron
wave packet diffracted by a femtosecond (10 15 second) standing wave pulse in a
pump-probe scheme, we observe so far unseen time-dependent diffraction
patterns. The fringe spacing in the observed pattern differs from that
generated by the conventional Kapitza-Dirac effect, moreover it decreases as
the pump-probe delay time increases. By exploiting this time-resolved
diffraction scheme, we gather access to the time evolution of the previously
inaccessible phase properties of a free electron.
