  In the early days of quantum mechanics Kapitza and Dirac predicted that
matter waves would scatter off the optical intensity grating formed by two
counter-propagating light waves [1]. This interaction, driven by the
ponderomotive potential of the optical standing wave, was both studied
theoretically and demonstrated experimentally for atoms [2] and electrons
[3-5]. In the original version of the experiment [1,5], only the transverse
momentum of particles was varied, but their energy and longitudinal momentum
remained unchanged after the interaction. Here, we report on the generalization
of the Kapitza-Dirac effect. We demonstrate that the energy of sub-relativistic
electrons is strongly modulated on the few-femtosecond time scale via the
interaction with a travelling wave created in vacuum by two colliding laser
pulses at different frequencies. This effect extends the possibilities of
temporal control of freely propagating particles with coherent light and can
serve the attosecond ballistic bunching of electrons [6], or for the
acceleration of neutral atoms or molecules by light.
