  Atomic motion dynamics during structural changes or chemical reactions have
been visualized by picosecond and femtosecond pulsed electron beams via
ultrafast electron diffraction and microscopy. Imaging the even faster dynamics
of electrons in atoms, molecules and solids requires electron pulses with
sub-femtosecond durations. We demonstrate here the all-optical generation of
trains of attosecond free-electron pulses. The concept is based on the periodic
energy modulation of a pulsed electron beam via an inelastic interaction with
the ponderomotive potential of an optical travelling wave generated by two
femtosecond laser pulses at different frequencies in vacuum. The subsequent
dispersive propagation leads to a compression of the electrons and the
formation of ultrashort pulses. The longitudinal phase space evolution of the
electrons after compression is mapped by a second phase-locked interaction. The
comparison of measured and calculated spectrograms reveals the attosecond
temporal structure of the compressed electron pulse trains with individual
pulse durations of less than 300 as. This technique can be utilized for
tailoring and initial characterization of sub-optical cycle free-electron
pulses at high repetition rates for stroboscopic time-resolved experiments with
sub-femtosecond time resolution.

Optics;Atomic Physics