  The quantum mechanical motion of electrons in molecules and solids occurs on
the sub-femtosecond timescale. Consequently, the study of ultrafast electronic
phenomena requires the generation of laser pulses shorter than 1 fs and of
sufficient intensity to interact with their target with high probability.
Probing these dynamics with atomic-site specificity requires the extension of
sub-femtosecond pulses to the soft X-ray spectral region. Here we report the
generation of isolated GW-scale soft X-ray attosecond pulses with an X-ray
free-electron laser. Our source has a pulse energy that is six orders of
magnitude larger than any other source of isolated attosecond pulses in the
soft X-ray spectral region, with a peak power in the tens of gigawatts. This
unique combination of high intensity, high photon energy and short pulse
duration enables the investigation of electron dynamics with X-ray non-linear
spectroscopy and single-particle imaging.
