  Studies of ultrafast dynamics along with femtosecond-pulse metrology rely on
non-linear processes, induced solely by the exciting/probing pulses or the
pulses to be characterized. Extension of these approaches to the
extreme-ultraviolet (XUV) spectral region opens up a new, direct route to
attosecond scale dynamics. Limitations in available intensities of coherent XUV
continua kept this prospect barren. The present work overcomes this barrier.
Reaching condition at which simultaneous ejection of two bound electrons by
two-XUV-photon absorption becomes more efficient than their one-by-one removal
it is succeeded to probe atomic coherences, evolving at the 1fs scale, and
determine the XUV-pulse duration. The investigated rich and dense in structure
autoionizing manifold ascertains applicability of the approach to complex
systems. This initiates the era of XUV-pump-XUV-probe experiments with
attosecond resolution.
