  How fast processes can we measure? Attosecond physics address the limit of
measurable time in science. Atomic X-ray excited states offers a way to study
extremely fast dynamics with chemical specificity. In black phosphorous an
X-ray excited electron can relocate in 22.7 attoseconds. Using the lifetime of
the P 1s core-hole as time-base, the radiationless decay spectrum can be used
to study charge transfer processes on the time-scale of the atomic unit of time
(24 attoseconds). We demonstrate that the technique can be extended to within a
few percent of the core hole's lifetime, an order of magnitude smaller than
previously thought.
