  Electron dynamics in water are of fundamental importance for a broad range of
phenomena, but their real-time study faces numerous conceptual and
methodological challenges. Here, we introduce attosecond size-resolved cluster
spectroscopy and build up a molecular-level understanding of the attosecond
electron dynamics in water. We measure the effect that the addition of single
water molecules has on the photoionization time delays of water clusters. We
find a continuous increase of the delay for clusters containing up to 4-5
molecules and little change towards larger clusters. We show that these delays
are proportional to the spatial extension of the created electron hole, which
first increases with cluster size and then partially localizes through the
onset of structural disorder that is characteristic of large clusters and bulk
liquid water. These results establish a previously unknown sensitivity of
photoionization delays to electron-hole delocalization and reveal a direct link
between electronic structure and attosecond photoemission dynamics. Our results
offer novel perspectives for studying electron/hole delocalization and its
attosecond dynamics.
