Attosecond spectroscopy of size-resolved water clusters

Electron dynamics in water are of fundamental importance for a broad range of phenomena,

but their real-time study has so far remained limited to the femtosecond time scale. 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 directly reflect 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 from electronic structure

to attosecond photoemission dynamics. Our results also bridge the technological, theoretical

and conceptual gaps between gas-phase and liquid-phase attosecond spectroscopies.