Electron dynamics in water are of fundamental importance for a broad range of phenomena^1-3, but their real-time study faces numerous conceptual and methodological challenges^4-6. 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^7-9 of water clusters. We find a continuous increase of the delay for clusters containing up to four to five 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 indicate a previously unknown sensitivity of photoionization delays to electron-hole delocalization and indicate a direct link between electronic structure and attosecond photoionization dynamics. Our results offer new perspectives for studying electron-hole delocalization and its attosecond dynamics.

We present the experimental observation of two-center interference in the ionization time delays of Kr_{2}. Using attosecond electron-ion-coincidence spectroscopy, we simultaneously measure the photoionization delays of krypton monomer and dimer. The relative time delay is found to oscillate as a function of the electron kinetic energy, an effect that is traced back to constructive and destructive interference of the photoelectron wave packets that are emitted or scattered from the two atomic centers. Our interpretation of the experimental results is supported by solving the time-independent Schrödinger equation of a 1D double-well potential, as well as coupled-channel multiconfigurational quantum-scattering calculations of Kr_{2}. This work opens the door to the study of a broad class of quantum-interference effects in photoionization delays and demonstrates the potential of attosecond coincidence spectroscopy for studying weakly bound systems.

Photoemission, in which photons cause atoms or molecules to emit electrons, is not an instantaneous process. The emitted electron may take hundreds of attoseconds to traverse the local potential of its parent atom or molecule after photoabsorption. The exact time is sensitive to the potential landscape the electron encounters, which causes a delay in when the photoelectron’s wave packet emerges from different parts of the molecule. Here, we experimentally explore this delay in the hitherto elusive frame of reference of the molecule, which allows us to observe photoelectron emission relative to specific orientations of the molecule.As a prototype process, we choose the photoionization of the inner valence electron of the NO molecule. During the photoelectron’s excursion, it experiences an asymmetric molecular potential landscape and a dynamical centrifugal potential. We introduce an approach to clock the photoionization dynamics of the NO molecule in its molecular frame. A key finding is an asymmetric photoemission delay difference, around 150 attoseconds, at the opposite ends of the molecule. Our theoretical simulation illustrates that the asymmetric delay originates from the interference between multiple photoionization pathways.Despite being demonstrated in a heteronuclear diatomic molecule, our methods and findings are applicable to the photoemission dynamics in many molecules, surfaces, and interfaces with asymmetric potentials. Our approach opens a new avenue for exploring the attosecond photoelectron dynamics in complex systems and investigating the time-resolved quantum dynamics in solutions, complex materials, and biologic tissues.