  We present the experimental observation of two-center interference in the
ionization time delays of Kr2. 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 Schrodinger
equation of a 1D double-well potential, as well as coupled-channel
multiconfigurational quantum-scattering calculations of Kr2. 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.
