Role of nuclear-electronic coupling in attosecond photoionization of H2

The separation of electronic and nuclear dynamics due to differing timescales is a useful concept for understanding ground-state molecular systems. However, coupling between these degrees of freedom is critical to understanding the evolution of most excited-state systems. We measure two-photon ionization delays of H2 and compare to calculations of the same measurement in a frozen-nuclei approximation. We find discrepancies between the vibrationally resolved measurement and bond-length-dependent theory, suggesting that nuclear motion affects H2 photoionization on attosecond timescales. We ascribe our observation to nuclear-electronic channel coupling between continuum vibrational states. Our results demonstrate that nuclear-electronic coupling cannot be neglected in the sudden ionization of molecules containing light atoms.