Attosecond spectroscopy of molecular charge transfer uncovers a 1.5-fs delay in population transfer.

The transfer of population between two intersecting quantum states is the most fundamental event in many dynamical processes in physics, chemistry, biology, and material science. Any two-state description of such processes requires population leaving one state to instantaneously appear in the other. We show that coupling to additional states, present in all real-world systems, can cause a measurable delay in population transfer. Using attosecond spectroscopy supported by quantum-chemical calculations, we measure a delay of 1.46 ± 0.41 fs at a charge-transfer crossing in CFI, where an electron hole moves from the fluorine atoms to iodine. Our measurements also resolve the other fundamental quantum-dynamical processes involved in the charge-transfer reaction: a vibrational rearrangement time of 9.38 ± 0.21 fs (during which the vibrational wave packet travels to the state crossing) and a population-transfer time of 2.3-2.4 fs. Our work shows that delays in population transfer readily appear in otherwise-adiabatic reactions and predicts them to be on the order of a single-femtosecond for molecular valence-state crossings. These results have implications for many research areas, such as atomic and molecular physics, charge transfer, or light harvesting.