Time-resolved study of roaming neutral H₂ in acetonitrile

Roaming reactions have garnered significant interest in recent years as they defy the conventional reactions that follow minimum energy pathways. Instead, these reactions involve flat regions of the potential energy surface where molecular fragments remain weakly bound and participate in long-range interactions mediated by relatively weak forces. The neutral character of the roaming fragment and its indeterminate trajectory have made it difficult for experimental identification and systematic studies. We will present a novel time-resolved approach to image the roaming H₂ neutrals leading to formation of H₃⁺ ions, using coincident Coulomb explosion imaging in combination with pump-probe spectroscopy. We demonstrate that by reconstructing dynamical information about the ‘invisible’ neutral roamers, it is possible to directly track and observe experimental signatures of roaming. Along with state-of-the art quantum chemistry calculations, our measurements and analysis provide a robust, kinematically complete picture of the roaming mechanism in acetonitrile.