Visualizing an elusive molecular reaction: Using time-resolved momentum imaging to detect neutral roaming processes

Roaming reactions have garnered significant interest in recent years since they defy the conventional reactions that follow minimum energy pathways. Instead, such 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. We will present the time-resolved study of H₃⁺ ions formed due to roaming H₂ neutrals in acetonitrile, using coincident Coulomb explosion imaging in combination with pump-probe spectroscopy. We demonstrate that by reconstructing dynamical information of 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 roaming mechanism in acetonitrile.