The Roaming Atom: Straying from the Reaction Path in Formaldehyde Decomposition

We present a combined experimental and theoretical investigation of formaldehyde (H 2 CO) dissociation to H 2 and CO at energies just above the threshold for competing H elimination. High-resolution state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H 2 . The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H 2 . Quasi-classical trajectory calculations performed on a global potential energy surface for H 2 CO suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.