Low-energy electrons ($<2\mathrm{eV}$) can fragment gas phase formic acid (HCOOH) molecules through resonant dissociative attachment processes. Recent experiments have shown that the principal reaction products of such collisions are formate ions (${\mathrm{HCOO}}^{-}$) and hydrogen atoms. Using first-principles electron scattering calculations, we have identified the responsible negative ion state as a transient ${\pi }^{*}$ anion. Symmetry considerations dictate that the associated dissociation dynamics are intrinsically polyatomic: a second anion surface, connected to the first by a conical intersection, is involved in the dynamics and the transient anion must necessarily deform to nonplanar geometries before it can dissociate to the observed stable products.

• Received 4 January 2006

DOI:https://doi.org/10.1103/PhysRevLett.96.213201