Bond Formation and Bond Scission Dynamics in Polyatomic Molecules Revealed by Momentum Imaging Experiments and Electron Scattering Calculations.*

We present combined experimental and theoretical studies of dissociative electron attachment (DEA) dynamics in methane and ammonia. DEA in each of these systems proceeds through electronic Feshbach resonances, where a valence electron is excited and captured with the incident electron in the lowest unoccupied orbital. In methane, one triply-degenerate resonance undergoes Jahn-Teller splitting through molecular distortions, leading to four observed final states, each having a 2-body and a 3-body dissociation with anionic products H$^{\mathrm{-}}$ and CH$_{\mathrm{2}}^{\mathrm{-}}$ and neutrals CH$_{\mathrm{3}}$, CH$_{\mathrm{2}}$, H$_{\mathrm{2}}$ or H. In ammonia, one resonance leads to H$^{\mathrm{-}} \quad +$ NH$_{\mathrm{2}}$ and NH$_{\mathrm{2}}^{\mathrm{-}} \quad +$ H, the latter resulting from non-adiabatic charge transfer. A higher energy resonance leads directly to H$^{\mathrm{-}} \quad +$ NH$_{\mathrm{2}}$* and indirectly to NH$_{\mathrm{2}}^{\mathrm{-}} \quad +$ H. We examine the dynamics of the transient anion in each of these processes.