Dynamics and Energy Relaxation of Formic Acid Dimers Following Inner Valence Ionization

Abstract We study gas-phase dissociation of the formic acid dimer dication as a model for DNA base-pair interactions after inner-valence 56 eV extreme ultraviolet (EUV) ionization. Employing reaction microscopy, we measure the 3D momenta of electrons and fragment-ions in coincidence to reconstruct channel-resolved electron kinetic energies, fragment kinetic energy release, and angular distributions. This work focuses on the symmetric 2-body (HCOOH+ + HCOOH+) and asymmetric 3-body (HCOOH+ + HCOO+ + H) breakup channels, reporting their relative yields and dynamics. Electron-energy signatures and angular correlations distinguish direct double ionization (DDI) from sequential ionization (SI) with a ratio of ∼9:5. In SI, low-energy electrons and their near-isotropic angular distributions indicate a competition between local intramolecular autoionization (AI) and nonlocal intermolecular Coulombic decay (ICD). Real-time simulations of the electron dynamics support AI/ICD-driven secondary ionization on sub-20 fs time scales. The SI process depends on whether the initially ionized orbital has carbonyl- or hydroxyl-oxygen character, with charge and energy transfer occurring between diagonally adjacent oxygen atoms rather than along hydrogen bonds.