Ultrafast and Ultraslow Proton-Transfer Dynamics Induced by Formic Acid Dimer Ionization

Proton-transfer dynamics in hydrogen-bonded dimers are important for understanding debated mechanisms of radiation damage to DNA base pairs. Using coincidence photofragment imaging in ultrafast extreme-ultraviolet pump and near-IR probe experiments on the formic acid dimer, we observed a transient enhancement (150 fs) of the protonated monomer signal. This correlates with ab initio molecular dynamics simulations of the ionization induced dynamics, showing concerted proton transfer and dimer ring opening in a metastable dimer. Coincidence analysis revealed the ultraslow mechanism of the metastable dimer cation on the microsecond time scale. The ultraslow dynamics were attributed to a barrier for the structural rearrangement of the deprotonated moiety from an HCOO to an OCOH geometry. Moreover, ultraslow channels of protonated monomer ions to form H₃O⁺ + CO and H₂O + CHO⁺ were observed and interpreted as dissociation of hot photoions, involving nontrivial hydrogen migration.