Unimolecular decomposition of formic acid cation (HCOOH+): Theory and experiment

The unimolecular decomposition of formic acid, FA, upon single-photon ionization is investigated by means of synchrotron radiation coupled to a photoelectron/Auger electron-photoion coincidence setup, allowing to identify the ionic products coming from state-to-state fragmentation upon ionization. We show that the C-O bond breaking and the proton ejection from ionized FA are the major processes, whereas H2O+ formation is a minor channel. For interpretation, we performed theoretical computations using post-Hartree-Fock ab initio computations to determine the neutral and ionic species formed during these experiments and the corresponding dissociation channels. We also mapped the potential energy surfaces of FA+ of doublet and quartet electronic states and its isomers along the reactive coordinates. Computations reveal that the major products are HCO+ or COH+ and HOCO+ or HCO2+ isomeric forms, depending on the initially populated parent cationic electronic state. Dissociation proceeds either directly or after intramolecular isomerization processes, via the HOCOH+ or H2OCO+ isomers. We also highlighted the importance of vibronic and spin-orbit couplings during these processes.