Hydrogen migration reactions via low internal energy pathways in aminobenzoic acid dications

Abstract

<p>Hydrogen migration is a ubiquitous phenomenon upon dissociation of organic molecules. Here we investigate the formation of a H₃O+ fragment after core-level photoionization and Auger decay in aminobenzoic acid molecules - a process- that requires the migration of at least two hydrogen atoms. Using photoelectron-photoion coincidence spectroscopy, the formation of a H₃O+ fragment is observed to be more probable in <em>ortho</em>-aminobenzoic acid than in <em>meta</em>- and <em>para</em>-aminobenzoic acid. Energy-resolved Auger electron-photoion coincidences are measured for the <em>ortho</em>-isomer to investigate the internal energy dependence of the fragmentation channels, most notably of those producing H₃O+. The corresponding fragmentation channels and their mechanisms are investigated by exploring the potential energy surface with <em>ab initio</em> quantum chemistry methods and molecular dynamics simulations. Excited-state modeling of dicationic <em>ortho</em>-aminobenzoic acid is used to interpret features in the Auger spectra and identify the electronic states contributing to the signals in the Auger electron photoion coincidence map. We show that populating low-energy excited states of the dication is sufficient to trigger hydrogen migration and produce H₃O+ efficiently.<br></p>