Investigation of H2 formation in electron-impact double ionization and fragmentation of tetrahydrofuran by multiparticle coincidence momentum imaging and ab initio calculations

We report the fragmentation dynamics of double ionization of tetrahydrofuran (THF) induced by electron impact (E₀=200 eV). Using a multiparticle coincidence momentum spectrometer, the hydrogen molecule (H₂) production channel, i.e., double ionization and fragmentation of THF⁺⁺→H₂+C₃H₅⁺+CHO⁺, is observed and identified using an ion-ion coincidence map and the correlated projectile energy-loss spectrum. The measured kinetic energy release, combined with ab initio molecular-dynamics simulations and high-level potential energy surface (PES) calculations, reveals the detailed fragmentation dynamics of the H₂ production channel. From the measured energy-loss spectrum, we determine the initial ionization mechanism of the fragmentation channel as a double-ionization excited state (3.3 eV above the ground state). The PES calculations reveal several H₂ formation pathways, which are all below the energy of the excited state and hence can proceed barrierless. We find that the H₂ formation and ejection occur on an ultrafast timescale (approximately $50$ fs), which is followed by ring opening via the C--O bond cleavage and ultimate Coulomb explosion. The present observation of the state-resolved fragmentation mechanism and dynamics is expected to provide valuable insights into the formation process of hydrogen molecules in the interstellar medium, as well as hydrogen storage and production.