Ultrafast Proton and Charge Transfer in the Coulomb Explosion of HCl Dimer

The ultrafast dissociation of hydrogen-bonded clusters offers a fundamental route to studying energy relaxation and intermolecular proton transfer. Here, we investigate the three-body fragmentation of the HCl dimer following strong-field ionization into tricationic states. Angle-resolved kinetic energy measurements reveal distinct breakup mechanisms: a direct Coulomb explosion and the two pathways mediated by intermolecular charge transfer and proton transfer. The kinetic energy distributions of the Cl⁺ and HCl⁺ fragments evolve from a single peak to a double-peak structure as the relative emission angle increases. Ab initio calculations of the tricationic potential energy surface indicate that this splitting arises from competing charge-transfer and proton-transfer driven dissociation pathways. These results illustrate how ultrafast charge redistribution and proton motion govern the strong-field dissociation dynamics of a prototypical hydrogen-bonded cluster.