Ultrafast proton transfer in femtosecond-laser-induced double ionization of HCOOH-H₂O complexes

We report the experimental observation of ultrafast proton transfer in the HCOOH-H₂O complex following double ionization induced by a linearly polarized two-color femtosecond laser field with controlled relative phase. This process leads to the formation of ionic fragments H₃O⁺ and HCOO⁺, which are detected in coincidence. The measured anisotropic momentum distributions of the fragments reveal a preferential emission direction aligned with the laser polarization. Analysis of molecular orbitals and potential energy surfaces indicates that both electrons involved in the proton transfer process predominantly originate from the HCOOH moiety. Ab initio molecular dynamics simulations show that proton transfer is initiated promptly after ionization and completes within 100 fs. Furthermore, by tuning the relative phase of the two-color laser field, we demonstrate control over the emission direction of the H₃O⁺ fragments. This directional emission is attributed to orientation-dependent tunneling ionization, as supported by simulations based on weak-field asymptotic theory.