Femtosecond Real-Time Observation of Tunneling Autodetachment from Dipole-Bound States Reveals Structural Dynamics of Sodium Halide Dimers

Femtosecond time-resolved photoelectron spectroscopy combined with <i>ab initio</i> electronic structure and molecular dynamics calculations reveals the real-time structural and electronic evolution of sodium halide dimers, (NaCl)₂ and (NaI)₂, following vertical electron detachment from their corresponding anions. Both pseudo-linear dimer anions transform into rhombic neutral geometries upon electron detachment. Despite the weak binding of the excess electron, the dipole-bound state (DBS) of each anion temporarily traps the electron within a finite potential well, creating a tunneling barrier that governs autodetachment. The estimated lifetimes of DBS are ∼4 ps for (NaCl)₂^- and ∼40 ps for (NaI)₂^-, in excellent agreement with theoretical tunneling barrier heights of ∼50 and ∼100 meV, respectively. These results constitute the first direct experimental observation of tunneling autodetachment from a DBS and demonstrate how femtosecond structural relaxation of the neutral core drives quantum mechanical electron decay.