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 ab initio 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.