Ultrafast Bimolecular Reaction in Acetylene Dimer Induced by Femtosecond Strong-Laser-Field Ionization

We report the observation of a femtosecond strong-laser-field-ionization-induced bimolecular reaction in the acetylene dimer (C_{2}H_{2})_{2}. Single ionization by the pump laser pulse produces a bound cationic intermediate that undergoes ultrafast intermolecular rearrangement involving hydrogen migration and carbon-carbon backbone reorganization, forming a covalently bound C_{4}H_{4}^{+} isomer. A delayed probe laser pulse drives secondary ionization, triggering further hydrogen migration and ultimately fragmenting the system into CH_{3}^{+} and C_{3}H^{+}. Time-resolved fragment yields, corroborated by molecular dynamics simulations, reveal a multistep reaction pathway characterized by bond formation and structural reorganization on the subpicosecond timescale. These results uncover a previously unobserved strong-laser-field-driven bimolecular reaction in a prototypical hydrocarbon dimer and demonstrate that time-delayed ionization provides a general route for steering chemical reactivity in weakly bound molecular systems.