We report the observation of a femtosecond strong-laser-field-ionization-induced bimolecular reaction in the acetylene dimer ${({\mathrm{C}}_2{\mathrm{H}}_2)}_2$. 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 ${\mathrm{C}}_4{\mathrm{H}}_4^{+}$ isomer. A delayed probe laser pulse drives secondary ionization, triggering further hydrogen migration and ultimately fragmenting the system into ${\mathrm{CH}}_3^{+}$ and ${\mathrm{C}}_3{\mathrm{H}}^{+}$. 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.

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• Atomic & molecular processes in external fields
• Molecular dissociation
• Multiphoton or tunneling ionization & excitation
• Strong field ionization & excitation
• Ultrafast femtosecond pump probe