Ultrafast Laser-Induced Isomerization Dynamics in Acetonitrile*

We will present the investigation of acetonitrile (CH₃CN) isomerization induced by laser ionization using pump-probe spectroscopy paired with ion-ion coincident Coulomb explosion imaging. Five primary channels indicating direct C-C breakup, single and double hydrogen migration, and H and H₂ dissociation in the acetonitrile cation were found. Unexpectedly, the hydrogen-migration channels dominate over direct fragmentation. This observation is validated by quantum chemistry calculations showing that isomerization through single and double hydrogen migration leads to very stable linear and ring isomers, most of them more stable than the original linear structure following ionization of the parent molecule. This is distinct from a previous investigation on ethanol using similar experimental technique. We have also determined the timescales of the corresponding dynamical processes by varying the delay between the pump and probe pulses and found that the isomerization occurs within a few hundred femtoseconds.