Facilitating role of hydrogen migration in the fragmentation dynamics of acetonitrile dications

We investigate the electron-impact double ionization and subsequent fragmentation dynamics of acetonitrile using multiparticle coincident momentum spectroscopy. Through the measurements of the kinetic energy release for the two-body Coulomb explosion processes and the potential energy calculations in the reaction coordinate, we identify several fragmentation channels that are typically not accessible via direct Coulomb explosion, necessitating prior isomerization, which dominates over the direct Coulomb explosion channel. The potential energy calculations reveal that hydrogen migration reduces the energy barrier of C-C bond cleavage, thereby promoting the fragmentation of acetonitrile dications. Moreover, we find that, instead of transferring to the nitrogen atom yielding the double hydrogen migration channel, the roaming ${\mathrm{H}}_{2}$ captures a proton or transfers an electron to the remaining moiety, leading to the formation of ${\mathrm{H}}_{3}{}^{+}$ and ${\mathrm{H}}_{2}{}^{+}$ cations. This work provides insight into the competitive interplay between hydrogen migration and Coulomb explosion mechanisms in molecular fragmentation.