Double ionization of small organic molecules followed by roaming-mediated isomerization has been proposed as a potential source of $\mathrm{H}{{}_3}^{+}$ in the interstellar medium. By integrating experimental and theoretical approaches, we report a different mechanism that significantly influences the $\mathrm{H}{{}_3}^{+}$ yields from methanol dication. The experimental investigation employed strong laser-field-induced double ionization of methanol, with fragment ions detected in coincidence using an ion imaging spectrometer. Notably, a non-negligible yield of the ${\mathrm{H}}^{+}+\mathrm{CH}{\mathrm{O}}^{+}$ coincident channel was observed, characterized by low kinetic energy release (KER) that deviates from predictions based on the Coulomb explosion model. Ab initio molecular dynamics simulations reveal that a portion of the generated $\mathrm{H}{{}_3}^{+}$ resides in highly energetic internal states, triggering a sequential fragmentation pathway that leads to the low-KER ${\mathrm{H}}^{+}+\mathrm{CH}{\mathrm{O}}^{+}$ channel. The simulations demonstrate that approximately 27% of the produced $\mathrm{H}{{}_3}^{+}$ dissociates into ${\mathrm{H}}^{+}+{\mathrm{H}}_2$ within a few-hundred femtoseconds. These findings offer insights into both the formation and degradation mechanisms of $\mathrm{H}{{}_3}^{+}$ in the interstellar medium.

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• Atomic & molecular structure
• Molecular dissociation