Investigating the behavior of polyatomic molecules in strong laser fields allows the examination of coupled nuclear and electronic dynamics. In this article we examine multibody fragmentation of formic acid dications, produced by ultrashort intense laser pulses, using coincidence momentum imaging and deuterium tagging to distinguish the hydrogen sites. Our measurements indicate that double hydrogen elimination leads to three- and four-body breakup, specifically $\mathrm{H}+\mathrm{D}+{\mathrm{CO}}_2^{2+}$ and $\mathrm{H}+\mathrm{D}+{\mathrm{O}}^{+}+{\mathrm{CO}}^{+}$, respectively. We find that the latter breakup channel also proceeds via the ${\mathrm{CO}}_2^{2+}$ metastable dication, but with shorter lifetimes than the ${\mathrm{CO}}_2^{2+}$ dications detected intact. This intriguing sequential breakup is identified using native frames analysis and simulations to confirm the signature of this process in spite of the inferior momentum resolution associated with breakup channels for which not all fragments are detected. In addition to the fragmentation channel above, we found no evidence of sequential breakup for single hydrogen elimination, $\mathrm{H}+{\mathrm{CO}}^{+}+{\mathrm{OD}}^{+}$. The elimination of three atoms, $\mathrm{H}+\mathrm{D}+\mathrm{O}+{\mathrm{C}}^{+}+{\mathrm{O}}^{+}$, shows evidence of both concerted fragmentation and sequential breakup via metastable intermediate ${\mathrm{CO}}^{2+}$ ions. Measuring the dissociation dynamics of these highly excited molecular ions yields information that can be used to test theoretical approaches to these challenging few-body systems.

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• Atomic & molecular processes in external fields
• Molecular dissociation
• Multiphoton or tunneling ionization & excitation