Comparing distinct dissociation pathways of ${\mathrm{C}}_2{\mathrm{H}}_2^{2+}$ under laser and electron impact

We report a comparative study of the dissociation dynamics of ethyne (C₂H₂) molecules and clusters under a strong laser field (9 × 10¹³ W/cm²) and 1 keV electron impact. Using coincidence momentum imaging, we identify several two-body dissociation channels of ${\mathrm{C}}_2{\mathrm{H}}_2^{2+}$, including C₂H⁺ + H⁺, ${\mathrm{CH}}_2^{+}+{\mathrm{C}}^{+}$, symmetric breakup into CH⁺ + CH⁺, as well as dimer dissociation channels such as ${\mathrm{C}}_2{\mathrm{H}}_2^{+}+{\mathrm{C}}_2{\mathrm{H}}_2^{+}$ and ${\mathrm{C}}_2{\mathrm{H}}_2^{2+}+{\mathrm{C}}_2{\mathrm{H}}_2^{+}$. By analyzing the kinetic energy release (KER) and branching ratios of the dominant fragmentation pathways, we demonstrate that double ionization under a strong laser field preferentially leads to dissociation on the singlet ground-state surface (¹Δ_g), whereas electron impact favors the triplet ground state (${}^3{\mathrm{\Sigma }}_{\mathrm{g}}^{-}$). This contrast is attributed to different ionization mechanisms: electron impact follows a propensity rule favoring the lower-energy triplet, while strong-field ionization aligned with the π orbitals promotes singlet formation due to the Pauli exclusion principle. Our findings provide clear evidence of excitation-condition-dependent dissociation pathways in acetylene dications.