Bond-breakage-dependent dissociative ionization of an asymmetric molecule in an intense femtosecond laser field

A coincidence momentum imaging method is used to investigate the dissociative ionization of carbonyl sulfide (OCS) irradiated by linearly polarized 800 nm femtosecond laser pulses. As a typical asymmetric molecule, kinetic-energy release (KER) distributions of two-body Coulomb explosion (CE) along C-S or C-O bond breakage from $\mathrm{OC}{\mathrm{S}}^{q+}$ ($q=2$, 3, 4) exhibit a different behavior when the parent charge state increases. Two processes, i.e., concerted enhanced multiple ionization and ladder-climbing-type sequential ionization, are proposed for the C-S and C-O bond breakage, respectively. With the help of the potential-energy curves (PEC) obtained from the multistate density-functional-theory method, the KER values are calculated for these CE channels of $\mathrm{OC}{\mathrm{S}}^{q+}$. The overall good quantitative agreement between the experiment and calculation confirms the validity of the ionization mechanism assumed. In our approach, the global minimum of cationic PEC for the OCS can be treated as ${R}_{c}$ for enhanced ionization. Furthermore, a bimodal KER distribution was observed for the channel of $\mathrm{OC}{\mathrm{S}}^{2+}\ensuremath{\rightarrow}{\mathrm{O}}^{+}+\mathrm{C}{\mathrm{S}}^{+}$, and with the help of PEC, we believe that both of the ionization processes are involved in the dynamics of its formation.