Charge-partition pathways in strong-field photoionization of carbonyl sulfide monomers and dimers

Strong-field photoionization of molecules and molecular clusters gives rise to a rich variety of fragmentation pathways governed by charge localization and redistribution on ultrafast timescales. Here, we report a velocity-map imaging study of the strong-field photoionization and fragmentation of carbonyl sulfide (OCS) monomers and dimers driven by femtosecond laser pulses at 775 nm. The images of the total-kinetic-energy and angular distributions of the OCS²⁺, S⁺, and CO⁺ products were interpreted with the help of electronic-structure calculations of the potential energy surfaces for OCS⁺ and OCS²⁺. We identify distinct dissociation pathways of singly and doubly ionized OCS, including two-body breakup channels of OCS⁺ into S⁺ + CO and CO⁺ + S, dissociation of OCS²⁺ into S⁺+ CO⁺ as well as higher-order three-body fragmentation. In addition, the images of the OCS²⁺ channel exhibit near-zero-momentum components, low-energy isotropic features, and highly anisotropic contributions at high kinetic energies that cannot be explained by monomer ionization alone. Analysis of the kinetic-energy release (KER) distributions and angular anisotropies indicates that these features originate from the breakup of multiply charged OCS dimers ((OCS)₂²⁺, (OCS)₂³⁺, and (OCS)₂⁴⁺) through charge-separation channels. Our results illustrate how dynamic signatures of strong-field fragmentation evolve from intramolecular dissociation in isolated molecules to intermolecular charge separation in weakly bound clusters providing a unified picture of charge-driven dissociation dynamics beyond the single-molecule limit.