Doubly ionized OCS bond rearrangement upon fragmentation - experiment and theory

The dissociation of OCS²⁺ ions formed by photoionization of the neutral molecule at 40.81 eV is examined using threefold and fourfold electron-ion coincidence spectroscopy combined with high level quantum chemical calculations on isomeric structures and their potential energy surfaces. The dominant dissociation channel of [OCS]²⁺ is charge separation forming CO⁺ + S⁺ ion pairs, found here to be formed with low intensity at a lower-energy onset and with a correspondingly smaller kinetic energy release than in the more intense higher energy channel previously reported. We explain the formation of CO⁺ + S⁺ ion pairs at low as well as higher ionization energies by the existence of two predissociation channels, one involving a newly identified COS²⁺ metastable state. We conclude that the dominant CO⁺ + S⁺ channel with 5.2 eV kinetic energy release is reached upon OCS²⁺ → COS²⁺ isomerization, whereas the smaller kinetic energy release (of ∼4 eV) results from the direct fragmentation of OCS²⁺ (X³Σ^-) ions. Dissociation of the COS²⁺ isomer also explains the existence of the minor C⁺ + SO⁺ ion pair channel. We suggest that isomerization prior to dissociation may be a widespread mechanism in dications and more generally in multiply charged ion dissociations.