Doubly ionized OCS bond rearrangement upon fragmentation - experiment and theory.

The dissociation of OCS 2+ 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 COS 2+ metastable state. We conclude that the dominant CO + + S + channel with 5.2 eV kinetic energy release is reached upon OCS 2+ → COS 2+ isomerization, whereas the smaller kinetic energy release (of ∼4 eV) results from the direct fragmentation of OCS 2+ (X 3 Σ - ) ions. Dissociation of the COS 2+ 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.