Photodissociation of negative ions and their clusters

Abstract

Studies of the dissociation dynamics of cluster ions provide insight into the process of energy disposal for mass-selected species. Detailed investigations are reported for two systems; one, CO^–₃(H₂O)_n(0 ⩽n⩽ 3), comprising a hetermolecular system and the other, (SO₂)^–₂, a homomolecular anion. In the case of CO^–₃, the unhydrated parent ion is observed to have a bound electronic excited state, through which absorption of a second photon proceeds to a repulsive state, leading to the ejection of O^–. Cluster dissociation is initiated by the ²A₁â†�²B₁ transition from the ground to a weakly bound excited state of the core ion, leading to the loss of all water molecules within the time of observation. This is in direct contrast to collisional dissociation processes, which lead largely to the loss of a single water molecule per collision. In the photodissociation of CO^–₃, CO^–₃(H₂O)_1,2,3, and (SO₂)^–₂, considerable excess energy is partitioned into relative translation of the photoproducts. Through studies of energy release in (SO₂)^–₂ with photons of parallel and perpendicular polarization, evidence has been obtained that the lifetime of the complex preceding photodissociation is less than a rotational period. The implications of the findings are discussed in terms of phase-space theory.