Low-energy electron attachment to mixed ozone/oxygen clusters

Electron attachment to a cluster beam formed by adiabatic expansion of a mixture of O3 (1%) and O2 (99%) is studied in the energy range 0–4 eV. Despite the initial large excess of oxygen molecules, the dominant attachment products are undissociated cluster ions (O3)m− including the monomer O3−, while oxygen cluster ions (O2)n− appear with comparatively low intensity. This behaviour is explained by an enrichment of ozone in the cluster formation process and the preferential formation of O3− from mixed clusters. The structured energy dependence of the cross section of O3− formation is interpreted in terms of three different mechanisms, in the low-energy region by s-wave capture, around 1 eV via Feshbach resonances, and above 1.5 eV by self-scavenging, i.e. inelastic scattering of the primary electron involving low-lying electronic states of neutral ozone and subsequent attachment of the slowed-down electron to another ozone molecule in the same cluster.