Photoabsorption of negative cluster ions near the electron detachment threshold: A study of the (O2)−n system

In order to better understand the propensity of negative cluster ion systems to photodissociate when excited near their photodetachment thresholds, we present a detailed photochemical study of the oxygen cluster negative ions, (O2)−n. Irradiation of the 3≤n≤6 parent ions at or slightly below their photodetachment thresholds in the near ir (1064 nm) is found to result in significant photofragmentation, even though the dimer is effectively photostable at this wavelength. The cross sections for n≥3 photofragmentation are approximately constant over the higher clusters and are about a factor of 3 larger than the O−2 photodetachment cross section. These observations suggest that photoabsorption of the higher clusters may result from a charge-transfer process between the O−4 core ion and the ‘‘solvent’’ O2 ligands.