Formation of negative cluster ions from (CO₂)_m in collision with high-Rydberg atoms

Negative ions of CO₂ clusters (CO₂)_m produced by electron transfer from highly excited Rydberg atoms were investigated by mass spectrometry. The relative intensities of (CO ₂)_n ^- (n≥3) (size distribution) showed a characteristic structure with magic numbers at 9, 14, and 16. The dependence of the intensities of (CO₂)_n ^- (n = 7-20) on the stagnation pressure of the cluster beam source was measured. The threshold pressure at which a cluster anion of a given size started to appear increased irregularly with the size. These observations suggest that evaporation of CO₂ molecules occurs by dissipation of the excess energy after electron attachment and that the number of evaporating molecules m-n depends on the size of the resulting ion. The evaporating molecules are fewer at n = 9, 14, and 16 than those at adjacent sizes. The cluster ions of these sizes are shown to have higher dissociation energies than other ions, and the magic numbers observed in the mass spectrum are ascribed to the difference in the stability of the ions.