Reactive scattering of sodium clusters with molecular oxygen

The first reactive differential scattering study for atomic clusters is reported. Oxidation of Na_x (x8) with O₂ is investigated in a crossed beam apparatus. Sodium oxide (Na_nO,n4) and sodium dioxide (Na_nO₂,n6) are produced with a total reactive cross section from 50 to 80 Ų, depending on the cluster size. The excess energies for these reactions are estimated by an SCF type ab initio calculation and range from 0.5 to 5 eV. The large cross section may then be understood quantitatively in terms of a harpooning mechanism as a first step in the reaction path. Angular distributions have been determined for the most abundant products, showing strong forward scattering. Two different schemes are discussed for the reaction: while the dioxides Na_nO₂ may be formed by an evaporative cooling process from a highly excited collision complex, formation of Na_nO appears to originate from a direct process. In both cases the experimental data suggest that most of the exothermicity remains in the reaction products.