Reactivity of Fen, Con, and Cun Clusters with O2 and D2 Studied at Single-Collision Conditions

This paper describes a method to study the reactivity of neutral clusters at single-collision-like conditions, which enables the determination of absolute numbers for the reaction probability ( S ) in a collision. A beam of clusters is produced in a laser vaporization source and skimmed and passes a cell with reactive gas, in which the clusters experience one or a few collisions with the gas molecules. The reaction products are detected with laser ionization and mass spectrometry. The depletion of pure clusters and the appearance of products are evaluated with a statistical model providing S for the first, second, etc., molecule adsorbed. The O 2 and D 2 reactivity of Fe n, Co n, and Cu n has been investigated for clusters in the approximate size range 10−60 atoms. The oxidation of transition metal clusters, here exemplified by Co n and Fe n, shows a simple S vs n dependence, where S increases almost monotonically as n increases from 10 to 20, while for larger n, S remains high and almost constant; S ≈ 0.7 for both Fe n and Co n . The low O 2 reactivity measured for the small transition metal clusters may be an effect of the products having a short lifetime due to the high exothermicity of the oxidation reaction. For copper clusters there are repeated minima in the O 2 reactivity appearing at cluster sizes that are known to have high IP and closed electronic shells. Co n is much less reactive toward D 2 than O 2, and S for D 2 on Co n exhibits large size to size fluctuations. Cu n and small Fe n appear unreactive (detection limit S ≈ 0.02) toward D 2, whereas the larger Fe n ( n ≥ 23) react with a low probability.