REACTIVITY OF FE-N, CO-N, AND CU-N CLUSTERS WITH O-2 AND D-2 STUDIED AT SINGLE-COLLISION CONDITIONS

This paper describes a method to study the reactivity of neutral duste rs at single-collision-like conditions, which enables the determinatio n of absolute numbers for the reaction probability (S) in a collision. A beam, of clusters is produced in a laser vaporization source and sk immed and passes a cell with reactive gas, in which the clusters exper ience one or a few collisions with the gas molecules, The reaction pro ducts are detected with laser ionization and mass spectrometry. The de pletion of pure dusters and the appearance of products are evaluated w ith a statistical model providing S for the first, second, etc., molec ule adsorbed. The O-2 and D-2 reactivity of Fe-n, Co-n, and Cu-n has b een investigated for clusters in the approximate size range 10-60 atom s. 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 rem ains high and almost constant; S approximate to 0.7 for both Fe-n and Co-n. The low O-2 reactivity measured for the small transition metal c lusters may be an effect of the products having a short lifetime due t o the high exothermicity of the oxidation reaction. For copper cluster s there are repeated minima in the O-2 reactivity appearing at duster 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 COn exhi bits large size to size fluctuations. Cu-n and small Fe-n appear unrea ctive (detection limit S approximate to 0.02) toward D-2, whereas the larger Fe-n (n greater than or equal to 23) react with a low probabili ty.