A detailed study of the reactions between size selected aluminum cluster ions, Al+n (n=3–26), and oxygen

Abstract

A detailed study of the reactions between size selected aluminum cluster ions and oxygen is presented. The experiments were performed using a low energy ion beam apparatus. Measurements of product distributions and total reaction cross sections at collision energies of 1.2 and 4.2 eV for aluminum cluster ions with between 3 and 26 atoms are reported. The total reaction cross sections increase with cluster size in a way which roughly correlates with the increase in the cluster’s physical size. The main products are Al⁺_n−4, Al⁺_n−5, Al⁺_n−6, and Al⁺. Only a very small fraction of the product ions contain oxygen. We suggest that the reaction occurs by chemisorption of O₂ onto the cluster followed by rapid loss of two Al₂O molecules to give Al⁺_n−4. If the Al⁺_n−4 fragment contains sufficient energy it will undergo further dissociation by loss of one or more aluminum atoms to give Al⁺_n−5, Al⁺_n−6, and Al⁺. RRKM theory is used to estimate the amount of energy above the dissociation threshold required to cause dissociation of the bare clusters on the experimental time scale. For the larger clusters this excess energy is remarkably large. Using this data we are able to deduce some information about energy disposal in the reaction. It is likely that the Al₂O molecules carry away a substantial fraction of the exothermicity arising from chemisorption of oxygen onto the clusters.