MIXED METALLIC IONIC CLUSTERS OF SILVER SILVER-IODIDE

We have sputtered isotopically enriched silver foil Ag-107 in the source of a double-focusing mass spectrometer (reverse geometry) in the presence of methyl iodide vapor. We have detected the formation of a range of positively and negatively charged iodinated silver clusters of formula [Ag(x)I(y)]k along with purely metallic silver clusters. The important observations are as follows: (1) The cluster mass peak intensities, in accord with the spherical jellium model, showed n-odd/n-even alternation in intensity and in some cases closed-shell behavior when plotted against the number of delocalized electrons, n, where n = X - Y - k and k is the overall charge on the cluster. This suggests that as the iodine atoms are added to the cluster, equal numbers of AgI ionic bonds are formed which localizes an equal number of previously delocalized silver valence electrons on the iodine atoms. (2) The n-odd/n-even intensity oscillation is more pronounced at lower X than at higher X. (3) Cluster species of unusually low intensity either have one delocalized electron in excess of that necessary to complete a jellium shell closing or are one delocalized electron deficient of a shell closing. (4) The purely metallic positively charged Ag clusters are formed with greater intensity in the presence of CH3I, suggesting that their formation and stability are enhanced by the reaction. (5) Although the positively charged iodinated clusters are stronger in intensity than the negatively charged iodinated clusters, the maximum number of iodines that are added is higher for the negatively charged metallic clusters than for the positively charged clusters which suggests the importance of a charge-transfer mechanism in the iodine addition process. (6) The intensity of all clusters decreases quasiexponentially with cluster size and with the number of halogens contained, suggesting that these clusters are formed from smaller ones in the gas phase by multiple collisions.