Tin clusters adopt prolate geometries

We have characterized the structures of Sn-n cations up to n = 68 using ion mobility measurements. Up to n similar to 35, tin clusters track the prola te growth pattern previously found for Si-n and Ge-n. However, the detailed size-dependent variations start deviating from those observed for Si-n abo ve n = 14 and Ge-n, above n = 21. Over the n similar to 35-65 size range, t in clusters gradually rearrange towards near-spherical geometries, passing through several intermediate structural families. Two or three isomers are resolved for some sizes in the n = 18-49 range. The observed geometries are independent of the He buffer gas temperature between 78 and 378 K and are not affected by collisional annealing. [S1050-2947(99)02608-6].