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∼35, tin clusters track the prolate growth pattern previously found for Si_n and Ge_n. However, the detailed size-dependent variations start deviating from those observed for Si_n above n=14 and Ge_n above n=21. Over the n∼35–65 size range, tin 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.