Measured and calculated mobilities of cluster ions drifting in helium and in nitrogen

New experimental results are presented for the mobilities of (1) the H3O+(H2O)(3) cluster ion drifting in He (the reduced zero-field mobility K-0((0)) is 11.5 +/- 0.4 cm(2) V-1 s(-1), (2) NO+(CH3COCH3)(n) cluster ions drifting in He (K-0((0)) = 7.5 +/- 0.4 cm(2) V-1 s(-1) for n = 2, K-0((0)) = 7.3 +/- 0.5 cm(2) V-1 s(-1) for n 3), and (3) the NO+(CH3CN)(2) cluster ion drifting in N-2 (K-0((0))) = 1.70 +/- 0.06 cm(3) V-1 s(-1)). These results together with other recently reported cluster ion mobilities in He are quantitatively explained using the hard-sphere collision model by von Helden et al. (J. Phys. Chem. 97 (1993) 8182-8192). The role of ion-induced dipole interactions is investigated and shown to be minor for the mobilities of all but the smallest ions in He. The existing data on cluster-ion mobilities in N-2. including the result here, are systematized. As a result of the stronger polarizability of N-2, the hard-sphere collision model does not agree with the data, and a better explanation is obtained using a simple model that includes ion-induced dipole interactions. The size of the cluster has only a minor effect on the mobility in N-2. The remaining discrepancies between the measured and calculated results are attributed to ion-quadrupole or dipole induced-dipole interactions. (C) 1997 Elsevier Science B.V.