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

Abstract

New experimental results are presented for the mobilities of (1) the H 3O +(H 2O) 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 +(CH 3COCH 3) 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 +(CH 3CN) 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.