Mass spectrometry and beam deflection studies of tin–lead nanoalloy clusters

Abstract

Photo-ionization mass spectrometry and electric beam deflection experiments were used to study isolated Sn_MPb_N clusters (7 ≤N + M≤ 13 for tin-rich clusters, 7 ≤N + M≤ 15 for lead-rich clusters) in a molecular beam apparatus. The observed mass spectra reveal a broad abundance distribution of the bimetallic clusters in which all possible cluster compositions can be identified within the investigated size ranges. Comparison of the relative cluster intensities between pure tin or lead clusters (Sn_N+M and Pb_N+M) and mixed Sn_MPb_N clusters indicate quite similar relative abundance distributions which can be smoothly shifted from one to the other extreme by changing the composition. The mass spectroscopic findings could be explained by assuming a substitution “alloy” formation in the Sn_MPb_N cluster system. In combination, the dielectric properties were determined by passing the bimetallic clusters through an inhomogeneous electric field. The observed polarizabilities are significantly increased for most of the bimetallic clusters. This can be explained in an adiabatic polarization model by the presence of permanent electric dipole moments. These observations demonstrate how the electronic properties are not only crucially influenced by the cluster size but also by the composition of this nanoalloy model system. In addition to the enhanced polarizability, most of the measured beam profiles for tin-rich clusters show detectable beam broadenings due to the permanent dipole moments, in contrast to lead-rich clusters which possess considerable smaller dipole moments. Molecular dynamic simulations of the measured beam profile for Sn₆Pb₁ taking theoretically calculated isomeric structures and dipole moments into account yields no completely satisfying outcome. Therefore we discuss possible reasons for the discrepancy between experimental and theoretical results.