Thermally induced polarizabilities and dipole moments of small tin clusters.

We study the influence of thermal excitation on the electric susceptibilities for Sn(6) and Sn(7) clusters by molecular beam electric deflection and Monte-Carlo simulations in conjunction with quantum-chemical calculations. At low temperatures (40 K), no field-induced broadening of the Sn(6) and Sn(7) cluster beams are observed, in agreement with vanishing permanent electric dipole moments due to their centro-symmetrical ground states. The electric polarizabilities of Sn(6) and Sn(7), as inferred from the field-induced molecular beam deflection, are in good agreement with the quantum-chemical predictions. At elevated temperatures of 50-100 K, increased polarizabilities of about 2-3 Å(3) are obtained. Also, we found indications of a field-induced beam broadening which points to the existence of permanent dipole moments of about 0.01-0.02 D per atom at higher temperatures. These results cannot be explained by thermal excitations within a harmonic oscillator model, which would yield a temperature-independent polarizability and fluxional, but not permanent, dipole moments. We analyze this behavior by Monte-Carlo simulations in order to compute average temperature-induced electric dipole moments. For that purpose, we developed a novel technique for predicting observables sampled on the quantum-chemical potential energy surface by an umbrella sampling correction of Monte-Carlo results obtained from simulations utilizing an empirical potential. The calculated, fluxional dipole moments are in tune with the observed beam broadenings. The cluster dynamics underlying the polarizability appear to be intermediate between rigid and floppy molecules which leads to the conclusion that the rotational, not the vibrational temperature seems to be the key parameter that determines the temperature dependence of the polarizability.