A unique property of size-resolved metal nanocluster particles is their superatom-like electronic shell structure. The shell levels are highly degenerate, and it has been predicted that this can enable exceptionally strong superconducting-type electr
on pair correlations in certain clusters composed of just tens to hundreds of atoms. Here we report on the observation of a possible spectroscopic signature of such an effect. A bulge-like feature appears in the photoionization yield curve of a free cold aluminum cluster and shows a rapid rise as the temperature approaches approximately 100 K. This is an unusual effect, not previously reported for clusters. Its characteristics are consistent with an increase in the effective density of states accompanying a pairing transition, which suggests a high-temperature superconducting state with Tc>~100 K. Our results highlight the promise of metal nanoclusters as high-Tc building blocks for materials and networks.
Water clusters embedding a nitric acid molecule HNO3(H2O)_{n=1-10} are investigated via electrostatic deflection of a molecular beam. We observe large paraelectric susceptibilities that greatly exceed the electronic polarizability, revealing the cont
ribution of permanent dipole moments. The moments derived from the data are also significantly higher than those of pure water clusters. An enhancement in the susceptibility for n=5,6 and a rise in cluster abundances setting in at n=6 suggest that dissociation of the solvated acid molecule into ions takes place in this size range.
The induced polarization of a beam of polar clusters or molecules passing through an electric or magnetic field region differs from the textbook Langevin-Debye susceptibility. This distinction, which is important for the interpretation of deflection
and focusing experiments, arises because instead of acquiring thermal equilibrium in the field region, the beam ensemble typically enters the field adiabatically, i.e., with a previously fixed distribution of rotational states. We discuss the orientation of rigid symmetric-top systems with a body-fixed electric or magnetic dipole moment. The analytical expression for their adiabatic-entry orientation is elucidated and compared with exact numerical results for a range of parameters. The differences between the polarization of thermodynamic and adiabatic-entry ensembles, of prolate and oblate tops, and of symmetric-top and linear rotators are illustrated and identified.
