Fullerene–C60 in Contact with Alkali Metal Clusters: Prototype Nano-Objects of Enhanced First Hyperpolarizabilities

The electric first hyperpolarizabilities (β) of a representative set of alkali metal droplets in contact with fullerene (C60) have been explored at the static limit via ab initio and density functional theory methods for the first time. We find that, when alkali metal droplets are adsorbed on the surface of C60, systems of enhanced static dipolar and/or octupolar hyperpolarizabilities are delivered. Both the type and the magnitudes of the first hyperpolarizabilities in such systems are dictated by the way the sodium atoms adsorb on the surface of fullerenes. One large metallic droplet of sodium atoms results to species with large hyperpolarizabilities of dipolar character. Smaller droplets adsorbed on the surface of C60 deliver systems of large octupolar contributions. In both cases two synergic polarization mechanisms have been detected by means of the classical version of configuration interaction singles (CIS) sum-over-states approach and a natural transition orbital analysis. The first comprises charge transport from the fullerene to the adsorbed droplets and resembles the polarization process met in simple diatomic molecules. In this case, C60 unconventionally functions as an electron donor at the excited states. The second, local in character, is related to the easily polarizable excess electrons maintained in the framework of the adsorbed clusters. From a certain point of view, such systems can be considered as hybrids that combine the basic characteristics of a classical donor/acceptor superstructure and systems with easily polarizable excess electrons.