Measurement of static electric dipole polarizabilities of lithium clusters: Consistency with measured dynamic polarizabilities

Static electric dipole polarizabilities of lithium clusters made of n $(n=2--22)$ atoms have been measured. The experiment consists of deflecting a collimated cluster beam through a static inhomogeneous electric field. The strong decrease per atom from Li to Li₃-Li₄ shows that electronic delocalization is reached for very small sizes. Moreover, directly measured polarizabilities are consistent with photoabsorption data. They thus confirm unambiguously the ``missing'' optical strength in lithium clusters.