Organometallic clusters of lanthanide (Ln = Nd, Er, Eu, and Yb) and 1,3,5,7-cyclooctatetraene (C 8 H 8 ) were produced by a combination of laser vaporization and molecular beam methods. In the mass spectra of [Lnn(C 8 H 8 )m], compositions of m = n + 1 were magic numbers. From mass spectrometry, photoionization spectroscopy, and photoelectron spectroscopy, we have concluded that these magic-numbered clusters take multiple-decker sandwich structures in which Ln atoms and C 8 H 8 molecules are alternatedly piled up. These sandwich clusters are formed through ionic bonds composed of multiply charged cations [Ln k+ (k = 2 and 3)] and multiply charged anions [C 8 H h− 8 (h = 1, 1.5, and 2)].PACS. 36.40.-c Atomic and molecular clusters -36.40.Mr Spectroscopy and geometrical structure of clusters -61.46.+w Clusters, nanoparticles, and nanocrystalline materials -71.24.+q Electronic structure of clusters and nanoparticles