Raman spectroscopy of naphthalene clusters. Evidence for a symmetrical trimer and an unsymmetrical tetramer

Mass-selective, ionization-loss stimulated Raman spectra of intramolecular and intermolecular vibrational transitions in isotopomers of naphthalene trimer and tetramer are reported. The intramolecular results provide strong evidence that the trimer has three equivalent, or near-equivalent monomer sites. The corresponding results for the tetramer show that that species has four inequivalent sites. The intermolecular spectra also exhibit clear manifestations of a high-symmetry geometry for the trimer and a low-symmetry one for the tetramer. In addition, they provide evidence that the most Raman-active intermolecular bands are those that involve librations of the naphthalene moieties.