VIBRATIONALLY RESOLVED PHOTOELECTRON SPECTRA OF ANNEALED ${\rm C}_n^-$ AND ${\rm Si}_n^-$ CLUSTERS

Photoelectron spectra of annealed carbon-cluster anions n=5–70, are presented which show vibrational fine structure. From the vibrational frequencies, theoretical results, and ion mobility measurements, the geometry of the clusters can be determined: clusters with n=5, 7, and 9 are linear chains; n=10, 12, 14, 16, and 18 build monocyclic rings; for n=20, 24, and 28 the vibrational features correspond best to bicyclic rings; and for even n>30 fullerenes are found. In contrast to carbon, the mass distribution of silicon-cluster anions (up to n≈30) does not change dramatically upon annealing. An enhanced abundance of typical fragmentation products with n=4, 6, 7, and 10 is found. The vibrational fine structure for agrees with that of a pentagonal bipyramid.