Raman spectra of size-selected silicon clusters and comparison with calculated structures

SMALL clusters of silicon, containing between 2 and 100 atoms, have been studied extensively both because of their intrinsic interest from the point of view of chemical structure and bonding and because of the potential technological applications of cluster-assembled materials^1-3. Ab initio quantum-chemical calculations predict that very small clusters should have structures markedly different from that of the crystalline phase^4-12. Experiments on two- (ref. 13), three- (ref. 14) and four-atom¹⁵ clusters have allowed some comparison with theoretical predictions, but structural information on larger clusters has been only indirect¹⁶. Here we report on structural studies of size-selected Si₄, Si₆ and Si₇ clusters prepared and isolated by low-energy deposition into a solid nitrogen matrix. Surface plasmon-polariton enhanced Raman spectroscopy^17-20 yields well resolved vibrational spectra for each of these clusters in which the vibrational frequencies agree well with those predicted for optimized structures calculated by ab initio methods. We confirm that Si₄ is a planar rhombus, and find that Si₆ is a distorted octahedron and Si₇ a pentagonal bipyramid.