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 materials1-3. Ab initio quantum-chemical calculations predict that very small clusters should have structures markedly different from that of the crystalline phase4-12. 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 spectroscopy17-20 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.