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By analyzing the vibrational eigenmodes of a model structure obtained previously by quantum molecular dynamics, the two sharp features appearing in the Raman spectrum of vitreous silica could conclusively be assigned to three-and four-membered ring structures embedded in the network. The projections of the eigenmodes onto oxygen breathing motions in these rings yield well defined peaks at frequencies in striking correspondence to the positions of the Raman lines. The good agreement between calculated and measured shifts upon isotopic substitution further confirms this assignment.