Quadrupolar vibrational mode of silver clusters from plasmon-assisted Raman scattering

Absorption and low-frequency Raman-scattering experiments have been perform ed on thin films consisting of small silver clusters embedded in a porous a lumina matrix. When the Raman excitation wavelength is close to the maximum (approximate to 420 nm) of the Mie band (dipolar surface plasmon resonance ) the Raman spectra exhibit a strong band located around 10 cm(-1), the max imum of which depends on the cluster diameter D at the maximum of the clust er-size distribution in the sample according to the approximate law omega(v ib)proportional to D-1. The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters, via the plasmon-phonon interac tion. Moreover, the maximum of the Raman band shifts towards lower frequenc ies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is shown to reflect the ellipsoidal sha pe distribution of part of the embedded clusters. [S0163-1829(99)02048-2].