Plasmon-phonon coupling and resonant raman scattering of silver clusters

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 mean cluster diameter (D) in the sample accord ing to the approximate law omega(vib) proportional to (D)(-1). The Raman ba nd corresponds to the excitation of the quadrupolar vibration mode of the c lusters. Moreover, the maximum of the Raman band shifts towards lower frequ encies when the excitation light is shifted to the red. This feature, as we ll as the rather large Mie-band width, is thought to reflect the ellipsoida l shape distribution of part of the embedded clusters.