Photophysics of nanometer sized metal particles: Electron-phonon coupling and coherent excitation of breathing vibrational modes

The wide variety of applications of metal nanoparticles has motivated many studies of their properties. Some important practical issues are how the si ze, composition and structure of these materials affect their catalytic and optical properties. In this article we review our recent work on the photo physics of metal nanoparticles. The systems that have been investigated inc lude Au particles with sizes ranging from 2 nm diameter (several hundred at oms) to 120 nm diameter, and bimetallic core-shell particles composed of Au , Ag, Pt and/or Pb. These particles, which have a rather narrow size distri bution, are prepared by radiolytic techniques. By performing time-resolved laser measurements we have been able to investigate the coupling between th e electrons and phonons in the particles, and their low frequency "breathin g" modes. These experiments show that for Au the time scale for electron-ph onon coupling does not depend on size, in contrast to metals such as Ga and Ag. On the other hand, the frequency of the acoustic breathing modes stron gly depends on the size of the particles, as well as their composition. The se modes are impulsively excited by the rapid lattice heating that accompan ies ultrafast laser excitation. The subsequent coherent nuclear motion modu lates the transmitted probe laser intensity, giving a "beat" signal in our experiments. Unlike quantum-beats in molecules or semiconductors, this sign al can be completely understood by classical mechanics.