Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

the specific system studied—Pt–Au core–shell particles—the steady-stateoptical properties are determined by the Au shell, but the dynamics aredominated by the Pt core. This occurs because Pt has a much larger density ofelectronic states than Au [19].The rapid electronic and lattice heating that accompanies ultrafast laserexcitation can also coherently excite the phonon mode that correlates with theexpansion coordinate of the particles. For spherical particles, the symmetricbreathing mode is observed in transient absorption experiments. The measuredperiods exactly match the predictions of continuum mechanics calculations[12,13]. In addition, the amplitude and phase of the modulations are inexcellent agreement with model calculations, where the expansion coordinateis treated as an harmonic oscillator [12,13,15,16] and the driving force forexpansion arises from the thermal stresses created by heating the electronsand the nuclei [38,39]. These results show that both direct and indirectcoupling between the laser excited electrons and the symmetric breathingmode are important. For rod-shaped particles, ultrafast laser excitationgenerates a Rayleigh surface wave, which has a frequency that depends onthe length of the rod [45]. This observation is very different compared torecent results from Perner and co-workers, who studied Ag ellipses in a solidmatrix [15]. This shows that the vibrational dynamics of nonsphericalparticles is very complex. Coherently excited vibrational modes can also beobserved for bimetallic particles. For core–shell particles composed ofmaterials with very different elastic properties, the interface between the coreand the shell must be explicitly taken into account in continuum mechanicscalculations of the breathing mode [17,48].ACKNOWLEDGMENTSThe work described in this chapter was supported by the National ScienceFoundation by grants No. CHE98-16164 and CHE02-36279. The author isextremely grateful to his students for performing the work (primarily JoseHodak), to Professor Arnim Henglein and Professor Paul Mulvaney for providingthe metal particle samples, and to Professor John Sader for performingthe contiuum mechanics calculations for the core–shell nanoparticles.REFERENCES1. Link, S.; El-Sayed, M.A. J. Phys. Chem. B 1999, 103, 8410.2. Hodak, J.H.; Henglein, A.; Hartland, G.V. J. Phys. Chem. B 2000, 104, 9954.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.