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

with the effect of the particle composition are presented in the first part of thischapter. The size, shape, and composition are the parameters that allow oneto tune the optical properties of metallic nanoparticles over the entire visibleand near-infrared spectral ranges.The decay of the plasmon absorption is due to ultrafast dephasing ofthe coherent electron motion, which occurs on a sub-10-fs timescale [14–16].The processes leading to this dephasing are also briefly discussed in thischapter.The interaction of metal nanoparticles with an ultrashort laser pulse hasbeen of great interest recently, as it allows the selective excitation of theelectrons in a metal nanoparticle [17–23]. The electron relaxation involveselectron–electron scattering, coupling of the hot electrons to lattice vibrations(phonons), and the energy exchange between the particles and the surroundingmedium. All of these processes can be studied by femtosecond transientabsorption spectroscopy. The emphasis in these studies has mainly been onthe investigation of the electron–phonon coupling as a function of a particlesize and shape. The excitation of coherent breathing modes has also been ofsignificant interest [24,25]. Although the latter topic is reviewed in great detailin Chapter 12, the energy relaxation after femtosecond excitation will bebriefly discussed here.We will also review the work on the shape transformation of gold nanorodsinto spherical nanoparticles induced by intense laser irradiation [26–30].We have studied the rod-to-sphere shape transformation in aqueous solutionsof nanoparticles as a function of a pulse energy and a pulse width and determinedthe minimal energy required to melt a gold nanorod as well as themelting time. We also discuss possible mechanisms for this transformation.II.SIZE, SHAPE, AND COMPOSITION DEPENDENCEOF SURFACE PLASMON RESONANCESAs already mentioned in Section I, the intense colors of metal colloids are dueto their surface plasmon absorption [8–10]. The surface plasmon absorptioncan be qualitatively explained in terms of collective oscillations of electrons inthe conduction band. The electric field of the incident light induces apolarization of free electrons with respect to a much heavier ionic core of aspherical nanoparticle. A net charge difference is only created at the nanoparticlesurface, and the surface charges generate a restoring force. In thisway, dipolar oscillations of the electrons are excited. The surface plays asignificant role in generating plasmons. Therefore, the surface plasmonabsorption is a unique feature of small nanoparticles and it is not observedfor either individual atoms (or even small clusters) or bulk metals.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.