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

12Time-Resolved Spectroscopyof Metal NanoparticlesGregory V. HartlandUniversity of Notre Dame, Notre Dame, Indiana, U.S.A.I. INTRODUCTIONIn the past decade, time-resolved laser spectroscopy has proven to be a powerfultechnique for probing the fundamental properties of metal particles [1–6]. In these experiments, a pump laser pulse is used to excite the electrondistribution, and a second probe laser pulse monitors how the system returnsto equilibrium. The major aim of these studies has been to obtain informationabout how confinement affects the couplings between the electrons andbetween the electrons and phonons. This work has recently been reviewed byseveral authors [1–6] and our current understanding is that confinement effectsare relatively small for electron–phonon coupling (at least for noblemetals like Ag and Au) and can be qualitatively described by consideringhow the electrons couple to surface phonon modes [7–9]. An unexpected resultfrom these experiments is the observation that ultrafast laser excitationcan actually coherently excite the acoustic phonon modes of the particles[11–13]. The aim of this chapter is to review these experiments—specifically,to describe the assignment and excitation mechanism for the vibrationalmodes and (more importantly) discuss what new physics can be learned fromthese measurements.A simple picture of these experiments is that the pump laser selectivelyexcites the electrons. The absorbed energy flows into the phonon modes on apicosecond timescale: The exact time depends on the electron–phonon couplingconstant, the electronic heat capacity, and the pump laser power [1,2,9,14]. This results in an increase in the lattice temperature, which causes theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.