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

show that effects due to the surrounding medium as well as interparticlecoupling are the issues that can be best addressed using single-particlespectroscopy.Another approach to circumvent the inhomogeneous broadening is touse nonlinear optical techniques [16,41,51]. For example, Heilweil andHochstrasser [41] found that both T 1 and T 2 are shorter than 48 fs for 20-nm colloidal gold nanoparticle solutions. Lamprecht et al. [51] measured T 2for lithographically fabricated 200-nm gold and silver nanoparticles using asecond-order nonlinear optical autocorrelation in the femtosecond regime.They obtained dephasing times of 6 fs for gold nanoparticles and 7 fs for silvernanoparticles. Liau et al. [16] combined nonlinear optical measurements(two-pulse, second-order interferometric autocorrelation) with high-spatialresolutionimaging to study optical responses from single silver nanoparticles.For a 75-nm particle, they obtained a dephasing time T 2 of 10 fs.This brief overview shows that the available experimental results onplasmon dephasing (obtained by different spectroscopic techniques) indicatethat coherent plasmon oscillations decay on ultrafast timescales of a fewfemtoseconds.IV.NONRADIATIVE DECAY OF THE SURFACE PLASMONRESONANCE AFTER LASER EXCITATIONThe use of femtosecond laser pulses to study electron dynamics in thin metalfilms [52] and metal nanoparticles [17–23] allows one to selectively excite theelectrons and then to follow energy relaxation due, for example, to interactionswith the lattice phonons. The size dependence of the electron–phononcoupling is an important issue in the problem of electron energy relaxation.An enhanced electron-surface scattering was already shown to have an effecton the plasmon bandwidth and, hence, on the dephasing time T 2 . The sameeffect can, in principle, influence energy relaxation, leading to more efficientenergy losses in smaller nanoparticles.Experimentally, the excitation of the electron gas in metal nanoparticlesleads to a bleach of the surface plasmon absorption band, as shown in Fig. 5afor 15-nm spherical gold nanoparticles (excitation wavelength is 400 nm) [20–22]. This bleach is caused by a decrease in the amplitude (damping) as well asby broadening of the surface plasmon resonance due to effective heating ofelectrons by laser pulses. The transient absorption spectra in Fig. 5a can bedescribed in terms of a difference between the plasmon absorption at the increasedelectron temperature (following the laser excitation) and the plasmonabsorption at room temperature (before the laser excitation). The latterspectrum (dotted line in Fig. 5a) shows that the plasmon bleach maximumCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.