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

oth the electronic and lattic contributions. Neglecting hot electron pressureeffects in Eq. (6) produces a phase difference of 45j between the experimentaland calculated DR/R versus time traces. Thus, hot electron pressure effectsplay a significant role in launching the coherant vibrational motion of theparticles. This conclusion is consistent with that from recent time-resolvedstudies of Ag particles in a solid matrix by Perner et al. [15] and Voisin and coworkers[16].IV.GOLD NANORODSStudies of the properties of nonspherical metal particles are extremelyimportant, as these materials are likely to be vital components in nanoelectronicsdevices [41]. Extensive work has been done on the optical properties ofcylindrical and ellipsoidal particles. For example, it is well known that theseparticles show two plasmon bands, corresponding to oscillation of theconduction-band electrons along the longitudinal or transverse directions[37]. Time-resolved studies of these systems have been performed by the E1-Sayed group [42–44], who studied electron–phonon coupling and laserinducedmelting in Au nanorods, and by Perner et al. who examined thecoherently excited vibrational modes of Ag ellipses [15]. In their work, Perneret al. observed that the period of the beat signal depended on the relativeorientation of the ellipse with the laser polarization vector. The measuredperiods are approximately given by 2d/c l , where c l is the longitudinal speed ofsound and d is either the length or width of the ellipse— depending on whetherthe probe laser interrogates the longitudinal or transverse plasmon band,respectively. They attributed this signal to expansin and contraction along themajor or minor axis of the ellipse.Figure 5 shows initial results from our ultrafast laser studies of Aunanorods [45] Modulations with a period on the order of 50–70 ps can beclearly seen in the transient absorption data. The inset in Fig. 5 shows themeasured period versus the length of the rod. The straight line corresponds toT = 2L/c R , where c R is the speed of Rayleigh surface waves in Au and L is thelength of the rod. This is approximately the period expected for a Rayleighsurface wave that propagates along a cylinder. The exact value of the perioddepends on the probe laser wavelength, which implies that different probewavelengths interrogate different length rods in the sample. The dependenceof the measured period on the laser wavelength has been analyzed to obtaininformation about the homogeneous and inhomogeneous contributions tothe spectra of the rods [45]. The quantitative differences between our results(T = 2L/c R ) and those of Perner et al. (T = 2L/c l ) show that the vibrationaldynamics of nonspherical particles is very complex. It is not at all clear whatCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.