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

in Fig. 7c. Because the average volume distribution of the spheres is comparableto that of the original nanrods, it can be concluded that each nanorodtransforms or ‘‘isomerizes’’ into a spherical particle without a significantfragmentation into smaller particles [26,27].The observed shape changes can be attributed to thermal process inwhich the electrons heat the lattice after intense laser irradiation [26,27]. Thelattice temperature reaches the melting point and a shape transformation intoa more thermodynamically stable spherical shape occurs before the latticeloses its energy to the surrounding medium. Due to a large amount of solvent,the temperature increase for the solution is minimal compared to the localtemperature of the nanorods.The shape changes can be monitored by monitoring the absorptionspectra of colloidal solutions, as illustrated in Fig. 8 [28]. This experiment wascarried out by placing 400 AL of a gold nanorod solution into a cylindricalcuvette, which was rotated so that a different portion of the solution wasexposed to each laser pulse. The wavelength of the femtosecond pulses was820 nm and the pulse energies were 20 and 0.5 AJ (Figs. 8a and 8b,respectively). The spectra were taken in time intervals of several seconds,which corresponded to several thousands of laser pulses (the laser repetitionrate was 1 kHz). One can see that the absorption intensity of the longitudinalsurface plasmon band at 800 nm decreases with increasing exposure time (i.e.,the number of laser pulses), whereas the absorption band at 515-nm becomesmore intense. The longitudinal surface plasmon mode is characteristic ofparticles of nonspherical shapes (e.g., cylinders) [8]. Therefore, the decrease inthe intensity of this mode indicates a transformation of rodlike particles intospheres, which is consistent with results of the TEM studies. The increase inthe amplitude of the 515-nm mode observed during this transformationindicates that the extinction coefficient for the surface plasmon absorptionin spherical nanoparticles is larger than that for the transverse surfaceplasmon mode in nanorods.In order to answer the question of whether this shape transformationrequires a single or multiple successive laser pulses, we plot the absorptionintensity at the maximum of the longitudinal surface plasmon absorption as afunction of the number of laser pulses (see insets to Figs. 8a and 8b). The solidlines are exponential fits of the depletion of the longitudinal surface plasmonabsorption. The characteristic constant s 1/e corresponds to the time that isFigure 7 Gold nanorods on a TEM grid deposited before (a) and after (b) irradiationwith 820-nm femtosecond laser pulses. (c) Comparison of the volume distributionof the nanorods prior to laser irradiation with that of the spherical nanoparticlesobtained after irradiation.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.