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

Melting into spheres having a comparable volume to the originalnanorods or partial melting (shorter and wider rods for femtosecond pulsesand ‘‘f-shaped’’ particles for nanosecond pulses) are, however, not the onlyproducts observed. When higher energy pulses are used, a fragmentation ofthe nanorods into small spheres can be observed [26,27]. Figures 11c and 11dcompare the different particle shapes obtained after fragmentation of the goldnanorods using femtosecond (Fig. 11c) and nanosecond (Fig. 11d) laserpulses. In the case of irradiation with femtosecond pulses, mostly irregularshaped nanoparticles are formed (Fig. 11c), whereas the particles are nearlyspherical after fragmentation caused by nanosecond pulses (Fig. 11d).Fragmentation with femtosecond pulses can be explained by a rapid explosionof the nanorods caused by their multiphoton ionization. The repulsion ofthe accumulated positive charges on the particles after multiphoton ionizationleads to fragmentation [27,58]. Because the initial excitation energy isreleased directly into the solvent by means of photoejected electrons, thelattice of the nanorods never becomes hot, which explains the irregularparticle shapes seen in Fig. 11c. In the case of fragmentation with nanosecondpulses (Fig. 11d), more regular spherical shapes of the fragments are observed.This was attributed to the absorption of additional photons by the hot latticewithin the nanosecond pulse duration [26,27]. Fragmentation would thereforenot necessarily lead to the total cooling of the fragmented parts, but atomicrearrangement into a more spherical shape is possible. Basically, hot particlesfragment in the case of nanosecond laser pulses.Fragmentation is not only observed for gold nanorods but also forspherical metal nanoparticles [58–63]. Irradiation of gold particles with 532nm nanosecond laser pulses was found to cause fragmentation, as reported byKoda et al. [59,60]. This was explained in terms of the slow heat release of thedeposited laser energy into the surrounding solvent, which, in turn, leads tomelting and vaporization of the nanoparticles, as estimated from the depositedlaser energy and the absorption cross section. As the nanoparticles cannotcool off as fast as they are heated, they fragment into smaller nanodots. On theother hand, in a study of silver nanoparticles that were irradiated with 355-nmpicosecond laser pulses, Kamat et al. [58] proposed that strong laser excitationcauses the ejection of photoelectrons. As the particles become positivelycharged, the repulsion between the charges then leads to fragmentation.Mafune et al. [61–63] combined the creation of gold nanoparticles by laserablation in aqueous solution with subsequent fragmentation in order toproduce size-selected gold nanoparticles. The ablation was carried out with1064-nm laser light, whereas the fragmentation was induced by 532-nm light,which was in resonance with the gold nanoparticle absorption. These examplestogether with the above-presented results demonstrate that the laser pulsewidth is an important factor that strongly influences the size and shape of theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.