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

equired to reduce the number of nanorods to 1/e of its initial value. For highlaser intensities (Fig. 8a), s 1/e correlates well with the time that characterizesthe increase in the absorption intensity at 515 nm. This result indicates thatthe absorption of a single laser pulse transforms all nanorods present in theexcitation volume into nanodots [28]. Because the spheres do not absorb at820 nm after the shape transformation is complete, the same particle cannotabsorb another laser pulse.For low-intensity excitation at 820 nm (Fig. 8b), a spectral ‘‘hole’’ at theexcitation wavelength is burned in a broad absorption band associated withthe longitudinal surface plasmon resonance [27,28]. The generation of thisspectral ‘‘hole’’ indicates a considerable inhomogeneous broadening of thelongitudinal surface plasmon band in rods, especially compared to a relativelysmall contribution from sample inhomogeneity into the plasmon linewidthfor samples of spherical nanoparticles (see above discussion). These measurementsalso allow us to establish the minimal energy required to induce shapechanges for an ensemble of nanorods in solution [28]. A plot of s 1/e as afunction of the laser pulse energy is shown in Fig. 8c. Time s 1/e is independentof the laser pulse energy above 5 AJ. For smaller pulse energies, s 1/e increasessignificantly, suggesting that there exists a threshold energy for the completemelting of all nanorods within the broad distribution with a single laser pulse.The reason for the constant value of s 1/e above the threshold pulse energy of 5AJ is that the energy in excess of the amount required for the shapetransformation simply heats the same particles to a higher temperaturewithout inducing further shape changes. Furthermore, the effect of extraenergy (extra pump pulses) is also reduced because of the reduced absorptionat the position of the longitudinal surface plasmon due to the shape change.Using this spectroscopically determined value of 5 AJ together with theoptical density of the sample and the concentration of nanorods in theexcitation volume, one can calculate the energy required to melt a single goldnanorod [28]. For nanorods with a mean aspect ratio of 4.1, a melting energyof 65 fJ is found. On the other hand, assuming bulk thermodynamicproperties of the gold nanorods, one can calculate the minimum meltingenergy, which yields a value of 16 fJ. As the nanorods are much larger than thesize range for which a decrease of the bulk melting temperature has beenobserved (