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

Figure 3 Change in radius (DR/R) versus time for 50-nm-diameter Au particlesafter ultrafast laser excitation: (.) experimental values determined from the transientabsorption data; (—) calculated using the harmonic oscillator model (see text fordetails).This behavior is analogous to a harmonic oscillator [12,13,15]. Considera ball at the minimum of a parabolic well. If the potential minimum issuddenly changed, the ball will initially be displaced from the new minimum.The ball will subsequently move toward the new minimum, pick up momentum,and overshoot, exactly as described earlier. The amount that the ballovershoots depends on the initial displacement and the friction in the system.This is shown schematically in Fig. 4. Note that this simple model predictsthat the amount of overexpansion should be approximately the same as theoverall increase in the radius due to lattice heating.A coherent response in the nuclei can also be induced by direct couplingbetween the expansion coordinate and the hot electrons [15,16]. This couplingarises from the pressure exerted by the hot electrons, [38,39] and dies offrapidly as the electrons equilibrate with the lattice. In terms of the harmonicoscillator model described earlier, the hot electron pressure effect is equivalentto a transient force that gives the nuclei an instantaneous kick at H = 0 [15].As shown in Fig. 4, this generates a very different response compared to latticeheating. First, the nuclei oscillate around the original equilibrium radius (notthe radius of the hot particles). Second, the oscillations induced by hotelectron pressure are 90j out of phase with those from heating the nucleiCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.