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is located at the position of the ground-state plasmon absorption maximum.The recovery of the bleach is due to the energy exchange between thephotoexcited electrons, the nanoparticle lattice vibrations, and the moleculevibrations in the surrounding solvent [20–22]. The dynamics of the plasmonbleach recovery at 520 nm (maximum of the plasmon band bleach) shows abiexponential decay with time constants of 3.1 and 90 ps. The initial fast decaywas attributed to the electron–phonon coupling (energy exchange betweenphotoexcited electrons and lattice vibrations) and was analyzed according tothe two-temperature model [53]. This first step in the relaxation leads tothermal equilibrium between the electrons and phonons. The second stepinvolves the cooling of the nanoparticle by energy exchange with the surroundingmedium on the f100-ps timescale [20–22]. Especially the electron–phonon relaxation time is strongly dependent on the laser pump powerincreasing with increasing pump power. This is due to the temperaturedependentelectronic heat [20–22] capacity, and relaxation times as short as700 fs to 1 ps have been measured for very weak excitation [21].The size and shape dependence of the plasmon bleach dynamics isanalyzed in Fig. 6, which compares the data obtained for 15- and 48-nmspherical gold nanoparticles and gold nanorods with an aspect ratio of 3.8[22]. The fact that the dynamics for all these samples are very similar indicatesthat the electron–phonon coupling is size and shape independent. These resultsare consistent with experiments carried out by Hartland and co-workers[21], who studied spherical gold nanoparticles in the size range between 2 and120 nm. The time constant obtained for nanoparticles are comparable tothose measured for bulk metals using similar time-resolved, transient-absorptiontechniques [52].The fact that energy relaxation dynamics in gold nanoparticles are sizeindependent is rather surprising considering that the mean free path ofelectrons is only f50 nm. In particles smaller than 50 nm, the electronsurfacescattering should become more frequent than the electron-latticescattering. Therefore, it was thought that the enhanced electron-surfaceinteractions should give rise to a pronounced size dependence of the electroncooling dynamics in particles smaller than the electron mean free path.However, this effect is not observed experimentally (see, e.g., Fig. 6). Usinga theory developed by Belotskii and Tomchuk [54], it can be shown that thecoupling constant between the electrons and acoustic and capillary modes ofthe surface phonons is determined by the ratio of the electron concentrationto the metal density [55]. Because of a small number of valence electrons ingold (one valence electron) and a large gold atomic mass (Au = 197), theelectron-surface scattering contribution to the overall rate of electron energylosses is small (