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

Figure 6 Ultraviolet–visible absorption spectra of Pt–Au core–shell nanoparticles.The molar ratios of Pt : Au are given in the figure. The plasmon band for thesecomposite particles occurs at 520 nm, close to the plasmon band of pure Au.different amounts of Au [29]. There are two important things to note aboutthese spectra. First, Pt has a very weak absorption in the visible/near-UVregion. Second, particles with a >1:1 ratio of Au to Pt have a strong plasmonband at 520 nm, which is very close to the position for pure Au (see Fig. 1).This is typical of core–shell particles: After the addition of enough shellmaterial, the UV-vis absorption spectra tends to resemble that of particles ofthe shell.Transient bleach data for Pt–Au core–shell and pure Au particles arepresented in Fig. 7a. These experiments were performed with 400-nm pumplaser pulses, with the probe laser tuned to the peak of the plasmon band [19].The bleach recovery corresponds to relaxation of the hot electrons by energyexchange with the phonon modes. The most noticeable feature of theseexperiments is that the relaxation time is much faster for the core–shellparticles compared to the pure Au particles. Similar results are observed forAu core –Pt shell particles [19].Figure 7b shows the characteristic timescale for electron–phononcoupling plotted again the mole fraction of Au. The presence of even a smallamount of Pt drastically shortens the relaxation time. The dashed line showsthe effective time constant for electron–phonon coupling calculated byaveraging the time constants for Pt and Au:1¼ ð1s effa AuÞs Ptþ a Aus Auð7ÞCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.