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that the particles with >80% Au have spectra that closely resemble pure Au(see Fig. 6) but have a dynamic response that is controlled by Pt. This occursbecause (1) the electron–phonon coupling constant for Pt is much larger thanthat for Au and (2) Pt has a much larger density of states at the Fermi levelthan Au [19].Coherently excited acoustic vibrational modes can also be observed forbimetallic particles. Pt and Au have very similar elastic modulii; therefore,the vibrational periods observed for Pt–Au core–shell particles are very similarto those for pure Au particles of the same size. A more interesting situationarises where the two metals have different elastic properties. Figure 8ashows transient bleach data for pure Au particles and for the Au particlescoated with 40 monolayers of Pb, where modulations due to the coherentlyexcited breathing mode of the particle can be seen [17]. Clearly, the addition ofthe Pb has a strong affect on the period.Figure 8b shows the measured frequency (in cm -1 units) versus thenumber of Pb atomic layers for all the samples investigated. It is important tonote that the decrease in the frequency with increasing Pb content cannot bereproduced by using Eq. (3) and average values of c l and c t , that is, theinterface between the core and shell must be explicitly taken into account [17].The solid line in Fig. 8b shows the results of continuum mechanics calculationsfor the breathing mode of a core–shell particle. In these calculations,the displacement and normal stress were set to be continuous at the interface,and the bulk elastic properties of Au and Pb were used, details of the theoryare given in Ref. 48. The experimental results and calculations are in excellentagreement. This shows that continuum mechanics gives an excellent descriptionof the elastic properties of core–shell nanoparticles. Furthermore, even ata few atomic layers (