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

particles to expand. Because the heating time is extremely rapid, the phononmode that correlates to the expansion coordinate can be impulsively excited[12,13]. For spherical metal particles, the symmetric breathing mode is excited.This mode contributes to the signal in transient absorption experimentsbecause it changes the volume of the particles, which changes the dielectricfunction of the metal and, therefore, the optical response of the system [11,13].This description of the excitation mechanism represents an indirect couplingbetween the electrons and the coherently excited phonon modes. A significantquestion that will be addressed in this review is whether the electrons candirectly couple to the symmetric breathing mode [15,16]. Other issues that willbe examined are how the period of the vibrational modes depends on the sizeand shape of the particles and what happens for bimetallic particles—inparticular, particles composed of two metals with significantly different elasticproperties [17].In addition to the vibrational studies, electron–phonon coupling in bimetallicparticles will be discussed. Compared to noble metal particles, muchless is known about the photophysics of multicomponent metal particles orparticles of transition metals, even though these materials are important componentsof catalysts [18]. At present, one of the limiting factors for studyingthese materials is that it is difficult to make multicomponent particles witharbitrary combinations of metals that have a well-defined structure (e.g.,core–shell compared to alloyed) and are in a controlled environment. In thischapter, our recent ultrafast studies of electron–phonon coupling in Pt–Aucore–shell nanoparticles will be reviewed [19]. These particles were preparedusing radiation chemistry techniques. The experimental results show that theeffective electron–phonon coupling constant in a bimetallic particle is an averageof the electron–phonon coupling constants of the pure metals, weightedby the density of electronic states. This is an intuitive result that was not at allobvious to us until we did these experiments!II.EXPERIMENTAL: SYNTHESIS AND LASER TECHNIQUESThe key factors in these studies (and really all studies) are to have highquality,well-characterized metal particle samples and to perform experimentswith high sensitivity. For most of the experiments described in this section, theparticles used were in the mesoscopic regime (10–100 nm in diameter). Anextremely versatile and controlled way of producing these materials is to useradiation chemistry [20]. All noble metal ions and many electronegative metalions can be reduced in aqueous solution by g-radiation. The radiation isprimarily absorbed by the solvent, generating reducing species (free radicals,aqueous electrons, and hydrogen atoms) and oxidizing OH radicals. TheseCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.