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

was frequency doubled in a 1-mm B- Barium Borate (BBO) crystal before thesample. Visible probe laser pulses were obtained by gently focusing the 10%portion of the output from the regen into a 3-mm sapphire window to generatea single-filament, white-light continuum. The pump and probe beamswere spatially overlapped at the sample, and the transmitted probe intensitywas monitored by a Si–PIN photodiode (Thorlabs, PA150). The probe laserwavelength was selected by a Jobin–Yvon Spex H-10 monochromator placedafter the sample. A normalization scheme using gated integration, an analogdivision circuit, and lock-in detection of the signal was used to reduce noisefrom fluctuations in the probe laser [14]. Normalization and a stable whitelight beam are crucial for the success of the experiments described in SectionIII, as they allow good signal-to-noise data to be collected at relatively lowpump laser power. For some of our experiments, the metal particle sampleswere flowed through a 3-mm-path-length sample cell; however, the airsensitivesamples were kept in sealed cuvettes (2-mm path length).III.COHERENT EXCITATION OF ACOUSTIC VIBRATIONALMODESTransient absorption data for 50-nm-diameter Au particles collected with550-nm and 510-nm probe laser pulses are presented in Fig. 1. These twoprobe wavelengths lie on the red and blue sides, respectively, of the goldplasmon band, which is shown in the insert of Fig. 1. The plasmon band is acollective oscillation of the conduction band electrons and occurs at 525nm for the particles in Fig. 1. The initial bleach signal (negative DA) is due tothe hot electrons created by laser excitation, which cause a strong broadeningof the plasmon band [1,2,14]. The decay time of the bleach providesinformation about the electron–phonon coupling constant for the particles.The modulations are due to the coherently excited vibrational modes. Notethat the period and damping times of the modulations are identical for thetwo scans, but the phases are very different: The modulations at 510 nm arecompletely out of phase with those at 550 nm. This implies that the modulationsarise from a periodic shift in the position of the plasmon band [9,11–13].The period of the modulations depends on the average size of theparticles. Figure 2 shows the frequency (in cm -1 units) versus the inverse of theradius, for a series of transient absorption experiments with different size Auparticles (all in aqueous solution). The frequency changes from 1 to 12 cm -1 asthe radius changes from 60 to 4 nm [13]. Also shown in Fig. 2 are the resultsfrom continuum mechanics calculations for the symmetric breathing mode ofa sphere. These calculations, which were first performed over 100 years ago,Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.