Softening of the Symmetric Breathing Mode in Gold Particles by Laser-Induced Heating ² Gregory V. Hartland* and Min Hu Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670 John E. Sader Department of Mathematics and Statistics, The UniVersity of Melbourne, Victoria 3010, Australia ReceiVed: December 3, 2002; In Final Form: January 10, 2003 The symmetric breathing mode in spherical gold particles has been examined by time-resolved spectroscopy using different intensity pump laser pulses. The results show that the period of the breathing mode increases as the pump laser power increases, up to pump laser powers of 2-3 μJ/pulse. This is attributed to softening of the elastic properties of the particles due to laser-induced heating. At pump laser powers greater than ∼3 μJ/pulse the period versus intensity data flatten off. This most likely arises from saturation of the sample absorption at high pump intensities. The particles studied in these experiments were relatively largesbetween 50 and 100 nm in diameter. Large particles were chosen because they have slower heat dissipation times, which means that the temperature in the particles is better defined during the course of the experiment. The particle temperatures were estimated from the laser power density, the heat capacity of gold, and the absorption at the pump wavelengthsassuming that the samples obey Beer’s law. This allows us to compare the experimental results to calculations of the period versus temperature, which are based on the known temperature- dependent elastic constants of gold. The experimental and calculated periods are in excellent agreement up to the melting point of gold, which is predicted to occur at ∼3 μJ/pulse (approximately the same point where the period versus intensity data flatten off). At higher powers the measured periods are significantly shorter than those predicted for molten gold particles. This implies that we can approach the melting point of gold, but we cannot completely melt the particles. Analysis of the damping of the beat signal indicates that we may form solid-core/liquid-shell particles at high laser powers. 1. Introduction The interaction of light with colloidal metal particles has been a major area of interest in physical chemistry since Faraday’s pioneering experiments. 1 Recently the use of metal particles for biomolecule detection, 2,3 plasmon propagation in metallic wires, 4-6 and optical limiting applications 7,8 has continued to make the spectroscopy of metal particles an important area of research. A useful way of studying the optical properties of metal particles is time-resolved spectroscopy. 9-13 In these experiments the pump laser primarily acts to heat the electron distribution of the particle. 14 The electrons subsequently equilibrate with the phonon modes, and the time scale for this process provides information about the electron-phonon coupling constant. 15-17 This sudden lattice heating can also coherently excite the phonon mode that correlates with the expansion coordinate, which for spherical particles is the symmetric breathing mode. 12,13 The coherently excited phonon mode appears as a modulation in transient absorption experiments. The period of the modulation depends on the speed of sound and size of the particles. 18,19 In turn the speed of sound depends on the density and elastic moduli of the material. 20 Thus, experiments that detect these modulations provide a way of investigating the material properties of nanometer-sized objects, which is difficult to do by conventional techniques. The systems that have been examined to date by time-resolved techniques include spherical silver, 18 gold, 19 gallium and tin particles, 21 bimetallic gold-lead particles, 22 ellipsoidal silver particles, 23 rod-shaped gold particles, 24 and (very recently) aggregated gold particles. 25 Semiconductor nanoparticles of PbS, 26 PbTe, 27 and InAs 28 have also been investigated. In almost all cases the measured period is exactly the same as that predicted by continuum mechanics calculations. The exceptions are the very small <2 nm InAs particles, 28 and the gold nanorod experiments of ref 24. For the InAs particles the deviation from predicted behavior is presumably due to the size dependence of the elastic constants of the material. 28 On the other hand, for the gold nanorods a satisfactory theory that describes the vibrational response of finite cylinders to impulsive heating does not exist at present. The majority of these experiments have been performed with relatively low pump laser fluences, so that the increase in the temperature of the lattice is several hundred kelvin at most. 29 However, it is relatively easy to provide pump laser pulses with enough power to melt or even fragment the particles. 30-38 The aim of this paper is to examine how the period and damping time of the coherently excited phonon modes change with pump laser intensity, using fluences that provide sufficient energy to (potentially) melt the particles. The particles chosen for this study are relatively large, with diameters on the order of 100 nm. Large particles were chosen for three reasons: (i) Large particles have better size distribu- ² Part of the special issue “Arnim Henglein Festschrift”. * To whom correspondence should be addressed. E-mail: hartland.1@ nd.edu. 7472 J. Phys. Chem. B 2003, 107, 7472-7478 10.1021/jp0276092 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/06/2003