Vibrational symmetry breaking of supported nanospheres

The elastic properties of nanoscale particles have been a matter of intense investigations in the recent past. On the one hand, noble metal nanoparticles can serve as model systems for accessing electron-phonon interactions. 1-3 On the other hand, nanoscale dynamics and structure are accessible via macroscopic optical tools to investigate size-dependent phenomena such as vibrations,4,5 vibrational damping, or melting.6,7 Nanoparticle vibrations play an interesting role in determining particle interaction with adsorbing species or surfaces8,9 or can be used as resonances for coherent control, 10,1 I laser cleaning,12 or determination of adhesion forces. 13 The goal of the present study is to understand the vibrational symmetry breaking of a particle attached on a surface, which represents the generalization of different aspects of adhesion.5.8 By comparing the results from femtosecond spectroscopy, time-resolved x-ray scattering, and molecular dynamics (MD) simulations, the assignment of the modes of adsorbed gold particles on a sUiface is clarified. The observed vibrations consist of a long-period translational mode, which is forbidden for a free particle. It adds to the uncoupled modes of sphelical and spheroidal symmetry. Time-resolved x-ray scattering has been performed by exciting gold particles with sub-monolayer coverage on a surface of silicon with pulses of an amplified femtosecond laser (frequency-doubled Ti :Sa radiation at 400 nm) and probing the lattice dynamics by synchronized pulses of monochromatic x rays at the storage ring of the European Synchrotron Radiation Facility (ESRF). 1416 Synchronization is achieved by an absorptive x-ray chopper at I kHz, whereafter the powder scattering is recorded by means of a charge-coupled device detector (CCD) and used to determine lattice expansion from the angular peak shift. 16,17 Time resolution is determined by the x-ray pulse length, which, depending on electron pulse charge, ranges from 90 to 120 ps in 16-bunch mode. A further reduction of pulse length can be observed in reduced charge modes, one of which occurs during hybrid pulse patterns . 15 PACS number(s): 63 .22.Kn, 61.43.Bn, 78.47.D-