Electron-Phonon Relaxation Dynamics of Hot Electrons in Gold Nanoparticles Are Independent of Excitation Pathway

We report on a pump wavelength-dependent study of electron-phonon relaxation in colloidal solutions of isolated gold nanospheres and nanorods. We varied the pump wavelength over a broad spectral range that covers plasmon-resonant and off-resonant excitations in 40 nm gold nanospheres with a plasmon resonance at 525 nm and 34 x 91 nm gold nanorods with a longitudinal plasmon resonance at 720 nm. We also performed an excitation power dependence at each pump wavelength and plotted the measured plasmon bleach recovery kinetics against the initial change in the electron temperature, as obtained from the incident fluence and gold nanoparticle absorption. These plots were all linear and revealed as they intercept the intrinsic electron-phonon relaxation time, which we found to be independent of pump wavelength and hence the same regardless of how the nanoparticles were excited. The plasmon bleach recovery dynamics at a given pump wavelength and excitation power simply describe the collective cooling of a hot thermalized electron distribution with the lattice to reach thermal equilibrium and are longer the larger the initial temperature rise is. Finally, we observed a nonlinear trend in the electron-phonon relaxation time when the nanorods were excited at the longitudinal mode with linearly polarized excitation, indicating a potential photoselection that created an even larger electron temperature. These results provide important guidance when comparing the dynamics of different plasmonic nanomaterials by using transient absorption spectroscopy.