Increased Damping of Plasmon Resonances in Gold Nanoparticles Due to Broadening of the Band Structure

For the first time, systematic investigations of the damping parameter A of gold nanoparticles as a function of photon energy are presented. A is an essential parameter that quantifies the size-dependent optical properties of metal nanoparticles in the dielectric function. To determine the damping parameter, the dephasing time T ₂ of gold nanoparticles has been systematically determined under ultrahigh vacuum conditions as a function of photon energy. Dephasing times ranging from  $T_2 = 5$ fs to $T_2 = 17$ fs were measured, and subsequently, the damping parameter has been extracted. We found a strong resonance-like damping of the plasmon resonance in the vicinity of the onset of the interband transition. While the damping parameter scatters statistically around a value of  $A = 0.19$ nm/fs for photon energies below  $h\nu = 1.70$ eV, it increases rapidly to 0.32 nm/fs for  $h\nu = 1.85$ eV. For higher photon energies, A decreases steadily to $A = 0.24$ nm/fs at $h\nu = 2.15$ eV. A comparison to former measurements as well as to theoretical predictions reveals surface scattering and a discretizing and broadening of the band structure that influences the interband transition as the most dominant size-dependent damping mechanisms. The latter, i.e., a damping via increased interband transitions, assumes a coherent damping process of the oscillating electrons and, as a consequence, the plasmon is treated as a two-level system. Thus, the results deliver new physical insight to the fundamental understanding of surface plasmons.