Theory of optical absorption in a small spherical shell: Investigation of quantum-size effect

To investigate the quantum-size effect on optical absorption spectra in a small metallic particle of a spherical shell shape, first-principles calculations are performed based on the Kubo formula. The resonant frequency of collective modes and their linewidth are studied systematically with Hartree-Fock and random-phase approximations, which are self-interaction-free methods. System-size dependence of the resonant frequency of the modes is in good agreement with the classical theory, while the linewidth reflects the quantum-size effect, which cannot be explained classically. The origin of the line broadening should be attributed to the Landau damping.