2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/26945Two phonon-states of alkali-metal clusters (treated as jellium spheres) are calculated by using a method based on a perturbative construction of periodic orbits of the time-dependent mean-field equations. Collective vibrations with various multipolarities in charged $Na^+_{21}$ are considered.15 pages of plain Latex and 2 figures (available from the authors), preprint IPNO/TH 92-80Condensed Mattertext