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Resonances in Small Fermi Systems

The systematics of the size dependence of the resonant response of small metal particles and nuclei to incident electromagnetic radiation is studied. The known radius$^{-1}$ variation of the full width at half maximum (FWHM) in matrix-embedded metal particles is qualitatively accounted for by a quantum calculation of the response within a simple model. In free clusters, the behaviour is more complicated, possibly because of thermal excitation of surface modes. For nuclei, the FWHM shows strong shell-structure-linked oscillations across the periodic table. Focussing on the lower envelope of the oscillations (magic nuclei), the downward trend of the FWHM is consistent with the radius$^{-1}$ variation. A schematic theoretical description of the systematics in nuclei is presented. If the FWHMs are scaled by the respective Fermi energies and the inverse radii by the Fermi wave vectors, the data sets for matrix-embedded metal particles and nuclei become comparable in magnitude.