Further Evidence for Collective Resonances in Monovalent Metals

Photoemission data from the alkali metals Na and K and the noble metals Cu and Ag are examined for evidence of collective resonances. In all cases the vacuum level is more than 1.5 eV above $E_F$, the Fermi energy. The photoemissive energy distributions taken with photon energies above 2.5 eV and up to 5 eV exhibit structure which can be interpreted in terms of an $s$-wave resonance 0.3 eV or less below $E_F$ and several $p$-wave resonances above $E_F$. Circumstantial reasoning based on the trends from Na to K, or from Cu to Ag, supports the assignment of the structure to collective resonances rather than to excitons or one-electron band structure.