It is argued that Ag should exhibit a multipole surface plasmon near ${\omega }_m\approx 0.8{\omega }_p\approx 7$ eV $({\omega }_p\approx 9$ eV is the $s$-electron bulk plasma frequency) since the density profile at the surface has predominantly $s$-electron character and since the multipole mode is much less affected by $s-d$ screening than the monopole surface plasmon. Calculations of nonlocal optical spectra based on the $s-d$ polarization model and the time-dependent density-functional approach support this picture.

• Received 23 July 1997

DOI:https://doi.org/10.1103/PhysRevB.57.3803