Optical Plasma-Resonance Absorption in Thin Films of Silver and Some Silver Alloys

Plasma oscillations in Ag, $\mathrm{Ag}$-Au, $\mathrm{Ag}$-Cd, and $\mathrm{Ag}$-In have been studied by optical transmittance measurements using $p$-polarized light at oblique incidence. The plasma resonance is observed as an increased absorption at the plasma frequency. The resonance disappears at about 40 at.% Au and at as little as about 3 at.% Cd or In. The energy of the resonance was found to be almost constant with varying impurity content. For the Ag-Au system this is in contrast with results obtained from electron energy-loss experiments and from optical data. It is argued that in some cases optical plasma-resonance absorption measurements are a more correct measure of the plasma frequency than are other experimental methods. The plasma resonance in pure Ag has also been studied in photoemission yield.