X-ray photoelectron spectra of Ag, O, and Cs and quantum yield distributions from surfaces prepared by evaporating Cs onto Ag oxidized in ultrahigh vacuum have been measured. The structure of this surface determined from these measurements consists of silver particles dispersed in a matrix of ${\mathrm{Cs}}_2$O with a layer of ${\mathrm{Cs}}_{11}$${\mathrm{O}}_3$ on top. The threshold at 1.2 μm is due to a lowering of the surface barrier by the ${\mathrm{Cs}}_{11}$${\mathrm{O}}_3$ and the yield in the visible and near infrared is explained as due to the excitation of Mie or optical resonance absorption in the silver particles.

• Received 2 April 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.204