Photoemission from adsorbate-covered Ag films: The dispersion relation for Ag plasma excitation

We have studied photoemission from an adsorbate-covered Ag film in an attenuated-total-reflection configuration and with direct illumination of the film. From the peaks in the photoemission as a function of angle of incidence of the light, P(ϑ), we have determined the dispersion relation for the excitation responsible for the peaks. There are three branches. The dispersion curve for the photoemission below 3.6 eV is the same as that for surface plasmon-polariton excitation. In the range between the surface and bulk plasmon limits, 3.6–3.8 eV, the dispersion curve bends backwards, i.e., with increased energy the angle for peak photoyield decreases. The third branch to the dispersion curve is above 3.8 eV and the peak photoyield occurs at increasing angle with increasing energy E. Large photoyield in both the second and third regimes requires a large imaginary part of the dielectric constant. The dispersion curve E(ϑ) and the shape of P(ϑ,E) agree with a calculation based on a model of Pepper [J. Opt. Soc. Am. 60, 805 (1970)]. We have found the temperature coefficient of the minimum in P(E) at ${\mathrm{\varepsilon }}_2$(ω)=0 to be -2.5×${10}^{\mathrm{-}4}$ eV/K.