Enhancement of surface and volume photoeffects in cesiated Al thin films by controlled excitation of surface plasma waves at low photon energies

At photon energies far below the surface-plasmon energy, photoemission from a metal surface may be described in terms of volume and surface photoeffects, whose respective contributions in absolute values are easily calculated with the help of phenomenological models with the condition, however, that several basic parameters have been previously determined experimentally. Aluminum thin films directly evaporated in ultrahigh vacuum onto a prism are smooth enough to exhibit a pure volume effect by illumination with $s$-polarized radiation, even after activation of their surfaces by a submonolayer of cesium. Surface plasma waves (SPW) excited at the metal-film-vacuum interface by the attenuated-total-reflection method allow an accurate determination of the thickness and of the local dielectric constant of the film before and after Cs deposition, respectively. After the determination of the electron escape length $L\left(\omega \right)$ in the photon energy domain $1.5<\mathrm{}\hslash \omega <4\mathrm{}$ eV and measurement of the spectral sensitivities of the film illuminated on its front face with $s$- and $p$-polarized light successively, the surface photoemission spectrum is obtained. A careful study of the respective contributions of the volume and surface effects is then carried out, in the presence of SPW excited in the film by illumination through the prism. The surface effect is more greatly enhanced than the volume effect by the SPW fields, and the respective variations of these effects as a function of the incidence angle are very different. At photon energies not too far from the threshold, the surface-effect photoemission far exceeds (even by one order for magnitude) the volume photoemission. Excellent agreement is obtained between measured and calculated values of total photoemission yield.