Photoemission from very thin films of strontium. Action of an electric field

Abstract

Properties of photoemission from very thin films of strontium have been studied on the one hand in terms of the films' structure, and on the other hand when the films were submitted to the action of an external electric field applied perpendicularly to the metallic surface. The thin films were prepared by thermal evaporation and condensation on a quartz substrate in an ultrahigh vacuum (5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ Torr). Mass thicknesses were between 0.5 and 33 nm. The electric field could reach 985 \ifmmode\times\else\texttimes\fi{} ${10}^{3}$ ${\mathrm{Vm}}^{\ensuremath{-}1}$. The work function varies with mass thickness of the thinnest films and exhibits two minimums for thicknesses ${d}_{1}=2.5$ nm and ${d}_{0}=7.5$ nm. It is constant, and equal to 2.64 eV, for all films thicker than 12 nm. A strong external electric field $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}$, of correct orientation, applied perpendicularly to the surface of the film brings on a decrease $\ensuremath{\Delta}{\ensuremath{\Phi}}_{E}$ of the work function. $\ensuremath{\Delta}{\ensuremath{\Phi}}_{E}$ varies linearly as a function of $\sqrt{E}$ and also depends on the thickness of the films; it is minimal for thicknesses ${d}_{1}$ and ${d}_{0}$. This set of results can be deduced from the variation of the electronic double layer on the surface.