Electron transmission in the energy gap of thin films of argon, nitrogen, andn-hexane

We report direct measurements of electron conduction within gap states of thin films of Ar, ${\mathrm{N}}_2$, and n-hexane. Since these materials have a conduction-band edge (the energy at the bottom of the conduction band) above the vacuum level, we have been able to study the transfer of electrons at ultralow energies (0.05–0.8 eV) within the energy gap by high-resolution electron transmission spectroscopy. We found the transmission to be strongly influenced by gap states. For instance, our Ar films which were fairly well crystallized nevertheless exhibited quantum tunneling with unusually long penetration lengths. We attribute this to a low density of gap states. Our ${\mathrm{N}}_2$ films, on the other hand, had a considerable density of gap states through which electrons could diffuse. We propose a model to explain the unusual behavior of the transmitted current near 0 eV and to estimate the diffusion constant of nonthermalized electrons in the gap states. In n-hexane films, finally, conduction via such states was accompanied by strong electron inelastic scattering events. These latter lead to a quick saturation of the current with film thickness. We generalize our model to account for this behavior. This semiphenomenological model can describe the gap-state motion of hot electrons through films.