By analyzing interference structures in low-energy electron-transmission spectra, we made a direct experimental determination of the electronic band structure in the (111) direction of argon films and possibly in the same direction for methane films in the energy range 0-10 eV relative to vacuum.PACS numbers: 71.25.Tn, 73.60.HyThere are few experiments which can deal directly with electron-transport properties and electronic density of states in the conduction bands of semiconductors and insulators in the range 0-10 eV relative to vacuum. As reported previously, 1,2 low-energy electrontransmission spectroscopy (LEETS) of thin films is an ideally suited technique to perform such investigations. Indeed, our past experience has shown that LEETS is quite sensitive to crystalline order and the electronic band structure. 3,4 More recently, attempts have been made to correlate the transmitted current with the density of electron states. 2 In this Letter we show that it is possible, in certain cases, to obtain the electron band energies as a function of the wave vector k [i.e., the dispersion curves E(k)] directly from the transmission spectra without any assumption about the shape of the band.Previously, 5 two of us and others reported LEETS results for very thin films of rare-gas solids. Oscillatory structure was observed in the energy dependence of the transmitted current as a function of film thickness. These were interpreted in terms of an interference phenomenon caused by the reflection of the electron wave at the vacuum-film and the film-metal interfaces. These investigations, unfortunately, had only been carried out for films up to 3 monolayers thick because of a rapid damping of the structures. In the present study, we have been able to keep track of the interference structures up to 10 monolayers for argon and methane.The measurements reported herein were performed in an apparatus described at length in previous publications. 1,4,5 The essential features are the following: A monoenergetic electron beam emerging from a trochoidal monochromator impinges at normal incidence on a molecular solid film deposited on a clean polycrystalline platinum substrate held at 20 K. After repeated cleaning by resistive heating at 1500 K, the Pt ribbon was observed to crystallize with a preferential (111) orientational normal to the surface. 6 The film was then grown using a precise layer-by-layer surfacecoverage method. 5 According to previous analysis, 5 interface structure can only occur if the films are crystalline. In fact, we verified in the present experiment that no such effects could be observed when the films were allowed to condense randomly on a disordered substrate. Moreover, it is well known that the rare gases grow with a close-packed distribution on a variety of substrates including those with the (111) face, and this independently of the topography of the surface underneath. 7 We can thus safely assume that our Ar films have at least a preferential (111) orientation perpendicular to the substrate. One can also sp...

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