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of remarkably reproducible thin potential wells and barriers, essentially rectangular and uniform to the order of a monolayer, can be created with molecular-beam epitaxy. The coupling behavior of the wells proves that synthetic superlattices can indeed be created. The molecularbeam-epitaxy technique for fabrication and the optical technique for energy-level determination should be applicable to additional configurations and compositions of interest for both basic and applied studies.We wish to acknowledge L. Kopf for the determination of the Al content of the layers and for expert technical assistance. We thank C. H. Henry and M. B. Panish for helpful discussions.Plasmons in the one-dimensional organic metal tetrathiafulvalene-tetracyanoquinodimethane were directly measured by high-energy Inelastic electron scattering in thin crystalline films at 300°K. For plasmons propagating along the conducting b axis the plasmon energy decreases from 0.75 to 0.55 eV and the width increases linearly as the plasmon momentum increases. Plasmons at 45° to b have an energy of 0.6 eV and show no dispersion.The organic charge-transfer salt tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) has been shown to have highly anisotropic electrical conductivity, and above 60°K has been characterized as a one-dimensional metal. ^ Since at high frequencies the room-temperature electronic behavior is certainly metallic in one dimension as established by the plasma edge reported in the normal-incidence reflectivity,^""* TTF-TCNQ provides an opportunity to study the elementary excitations of a one-dimensional electron gas. We report here the first known direct measurements of plasmon dispersion and damping in such a material. The experiment, per-formed at Princeton University, utilized high-energy inelastic-electron-scattering spectroscopy as the most sensitive and direct method to study dispersion and damping of plasma oscillations. NEpitaxial films of TTF-TCNQ (1000 A thick) were grown on the (100) cleaved face of NaCl and had the same bioriented nature as previously observed.^ That is, the film consisted of irregularly shaped regions (20 ]um across) within each of which the conducting b axis of the crystals point-ecj^long one of the two orthogonal [llO] directions in the face of the salt substrate. Our sample thus was equivalent to two TTF-TCNQ crystals at right angles to each other (both having the 1330

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The first direct observation of electric monopole transitions in solids is reported for tetracyanoquinodimethane using an inelastic electron scattering experiment. Good agreement is obtained with a semiempirical self-consistent molecular-orbital calculation which predicts dipole allowed transitions near 3 and 7 eV and optically forbidden monopole transitions near 5 eV. We find an anomalous momentum dependence in the shape of the first dipole transition, possible due to local field effects.High-energy inelastic electron scattering has long been considered a useful supplement to optical spectroscopy since with a single spectrometer one can obtain the dielectric response function over a range of energies extending from the infrared to the soft x-ray region. 1 However, with the exception of extensive measurements of plasmon dispersion, 2 comparatively little use has been made of the ability to transfer momentum in the scattering event in solid-state studies. Momentum transfer produces nonvertical transitions between energy bands and gives rise to new selection rules which greatly expand the information available through optical spectroscopy. In low-energy electron scattering experiments optically forbidden transitions have been observed for atoms and simple molecules in the gas phase, 3 ' 4 but in solids they are only observed through the apparent breakdown of selection rules at some suitably low energy. 5 In this paper we report results of a high-energy electron scattering experiment which yields unambiguous identification of monopole transitions in solid tetracyanoquinodimethane (TCNQ) as well as an anomalous momentum dependence of the first dipole transition possibly due to local field effects.Within the Born approximation the differential scattering cross section per unit energy loss per unit solid angle for high-energy electron scattering is 6whereto and 1^ are the initial-and final-state wave functions with energies E 0 and£ /7 respectively, and q is the momentum transferred in the inelastic event. For q