Detection of Optical Surface Guided Modes in Thin Graphite Films by High-Energy Electron Scattering

Abstract

excitation-exchange scattering among the sites during the nonradiative process. Thus, in the present case, our measurement also puts a limit of about 0 o 2 nsec on the time for spectral diffusion in low-temperature pentacene.In this Letter, we report the observation for the first time of peaks in characteristic electron energy-loss spectra which can be clearly identified with the excitation of guided surface waves in a thin-film specimen. Such waves can most readily be visualized as electromagnetic waves propagating in the thin film in a wave-guide fashion. These excitations are contained within the usual surface-plasmon formalism 1 and become significant at long wavelengths in thin films of materials in energy regimes where the dielectric constant is greater than unity. Thus in the neighborhood of a strong absorption edge, the surface excitations above the edge are of surface-plasmon nature [provided the dielectric constant becomes sufficiently negative, i.e. e((J) <-1], whereas below the edge they become guided surface waves. In the infrared, in materials where a strong absorption edge arises as a result of surface-phonon effects, the equivalent excitation to the surface plasmon at that edge is the surface polariton, whereas the guided-mode equivalent has been identified as a wavelike "bulk" mode. 2 In the present case we are concerned with a situation in which the basic physical cause of the absorption edge is a normal interband transition. However as the system in question, graphite, is anisotropic some complexities are introduced into this simple picture.Specifically, we report high-angular-resolution, energy-loss data as a function of scattering angle 0 for a specimen of graphite, 75 nm thick, tilted so that the c axis (normal to the specimen plane) makes an angle of 60° with the incident beam. We demonstrate the existence of three surface modes 11 Unfortunately, the ShpoPskii-matrix measurements reported by Kohler et al. (see Ref. 4), were instrumentlimited, with resolution on the order of 0.4 cm' 1 , not narrow enough to pin down the residual homogeneous width of the lines measured. in the energy range between 4 and 11 eV at the smallest scattering angles. At large angles a single peak corresponding to a volume excitation is observed. In between, the volume and surface terms in the scattering probability cancel each other™a corresponding gap is seen in the excitation spectrum. The volume peak occurs precisely at the point where disagreement arises between the electron energy-loss measurements of Zeppenfeld 3 and the optical observations of Willis, Feuerbacher, and Fitton 4 using photoemission and secondary-electron emission and Klucker, Skibowski, and Steinmann 5 using reflectivity measurements with synchrotron radiation. The observations reported here resolve that discrepancy and offer evidence in favor of the dielectric constants proposed by Tosatti and Bassani. 6 The interpretation demonstrates the necessity for a complete calculation of the energy-loss probabilities including both anisotropic dielectr...

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