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Absolute-reflectance measurements of single-crystal Re(>3 in the spectral range 0.2-22 eV are analyzed by Kramers-Kronig relations to give the real and imaginary parts of the dielectric constants. The dielectric constant is characteristic of free-electron behavior below the sharp plasma edge at 2.1 eV, and confirms the metallic nature of the lustrous red oxide. Interband transitions dominate the optical spectrum above the plasma edge. From our analysis we derive an optical effective mass of 0.86mo. The metallic properties and interband transitions are discussed in terms of proposed band models.

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