  We present a detailed optical study of single crystal bismuth using infrared
reflectivity and ellipsometry. Colossal changes in the plasmon frequency are
observed as a function of temperature due to charge transfer between hole and
electron Fermi pockets. In the optical conductivity, an anomalous temperature
dependent mid-infrared absorption feature is observed. An extended Drude model
analysis reveals that it can be connected to a sharp upturn in the scattering
rate, the frequency of which exactly tracks the temperature dependent plasmon
frequency. We interpret this absorption and increased scattering as the first
direct optical evidence for a charge carrier interaction with a collective mode
of purely electronic origin; here electron-plasmon scattering. The observation
of a \emph{plasmaron} as such is made possible only by the unique coincidence
of various energy scales and exceptional properties of semi-metal bismuth.
