The optical constants of an opaque electroplated gold film (Laser Gold from Epner Technology Inc.), were determined by spectroscopic ellipsometry at room temperature over the spectral range from 0.142 mu m in the vacuum ultraviolet to 36 mu m in the infrared (photon energy range 0.034-8.75 eV). Data from two separate ellipsometer instruments covering different spectral ranges were analyzed simultaneously. The optical constants n&k or epsilon(1)&(2) were determined by fitting an oscillator dispersion model combining Drude, Gaussian, and Sellmeier dispersion functions to the experimental Psi and Delta data. The data were analyzed using both an ideal bulk substrate model and a simple overlayer model to account for surface roughness. Including the optical surface roughness layer improved ellipsometric data fits in the UV, and using a separate Drude function for the surface layer improved fits in the infrared. The surface roughness was also characterized using an Atomic Force Microscope. Using an oscillator dispersion model for the optical constants determined in this work allows for more realistic extrapolation to longer infrared wavelengths. Extending optical constants out to 50 mu m and beyond is important for calibrating far-infrared reflectance measurements. Applications include understanding the thermal performance of cryogenic space-based instruments, such as the James Webb Space Telescope (JWST).