  To better understand the many effects of temperature on the optical
properties of metals, we experimentally and theoretically quantify the electron
vs. phonon contributions to the thermoreflectance spectra of gold. We perform a
series of pump/probe measurements on nanoscale Pt/Au bilayers at wavelengths
between 400 and 1000 nm. At all wavelengths, we find that changes in phonon
temperature, not electron temperature, are the primary contributor to the
thermoreflectance of Au. The thermoreflectance is most sensitive to the
electron temperature at wavelength of ~480 nm due to interband transitions
between d-states and the Fermi-level. In the near infrared, the electron
temperature is responsible for only ~2% of the total thermoreflectance. We also
compute the thermoreflectance spectra of Au from first principles. Our
calculations further confirm that phonon temperature dominates
thermoreflectance of Au. Most of Au's thermoreflectance is due to the effect of
the phonon population on electron lifetime.
