  Ultrafast pump-probe measurements of plasmonic nanostructures probe the
non-equilibrium behavior of excited carriers, which involves several competing
effects obscured in typical empirical analyses. Here we present pump-probe
measurements of plasmonic nanoparticles along with a complete theoretical
description based on first-principles calculations of carrier dynamics and
optical response, free of any fitting parameters. We account for detailed
electronic-structure effects in the density of states, excited carrier
distributions, electron-phonon coupling, and dielectric functions which allow
us to avoid effective electron temperature approximations. Using this
calculation method, we obtain excellent quantitative agreement with spectral
and temporal features in transient-absorption measurements. In both our
experiments and calculations, we identify the two major contributions of the
initial response with distinct signatures: short-lived highly non-thermal
excited carriers and longer-lived thermalizing carriers.
