  Temperature and kinematic line broadening are the primary contributions to
the width of the proton energy spectrum measured in cluster-impact fusion
experiments. By ascertaining these two contributions, we have determined an
effective temperature for the high-velocity deuteron component that is
responsible for the measured fusion yield. The extracted effective temperature
is substantially higher than conventional estimates., and implies that
cluster-impact fusion is hot fusion on an atomic scale. The proton spectrum
rules out contaminants in explaining the high yield.
