  We investigate the role of the effective nucleon-nucleon interaction in the
description of giant dipole resonances in hot nuclei. For this purpose we
calculate the response function of hot nuclear matter to a small isovector
external perturbation using various effective Skyrme interactions. We find that
for Skyrme forces with an effective mass close to unity an undamped zero sound
mode occurs at zero temperature. This mode gives rise in finite nuclei
(calculated via the Steinwedel-Jenssen model) to a resonance whose energy
agrees with the observed value. We find that zero sound disappears at a
temperature of a few MeV, leaving only a broad peak in the dipole strength. For
Skyrme forces with a small value of the effective mass (0.4- 0.5), there is no
zero sound at zero temperature but only a weak peak located too high in energy.
The strength distribution in this case is nearly independent of temperature and
shows small collective effects. The relevance of these results for the
saturation of photon multiplicities observed in recent experiments is pointed
out.
