It is shown that the complex admittance, which describes the dissipation of the giant dipole resonance (GDR) in hot nuclei, can be derived from the microscopic double-time Green function for the propagation of the GDR. The damping width of the GDR is calculated directly from the complex admittance without explicitly solving the equation for the poles of the Green function. Using this method, a systematic study of the width of the GDR as a function of temperature $T$ is carried out in ${}^{120}$Sn and ${}^{208}$Pb. The quantal width, caused by the coupling to $\mathrm{ph}$ configurations decreases slowly with increasing $T$. The thermal width, caused by the coupling to $\mathrm{pp}$ and $\mathrm{hh}$ configurations at $T\ne 0$, increases sharply at low temperatures up to $T\sim$ 3 MeV, and slowly at high temperatures, where it reaches a saturation in the region of $T>\mathrm{}$ 3–4 MeV. The calculated values of the total damping width of the GDR are found in reasonable agreement with the experimental data in heavy-ion fusion reactions and inelastic $\alpha$ scattering. The mechanism of the “disappearance” of the GDR at high temperatures is analyzed. The evidence of motional narrowing in the hot GDR is investigated.

• Received 18 April 1997

DOI:https://doi.org/10.1103/PhysRevC.57.3032