Pre-Equilibrium Effects in the Population of Giant Dipole Resonances

High-energy {gamma} rays associated with the decay of the giant dipole resonance have been measured for two fusion reactions leading to the {sup 140}Sm compound nucleus at an excitation energy of 71 MeV. The observed yield increases with the asymmetry in the ratios of the number of neutrons to protons in the entrance channel. This is interpreted as resulting from giant dipole phonons excited at the moment of collision in an {ital N}/{ital Z} asymmetric reaction. {copyright} {ital 1996 The American Physical Society.}