Time structure and atomic excltation spectra in heavy-ion collisions with nuclear contact

From a quantum mechanical model for quasielastic nuclear scattering, employing a pocket in the internuclear potential at close distances, a distribution of nuclear delay times is derived. The influence of this time structure on atomic excitation spectra is demonstrated using positron emission from supercritical collisions as an example. In a narrow regime of beam energies close to the Coulomb barrier, a considerable probability for collisions with long nuclear delay times is found, associated with a sharp peak in the positron spectrum which is due to enhanced spontaneous positron production. It is pointed out that quasielastic nuclear scattering alone cannot account for the absolute numbers of spontaneous positrons as extracted from recent experiments. A possible generalization of the theory to include inelastic nuclear processes is briefly discussed.