Fusion from an excited state

Abstract

Heavy-ion fusion reactions at energies below the Coulomb barrier are typical examples of the quantum tunneling, where intrinsic degrees of freedom strongly influence the barrier transmission probability of a macroscopic motion. A related example is the low energy nuclear reaction in laboratories, where the bound electrons in the target nucleus modify the reaction rate through the screening effect on the fusion barrier. Here we discuss the characteristics of the effects of intrinsic degrees of freedom on the barrier transmission probability when they are initially in an excited state using a two-level coupled-channels calculations. In the case of fast tunneling, where the intrinsic motion is slow, the barrier transmission probability is affected in the same manner irrespective of whether the initial intrinsic state is the ground or an excited state. On the other hand, a fast intrinsic motion hinders the barrier transmission for whole energy region if the initial state is an excited state contrary to the enhancement in the case when the intrinsic motion starts from the ground state. We show also that a significant transition from the excited state to the ground state takes place in low energy region even for the cases, where one would expect almost no non-adiabatic transitions because of the large excitation energy of the intrinsic motion.