Excited eigenstates and strength functions for isolated systems of interacting particles

Eigenstates in finite systems such as heavy nuclei and atoms, atomic cluste rs and quantum dots with few excited particles are known to be chaotic supe rposition of shell model basis states. Here we develop a method for descrip tion of this kind of eigenstates (ES) as well as of strength functions (SF) . Using the model of n randomly interacting particles distributed over m or bitals we show that the average form of ES and SF in energy representation is given by the Breit-Wigner formula with the width Gamma which has a Gauss ian dependence on energy. This explains evolution of ES and SF from the Bre it-Wigner form for weak interaction to Gaussian form for strong interaction .