Isoscalar giant monopole resonance and its overtone in microscopic and macroscopic models

We calculate the transition density for the overtone of the isoscalar giant monopole resonance (ISGMR) from the response to an appropriate external field ∼f̂_ξ(r) obtained using the semiclassical fluid dynamic approximation and the Hartree-Fock (HF) based random phase approximation (RPA). We determine the mixing parameter ξ by maximizing the ratio of the energy-weighted sum for the overtone mode to the total energy-weighted sum rule and derive a simple expression for the macroscopic transition density associated with the overtone mode. This macroscopic transition density agrees well with that obtained from the HF-RPA calculations. We also point out that the ISGMR and its overtone can be clearly identified by considering the response to the electromagnetic external field ∼j₀(qr).