The monopole excitations ($0_{\mathrm{g}.\mathrm{s}.}^{+}$ $\to$ $0_{\mathrm{ex}}^{+}$) of light neutron-excess systems ${}^{12}\mathrm{Be}$ $=\alpha +\alpha +4N$ are investigated by the generalized two-center cluster model. We analyze the mechanism of an enhancement of the monopole strength in connection to structures of adiabatic energy surfaces as a function of an $\alpha$-$\alpha$ distance. The crossing of two adiabatic energy surfaces, which have different intrinsic configurations, increases the monopole strength. Furthermore, we find that the monopole strength is strongly enhanced by a smooth connection in the adiabatic energy surface from the ground $0^{+}$ state to the excited $0^{+}$ state. A comparison of the calculated strength with the respective single particle limit is also discussed.

• Received 13 August 2010

DOI:https://doi.org/10.1103/PhysRevC.83.044319