Core excitations in He8 and He7

The He8 and He7 nuclei are investigated in a microscopic cluster model involving He6+n+n and He6+n configurations, respectively. The wave functions are defined in the Generator Coordinate Method, with excitations of the He6 core nucleus. In the He8 ground state, we find that the He6(2+)+n+n configuration plays a major role, in agreement with recent data. A similar conclusion holds for He7, unbound against neutron emission.