Electronic shell and supershell structures in gallium clusters containing up to 7000 electrons

Gallium clusters have been studied by near-threshold photoionization up to 2400 atoms. An electronic shell structure is clearly observed and may be interpreted by considering three electrons per atom. A quantum supershell corresponding to the interference between the contributions of triangular and square classical electronic orbits is obtained at about 2500 electrons. This number is considerably larger than expected from self-consistent homogeneous jellium calculations and is interpreted by introducing an inhomogeneous jellium approach.