Isomeric Excitation Energy for 99In^m from Mass Spectrometry Reveals Constant Trend Next to Doubly Magic 100Sn

The excitation energy of the 1/2- isomer in 99In at N=50 is measured to be 671(37) keV and the mass uncertainty of the 9/2+ ground state is significantly reduced using the ISOLTRAP mass spectrometer at ISOLDE/CERN. The measurements exploit a major improvement in the resolution of the multireflection time-of-flight mass spectrometer. The results reveal an intriguing constancy of the 1/2- isomer excitation energies in neutron-deficient indium that persists down to the N=50 shell closure, even when all neutrons are removed from the valence shell. This trend is used to test large-scale shell model, ab initio, and density functional theory calculations. The models have difficulties describing both the isomer excitation energies and ground-state electromagnetic moments along the indium chain.