Isomeric excitation energy for $^{99}$In$^{m}$ from mass spectrometry reveals constant trend next to doubly magic $^{100}$Sn

The excitation energy of the <math display="inline"><mrow><mn>1</mn><mo>/</mo><msup><mrow><mn>2</mn></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> isomer in <math display="inline"><mrow><mmultiscripts><mrow><mi>In</mi></mrow><mprescripts/><none/><mrow><mn>99</mn></mrow></mmultiscripts></mrow></math> at <math display="inline"><mi>N</mi><mo>=</mo><mn>50</mn></math> is measured to be 671(37) keV and the mass uncertainty of the <math display="inline"><mrow><mn>9</mn><mo>/</mo><msup><mrow><mn>2</mn></mrow><mrow><mo>+</mo></mrow></msup></mrow></math> 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 <math display="inline"><mrow><mn>1</mn><mo>/</mo><msup><mrow><mn>2</mn></mrow><mrow><mo>-</mo></mrow></msup></mrow></math> isomer excitation energies in neutron-deficient indium that persists down to the <math display="inline"><mi>N</mi><mo>=</mo><mn>50</mn></math> 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.