First two energy levels in ¹⁵F

The ground state and first excited state of ¹⁵F were measured by the method of elastic resonance scattering in inverse kinematics. A secondary beam of 115 MeV/nucleon ¹⁴O was slowed down to 8 MeV/nucleon and energy bunched before stopping in a C₂H₄ target. The ¹⁵F excitation energy spectrum was extracted from elastically scattered protons at 0°. The 1/2⁺ ground state resonance of ¹⁵F was determined to be unbound with respect to single-proton emission by 1.51±0.11 MeV, corresponding to a mass excess of 16.81±0.11 MeV. The 5/2⁺ first excited state resonance is unbound by 2.853±0.045 MeV leading to an excitation energy of 1.34±0.15 MeV. A comparison with systematics of single-nucleon separation energies and theoretical models suggests that ¹¹N should be unbound by about 1.5±0.15 MeV.