Structural properties of the negative strontium ion: Binding energy and fine-structure splitting

The binding energy and fine-structure splitting of the ground state of the Sr^-n(5s²5p²P) ion have been measured by means of the laser photodetachment threshold technique combined with state-selective detection of neutral atoms using resonant ionization spectroscopy. We find binding energies of 32.17±{}0.03 meV (J=3/2) and 52.06±{}0.06 meV (J=1/2), yielding a fine-structure splitting of 19.89±{}0.07 meV. This represents an improvement of two orders of magnitude compared to the recent data based on a combination of laser photodetachment and accelerator mass spectroscopy [D. Berkovits et al., Phys. Rev. Lett. 75, 414 (1995)].