Observation of the magnetic-dipole fine-structure transition in the tellurium negative ion

Two-colour two-photon detachment of via the magnetic dipole transition has yielded an accurate value for the fine-structure splitting of the ion: (0.620586(13) eV). The work clearly demonstrates the applicability of forbidden transitions in the study of the structure of negative ions. One-photon detachment from the level also provided a threshold value which, in combination with the well known binding energy, served as an independent measurement of the fine-structure splitting.