The excitation spectrum of the neutral orthorhombic (C2v) FeIn center was studied by means of Fourier transmission infra-red spectroscopy. The Zeeman results showed that the electronic structures of the ground and excited states were similar. Both of them comprised 2 close Kramers doublets which were derived from a 4B1 term; split by spin/orbit interaction with a 4B2 term. The crystal-field and spin-orbit parameters for the ground state were close to those previously determined, by using electron paramagnetic resonance methods, for the orthorhombic FeIn center. Previous electron paramagnetic resonance results for the ground state could be accounted for by considering the electronic structure of an Fei ion; as perturbed by a nearby inert In ion. Almost identical sets of electronic parameters were deduced for the ground and excited states. The excitation lines did not split under a uniaxial stress, and this was explained in terms of the existence of equal stress-induced energy shifts for the initial and final states. The Zeeman and stress results therefore strongly indicated that the excitation lines were due to internal so-called d-transitions at the interstitial Fe ion.

M.Kleverman, P.Tidlund: Physical Review B, 1997, 56[24], 15685-90