Extremely narrow optical absorption and emission lines were observed from 3T1(F) 3T1(P) transitions. The well-resolved isotope shifts and non-linear magnetic field splittings were interpreted here in terms of a moderate Jahn-Teller coupling to local vibrational modes of T2 symmetry. In an initial calculation, one local vibrational mode was coupled and the observed fine-structure transition lines were used to fit the energy of the local vibrational mode, the Huang-Rhys factor, and the spin-orbit coupling parameter to each of the 3T1 multiplets. On the basis of these fine-structure wave functions, parameter-free calculations were made of the magnetic field splitting. These agreed with the observed non-linear Zeeman behavior. Calculations were also made, of the dependence of the local vibrational mode upon the Ni mass, by using the Keating-Kane valence-force model for the interatomic forces and the isotopic shift of the vibronic energy levels. It was found that the observed isotope shifts could not be understood in terms of coupling to a single local vibrational mode. Therefore, calculations were made of the Jahn-Teller coupling of 2 different local vibrational modes, with T2 symmetry, whose energies were equal to those, of 0.0262 and 0.0389eV, which had been found by Raman spectroscopy. The observed fine structures and magnetic field splittings could also be interpreted in terms of coupling to 2 local vibrational modes of T2 symmetry. However, the isotope shift depended sensitively upon a correct description of the electron-phonon coupling.
C.Schrepel, J.Schöpp, R.Heitz, A.Hoffmann, U.Scherz: Materials Science Forum, 1995, 196-201, 743-8