We report uniaxial stress measurements on the 1.693 eV zero-phonon line, ob
served on nickel- and nitrogen-containing synthetic diamonds after annealin
g at 1600 degrees C. We show that the line is an electric dipole transition
which occurs at a defect of rhombic I symmetry. The shape of the vibronic
sideband and the temperature dependence of the zero-phonon intensity may be
understood assuming linear coupling to totally symmetric vibrational modes
. Quadratic coupling to a single quasi-localised non-symmetric mode is invo
ked to explain a temperature-induced transition 7 meV below the zero-phonon
line energy. (C) 1999 Elsevier Science B.V. All rights reserved.