Optical studies of the 1.945 eV vibronic band in diamond

The 1.945eV band is the dominant end product of annealing radiation damage in diamonds containing isolated substitutional nitrogen atoms. In this paper uniaxial stress experiments are described which show the band to be a transition between A1 (ground) and E (excited) electronic states of a trigonal centre, the E state being coupled predominantly to A1 modes of vibration. Annealing experiments suggest that the centre is either a substitutional nitrogen atom plus an interstitial nitrogen atom, or a substitutional nitrogen atom plus a vacancy. Small differences between the luminescence and absorption bands are shown to be consistent with the double minima vibrational potential of the nitrogenvacancy model.