THE EFFECTS OF CARBON AND NITROGEN ISOTOPES ON THE N3 OPTICAL-TRANSITION IN DIAMOND

The N3 centre in diamond consists of three nitrogen atoms and one vaca ncy. We show that the N3 zero-phonon line, at 2.985 eV in natural diam ond, is shifted by +0.2 +/- 0.1 meV in C-12 diamond doped with N-15, w ith no detectable change in the vibronic bandshape. In C-13 diamond, t he energies of all the phonons seen in the vibronic band appear to be reduced from C-12 diamond in the ratio root 12/13, and the zero-phonon is shifted by +4.5 +/- 0.2 meV. The carbon isotope shift can be under stood in terms of contributions from the lattice expansion, of 0.36 +/ - 0.08 meV, 2.7 +/- 0.4 meV from the different vibrational frequencies in the ground and excited electronic states, and similar to 1.1 meV f rom the Jahn-Teller effect in the excited state.