The TR12 vibronic band in diamond

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Abstract
The TR12 vibronic band zero-phonon line at 2.638 eV is produced by radiation damage of relatively pure diamond. Its luminescence band is dominated by coupling to a mode of quantum 67 meV, but the absorption band does not show a corresponding feature. The authors present detailed uniaxial stress data on the TR12 band. The data show that the electronic states of the transition are orbitally non-degenerate, but otherwise the measurements cannot be explained in terms of an optical centre of fixed symmetry. Temperature dependence data (7<T<400K) do not reveal any abnormalities caused by states nearly degenerate with the TR12 energy levels. They identify the existence of a local mode of vibration (h(cross) omega approximately=90 meV) at the centre. The luminescence bandshape is shown to be understandable in terms of totally symmetric electron-phonon interactions. The anomalies in the absorption bandshape can be explained on the basis of a softening of one vibrational mode in the excited electronic state, together with a strong dependence of the dipole moment of the transition on the displacement in this mode.

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