Optical studies of the 2.367 eV vibronic absorption system in irradiated type I b diamond

Abstract

A vibronic centre with a zero phonon line at 2.367 eV is produced in type I b diamond by electron irradiation. The absorption system associated with the centre is strongly photochromic, and at low temperature (less than 200 K) may be bleached to less than 10% of its original strength; as the 2.367 eV system is bleached an absorption band with a zero phonon line at 1.979 eV is produced. The process may be reversed by warming the sample to room temperature. The temperature dependence of the thermal recovery process has been analysed to obtain the associated activation energy, and the temperature dependences of the intensities of the 1.979 and 2.367 eV zero phonon lines have been investigated in the temperature range 5–100 K. Analysis of the data shows that both zero phonon lines are due to transitions from split ground states, and the splittings and relative degeneracies of the levels in the two ground states have been determined. No photoconductivity associated with absorption at the 2.367 eV centre has been detected, suggesting that the bleaching phenomena are not due to a change of charge state of the centre or a charge transfer process. It is therefore proposed that the 1.979 and 2.367 eV centres are the same defect in two different orientations with respect to the diamond lattice. The acti­vation energy associated with the thermal recovery of the 2.367 eV system after bleaching may then be identified with the reorientation energy of 1.979 eV centres. The 2.367 eV centre is unstable on thermal annealing, being virtually destroyed at 200 °C. A probable model for the centre is a substitutional nitrogen–interstitial complex, involving at least two interstitials.