Mechanisms of Exciton Trapping in Oxides

The localisation energy required for intrinsic self-trapping of excitons increases with increasing bandwidth and calculation suggests that oxides are marginal cases for self-trapping. We have used ODMR of recombination radiation of oxides to study exciton trapping mechanisms. Work carried out in the past has shown that in oxides such as YAG1 and YA1032 excitons are trapped near defect sites. More recently we have studied the well-known blue emission of quartz at 4K3. Our results show that part of this emission arises from a triplet state of a trapped exciton and we have determined the principal magnetic axes of the centre. Comparison with other work4 suggests that the exciton is trapped in an efficient intrinsic radiolytic process involving the transient production of an oxygen Frenkel pair. The fine-structure splitting of the triplet state is unusually large (D = 22.6 GHz), consistent with an oxygen molecular configuration involving an interstitial oxygen. The model for this centre requires the interstitial oxygen and vacancy to be close together and to recombine in the process of radiative decay.