Optically detected magnetic resonance and zero-field splitting energy of the self-trapped exciton in NaBr

The triplet states of the type I self-trapped exciton (STE) in NaBr have been studied using a pulsed X-band (9.4 GHz) ODMR technique for the magnetic field H//[110] under the two-photon excitation with KrF laser pulses: An EPR signal from the Mz=O sublevel in the [110]-oriented STE was time-resolved at 5.4 K, by monitoring a transient change induced in the decay curve of triplet emission (''x band'' at 4.6 eV) when a microwave pulse (10 mu s duration) was applied at 20 mu s after the laser excitation. The low-field transition of (Mz=0 double right arrow Mt=1) was confirmed to appear around 1.44T as a broad EPR band with partially resolved seven lines. The Zero-field splitting was unambiguously determined to be D=0.2 meV, in good agreement with our previous study of magnetic field effect on luminescence lifetimes. The D value thus determined is one order of magnitude larger than that of the type ill STE in KBr (0.03 meV). Such a large D value comes most likely from a geometry of on-center type, i.e. [V-K + e] configuration, though it is still open problem whether the type I STE in NaBr strictly keeps the D-2h symmetry.