Identification of a Tl dimer centre in alkali halides by ODMR

In Tl + -doped alkali halides laser-active centres can be produced by ionising irradiation. This treatment inevitably produces other Tl-associated centres, which might degrade the laser performance. One of these centres that absorbs in the wavelength region where the laser centre is pumped was identified in the hosts KCl, RbCl and KBr by measuring its magnetic circular dichroism (MCD) tagged by electron spin resonance (ESR). The positions of the lowest-energy absorption and emission bands as well as sign and magnitude of the MCD resemble closely those of the laser centre. From the analysis of the optically detected ESR the structure of this new centre was determined. It consists of two neighbouring Tl + cations adjacent to an anion vacancy, which share an electron. The ESR spectrum is explained by a fast hopping of the unpaired electron between the two Tl + positions.