Spin-polarization and spin-memory effects in theF-center absorption andF′conversion of NaI and NaBr crystals

Abstract

The low-temperature optical conversion of $F$ centers in NaBr and NaI into defects with absorptions at equal or higher energies was studied under magnetic fields up to 80 kG. A strong reduction of the optical conversion was observed under fields giving clear evidence that the reaction product of the conversion is an $F^{\prime }$ center with two electrons in a singlet state, the formation of which becomes forbidden in spin-polarized $F$-center systems. This magnetic field quenching of the $F^{\prime }$ conversion ("Porret-Luty effect"), however, is incomplete even under full spin polarization indicating the existence of some loss mechanisms. To clarify the latter, we measured the spin-memory loss during the optical cycle by monitoring the magnetic circular dichroism (MCD) under known pumping conditions, obtaining spinmixing parameters of $\epsilon =0.03\mathrm{} \mathrm{and} 0.12$ for NaBr and NaI, respectively. These $\epsilon$ values can explain only in part the incompleteness of the magnetic field effect, hinting towards the existence of a bound triplet $F^{\prime }$ state. From the MCD measurement, the spin-lattice relaxation time of the $F$ ground state as a function of magnetic field and the spin-orbit—splitting parameter $\Delta$ of the $F$ center were determined too.