Study of the photoexcitation mechanisms of Tm⁺³ luminescent centres in ZnS:Tm

Conclusion

In conclusion we point out that the Tm⁺³ ions in ZnS can obtain the energy of excitation by the three mechanisms discussed above: i.e. a direct absorption of the excitation energy in Tm⁺³ ions, a recombination of non-equilibrium carriers in Tm⁺³-centres and a resonance transfer of energy from a sensibilizer which accepts energy of excitation by some recombination mechanism. The investigation of the emission and excitation spectra elucidate which of the mechanisms dominates under certain conditions. The experiments described above proved a possibility of direct excitation of the¹ I ₆ level of the Tm⁺³ ions by the photons of energy higher than the ZnS band gap.

The emission bands corresponding to the transitions¹ D ₂ →³ F ₂ and¹ D ₂ →³ F ₃ point out that the¹ D ₂ level is located below the ZnS conduction band, the fact which indicates that the ground state of Tm⁺³ ions, i.e. the³ H ₆ state is situated at the energy less than 0·3 eV above the valence band top of ZnS. Apart from the recombination mechanism a resonance transfer of the excitation energy from the sensibilizers which accept the excitation energy by some recombination process to the Tm⁺³ centres plays a significant part. The centres of the self-activated luminescence that are always present in ZnS fulfil a function of sensibilizers.