Optical properties of the Tl + center doped in NH 4 Cl crystals with the CsCl structure

We have investigated the optical properties of the Tl + center doped in NH 4 Cl single crystals with the CsCl structure. The A, B and C absorption bands due to the cationic transition in the Tl + ion are observed at 5.22, 6.22 and 6.59 eV at low temperature, respectively. When the A absorption band is photoexcited, the A X luminescence band appears at 4.35 eV, and the A T luminescence band at 3.37 eV. As the temperature increases, the A T band that is dominant at 13 K decreases around 20 and 60 K. On the other hand, the A T band exhibits an increase that compensates the decrease of the A X luminescence intensity. The blue‐shift of the peak energy and the narrowing of the halfwidth with increasing temperature are observed in the temperature ranges from 18 to 25 K for the A X band, and from 25 to 45 K for the A T band. Such complex temperature behaviors of the A X and A T bands are explained on the basis of the thermally activated non‐radiative transitions among four relaxed excited states introduced semiempirically. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)