Optical properties of the Tl⁺ center doped in NH₄Cl crystals with the CsCl structure

Abstract

We have investigated the optical properties of the Tl þ 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 þ 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 X 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.

Dynamical processes on relaxed excited states in NH4Cl:Tl+ single crystals

“…The Tl + center doped in NH 4 Cl crystals exhibits only the A T and A X luminescence bands, as reported in our previous paper [19]. The energy order of the A T and A X bands in NH 4 Cl:Tl + is reversed with respect to that of the Tl + centers doped in NaCl-type alkali halides, but is the same as that of the Tl + centers doped in CsCl-type cesium halides [22,24,25].…”

“…As reported in previous papers [19,27], the Tl + center doped in NH 4 Cl crystals exhibits the A, B, and C absorption bands at 5.22, 6.22, and 6.59 eV, respectively. Figure 1 shows the integrated luminescence spectra at various temperatures under the excitation of the ArF excimer laser (193 nm), whose wavelength corresponds to the lower energy tail of the C absorption band.…”

“…The value of 12 meV corresponds to the effective energy of the phonon interacting with the relaxed excited state, which was estimated by us from the temperature dependence of the halfwidth of the luminescence bands in the previous paper [19]. The estimated effective phonon energy is almost equal to the energy (11.4 meV) of the TA phonon of NH 4 Cl bulk crystals [29].…”

“…In a previous paper [19], we have reported the temperature dependence of the A T and A X luminescence bands in the Tl + center doped in ammonium chloride crystals. Ammonium chloride crystals are ionic crystals in the same way as alkali halides and have the CsCl crystal structure, where the Tl + ion is surrounded by eight nearest chloride ions.…”

Influence of the Next-Nearest-Neighbor Cations on Tl+ Ions Doped in Ammonium Chloride Crystals

“…[4][5][6][7][8][9][10] Among the Tl þ -type impurity ions, Tl þ ions have an ion radius of 0.144 nm, which is almost equal to that of NH 4 þ ions (0.148 nm). Furthermore, NH 4 Cl and TlCl have the same CsCl-type cubic structure and nearly the same lattice constants.…”