Paramagnetic centers in Tm-doped single crystals were characterized by means of electron spin resonance spectroscopy. Most of the dopant was found to be present in the crystal as Tm3+ which substituted for Zn2+, and yielded no electron spin resonance spectrum. However, a low concentration of Tm2+ centers with the 4f13 electron configuration and S = 1/2 was detected, with C1 symmetry. The intensity of this center was greatly enhanced, by 366nm ultra-violet irradiation at 77K, due to electron capture. The low symmetry was attributed to an associated defect. This was probably a Zn vacancy which had contributed to local charge compensation of the original Tm3+ ion, where the vacancy remained at the Tm ion even after electron capture at low temperatures. This center was described as being a Tm2+-VZn center. Some paramagnetic hole-type defects were also observed, using electron spin resonance, after ultra-violet irradiation at 77K. One of these was suggested to a center which was near to a Tm3+ Zn-vacancy pair and was denoted as being a O-VZn-Tm3+ center.

A.Watterich, L.A.Kappers, O.R.Gilliam: Solid State Communications, 1997, 104[11], 683-8