“…There are 8 formula units per cubic unit cell in YAG, which gives 24 different sites for the Tm ions but only 6 different orientations of the sites. 8,9 Tm has a nuclear spin of 1͞2, which gives no zero-f ield hyperf ine splitting of the levels. When a magnetic f ield is applied, both the ground and the excited states are split into two Zeeman-type levels through the coupling of the nuclear magnetic moment to the f ield.…”