“…1). In accordance with the energy level diagram and the literature data [19][20][21][22], all the observed absorption bands (see Fig. 1) were assigned to appropriate f-f electronic transitions of Er 3+ ions from the 4 I 15/2 ground state to the following excited states: 4 G 9/2 , 4 G 11/2 , 2 G 9/2 , 4 F 3/2 , 4 F 5/2 , 4 F 7/2 , 2 H 11/2 , 4 S 3/2 , 4 F 9/2 , 4 I 9/2 , 4 I 11/2 and 4 I 13/2 .…”