could be the formulae where n ¼ 1/2 for allowed transitions, B is the constant and Eopt the direct optical band gap, for
indirect transitions aðhnÞ ¼ Bðhn EoptÞn=hn where n ¼ 2 for allowed transitions and Eopt is the indirect optical bond gap by using the above two equations, by plotting ðahnÞ12 and ðahnÞ2 as a function of photon energy ðhnÞ, optical energy band gaps (Eopt) for indirect and direct transitions are determined. From UV-absorption spectra (Fig. 2) of Mn2+ glasses, both direct and indirect band gaps have been calculated[22,23]. The visible absorption spectra of Mn2+:ZPB glasses are shown in Fig. 3, with a broad absorption bond at 470 nm ( 6
A1g (S)- 4T1g (G)) of Mn2+ ions in octahedral symmetry [17,24]. These transitions are spin and parity forbidden for electric dipole radiation in an octahedral environment, hence the absorption bands are very weak. Fig. 4 shows the excitation spectra of Mn2+:ZPB glasses. We are aware of the fact that Mn2+ d–d absorption transitions are difficult to be pumped as those are forbidden by spin and parity for