Even-parity excited states of Ag^-centers in alkali halides

Two-photon excitation of the Ag^- centers in RbCl, RbBr, and KCl have been measured. From the dependence of luminescence on the polarization of the exciting beams, the symmetry of the final levels could be obtained. In all three host crystals, we observed an intense band assigned to the ¹A_1g→¹A_1g transition. At higher energies ¹A_1g→¹E_g and ¹A_1g→¹T_2g transitions could be identified but their presence and intensity change in the different crystals. On the high-energy side of the investigated range, we could observe an intense luminescence signal arising from a two-step excitation process; the exciting photon energy becoming resonant with the ³T_1u state (A band).