RADIATIVE AND NONRADIATIVE PROCESSES AFFECTING HIGHER-EXCITED-STATE LUMINESCENCE OF NI2+ IN SEVERAL CHLORIDE LATTICES

The 7 K luminescence spectra of Ni2+ lightly doped into MgCl2, CsMgCl3, CdCl2, and CsCdCl3, produced by excitation into the 3A2g --> 3T1g (3P) absorption band of Ni2+, are reported. Luminescence in these compounds originates from two metastable states of Ni2+: the first excited state, 3T2g; and a higher excited state, 1T2g. Emission from 1T2g, corresponding to transitions to all lower-lying levels, is observed. Rates corresponding to the various radiative and nonradiative transitions from 3T2g and 1T2g at 5 K are given. At low temperature, relaxation from 3T2g is largely radiative in nature, whereas multiphonon emission makes a significant contribution to 1T2g relaxation down to the lowest temperature studied (5 K). In Ni2+:CsCdCl3, there appears to be a significant nonradiative pathway out of 1T2g directly to the ground state, bypassing all intermediate excited states. The temperature dependence of multiphonon emission from 1T2g is modeled using the single configurational coordinate (SCC) approximation, assuming harmonic potentials with equal force constants. As the Ni2+ concentration is increased in these lattices, 1T2g luminescence is deactivated due to Ni2+ -Ni2+ cross relaxation. The mechanism of cross relaxation in Ni2+:CsCdCl3 is investigated by analyzing the decay curves of 1T2g luminescence according to the theories of Dexter and Inokuti and Hiroyama. Cross relaxation in Ni2+:CsCdCl3 is best described by a dipole-dipole mechanism. The cross relaxation process is shown to increase the quantum efficiency of 3T2g luminescence in Ni2+:CsCdCl3 and Ni2+:CsMgCl3 at 5 K.