The temporal dependence of the fluorescence line shape of Pr ion diluted in La
F3 has been studied in detail. The line-shape study is based on the assumption that the cross relaxation between Pr ions dominates the nonradiative process. The ion-ion interaction has been directly probed by the two-ion cooperative absorption process under which a
3P0 level lifetime dilation has been observed. The dilation effect can be suppressed when the spectral energy transfer is switched on by raising the temperature of the host crystal. The temporal behavior of the fluorescence line shape of the
1D2 level has been systematically investigated. The "inhomogeneous" linewidth is found to be composed of a main central peak and a series of ion-pair satellites. The shorter decay lifetime of the ion-pair states results in the narrowing of the emission line shape at later times. The energy positions of the ion-pair states have been measured from the
1D2 ion pairs up-conversion fluorescence process. The lifetime of the ion-pair states is determined by the coupling strength of the cross relaxation from an excited ion to its partner ion and hence varies according to the ion-pair separation. The decay under the inhomogeneous width is nonexponential in general. Based on the number of the distinguished ion pair levels, a concept of the sphere of influence is introduced. Following this model, an approach has been developed which allows us to study the decay characteristics. Systematical trends emerge from such analysis. The cross-relaxation coupling constants for ion pairs have been extracted from the decay curves and profiles of the ion-pairs line shape have been mapped out. Their peak positions agree with those obtained from
1D2 ion-pair up-conversion fluorescence measurements.