Vibronic relaxation processes of FA centers in KCl:Li

The vibronic relaxation process of FA centers in KCl:Li has been studied for the first time by measuring the hot luminescence spectrum from optically excited FA centers with time-resolved spectroscopy. It is shown that the FA center in KCl:Li has a relaxation process belonging to a Type I center relaxation behavior from its absorption at ∼670 nm up to ∼840 nm, although it is classified as a Type II center because of the large Stokes-shift of its emission band at ∼2.7 μm. The relaxation after FA2 band excitation undergoes an anticrossing process due to the vibronic mixing effect of 2s and 2px,(y) states, similarly to that of F centers in KCl. For FA1 band excitation, the relaxation proceeds along the adiabatic potential of 2pz state, at least within the energy range investigated here. From the data obtained in the present study, the adiabatic potential energy curves of 2s and 2p states have been tentatively deduced.