Restoration of Fluorescence from the Lowest Singlet State in the Self-Trapped Exciton by Perturbation with Monovalent Cation Impurities in Alkali Halides

The electronic structure and intersystem crossing at excited states of the relaxed exciton, ( V k e ) A , perturbed by monovalent cation impurity in KCl and KBr have been studied by pulsed laser irradiation of samples populated with the lowest triplet ( V k e ) A . It is found that the σ-polarized luminescence, at 3.03 eV and 2.93 eV in KCI:Na and KBr:Na, respectively, that decays in less than 10 ns is produced upon the photoexcitation of the lowest triplet ( V k e ) A . The fast luminescence is ascribed to the fluorescence (referred to as 1 -luminescence) from the lowest singlet ( V k e ) A . The degree of polarization of the optical absorption due to the lowest triplet ( V k e ) A is determined and it is shown that the electron orbitals of ( V k e ) A are in the order of b 2 u , b 3 u , b 1 u and b * 3 u from lower energy. The S 1 luminescence is found to be produced most efficiently by the electron excitation from the a g orbital to the b * 3 u orbital.