Orientation of the Optical Dipole Moments of the Lowest-to-Higher-States Transitions of Self-Trapped Excitons in Alkali Chlorides

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Abstract
Measurements have been made of the dichroic changes, induced by polarized laser-light irradiation, of the specimens populated with the self-trapped excitons in the π-luminescence and in the optical absorption due to the lowest self-trapped excitons in NaCl, KCl and RbCl. The degree of polarization was determined as a function of the photon energy of the laser light and was analyzed by taking it into account that the electron transitions from the lowest a 1 g orbital to the b 1 u , b 2 u and b 3 u orbitals overlap each other. The peak positions and the half widths of each transition were determined, and it is shown that the electron orbitals of the self-trapped excitons are in the order of b 2 u , b 3 u , b 1 u and b 3 u * from lower energy. It is also pointed out that the width of the band due to the transition to the b 3 u orbital is nearly the same or even larger than that to the b 1 u orbital. These results are compared with the optical absorption of the F 2 center. The implication of the results on the mechanism of the Frenkel-pair formation is also discussed.