Lattice Relaxation of Self-Trapped Excitons in Binary Mixed Crystals of KCl and KBr

Change of emission bands in KCl 1- x Br x mixed crystals has been investigated at liquid helium temperature (LHeT) to make clear the nature of lattice relaxation of the self-trapped exciton (STE). When x <0.03, two emission bands appear at 3.60 eV and 4.88 eV. These emission bands have been confirmed to originate from a triplet state and a singlet state of localized excitons created at Br - dimer centers, respectively. With increase of x , the 4.88 eV band shifts gradually toward the low energy side to become finally the σ emission band in KBr ( x =1.0), but the 3.60 eV band diminishes on the way. Instead, a new emission band, which appears in the lower energy side, is connected to the π emission band in KBr. The discontinuity between the 3.60 eV band and the π band has been attributed to an additional lattice distortion characteristic to the initial state of the π emission. An off-center displacement of the Br 2 - core of the STE is suggested as a candidate for the type of the additional distortion.