Resonant Raman scattering and photochromic effects in KI:MnO 4 ?

A study is reported of photoinduced bleaching in the resonant Raman (RR) scattering spectra of KI:MnO ₄ ⁻ crystals under laser excitation of the MnO ₄ ⁻ centers in resonance with the ¹ A ₁→¹ T ₂ optical transition. The bleaching effect is found to exist for all RR lines and is studied in detail as a function of time, temperature, and laser excitation power for the ν ₁ line. A reversible and an irreversible process are observed, whose contributions to the total effect depend on temperature. It is established that the decrease in RR intensity is accompanied by a decrease in integrated impurity absorption. The observed photochromic effects are attributed to photoinduced instability of the MnO ₄ ⁻ ions. The results find qualitative interpretation within a three-center model including the following photoelectronic processes: photothermal ionization of the permanganate, electron ejection from the excited state of the impurity center into the conduction band, and electron capture by traps of two types, with subsequent trap reactivation. The intensity distribution in a multiphonon RR scattering spectrum obtained at 5 K is used to find the main electron-phonon interaction constants. The introduction of the permanganate ion is found to excite a local lattice vibration, which is observed both near the exciting line and in combination with the fully symmetric intramolecular vibration of the impurity ion itself. Fiz. Tverd. Tela (St. Petersburg) 39, 1567–1574 (September 1997)