Anharmonic metastable charge transfer vibronic exciton in potassium tantalate

Abstract

Charge Transfer Vibronic Excitons (CTVEs) are bi-polaronic excitons characterized by specific recombination luminescence. The latter has been investigated with respect to the influence of IR pumping and led to the experimental evidence of an non-linear CTVE. Such an essentially anharmonic CTVE with respect to the charge transfer and lattice displacements was predicted recently [Vikhnin, 2003 Solid State Commun. 127 2832], and should manifest a well-defined metastable behavior. It is shown that an IR pumping recalls practically the same CTVE-recombination emission band as that induced initially by a UVpumping. Such an effect is characterized by relaxation time τ after the UV-pumping (for example, τ ∼ 60 sec. at 3 K ). This behavior is explained as a result of occupation of a metastable anharmonic CTVE minima. An additional IR-pumping could transfer the occupation from the anharmonic CTVE state to the harmonic CTVE which is active for recombination luminescence. The temperature dependence of τ for the anharmonic CTVEs with metastable behavior shows that the relaxation time is controlled by resonance as well as by phonon-induced tunneling.