TITLE: Ph. D. Thesis presentation: Subpicosecond absorption study of an electron polaron and its formation time in KNbO₃ AND LiNbO₃

SPEAKER: Yong Qiu,

TIME: Tuesday Apr. 26, 2005 at 1:30 PM

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

Subpicosecond spectroscopy was applied to study the small electron polaron optically induced in LiNbO₃ and KNbO₃ crystals. The polaron absorption was measured in the infrared range of 1000 nm to 1600 nm. The absorption band was assigned to the intrinsic small electron polaron in stoichiometric crystals. The experimental absorption band at 0.8 eV was consistent to the prediction of 0.78 eV by computational work on KNbO₃ crystals. Congruent LiNbO₃ has lower polaron absorption optical density than the stoichiometric LiNbO₃. The dopant atom like Mg in LiNbO₃ shifts the absorption peak to higher energy, consistent with theoretical calculations on Mg-bound polarons in KNbO₃. The time-dependence of polaron formation was measured experimentally as a function of temperature. It was then modeled in two stages conduction electron relaxation and vibrational relaxation in a configuration coordinate model. A quantitative fitting of the polaron formation time was carried out based on the application of Jortner guest molecule relaxation theory, and the computational results showed some consistency with the experimental data.

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