Spectroscopic Investigation Of Electronic Relaxation Processes in Isolated Pairs of Trivalent Lanthanide Ions CsCdBr3, CsMgCl3, and CsMgI3

Author

Yizhe An, South Dakota State University

Document Type

Dissertation - University Access Only

Award Date

2004

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry and Biochemistry

First Advisor

Stanley May

Abstract

These projects were focused on the laser spectroscopy of trivalent lanthanide ions doped or co-doped in the CsCdBr3 crystal lattice in which Ln^{3 +}-Ln³⁺ pairs are formed due to the unique structure and charge compensation requirements. Following are the outlines of the three research projects.

The time-dependence of thermalization between the ³P₀ and ³P₁ electronic states of Pr³⁺ in CsCdBr₃ was investigated. The energy gap between the ³P₀ and 3 P₁ states of Pr³⁺ doped in CsCdBr₃ is about 570 cm^-1, which can be bridged by three or four phonons of the host matrix. In order to investigate the multi phonon absorption and emission involving these two levels, Pr³⁺ ions were co-doped with Gd³⁺ ions in CsCdBr₃ with a high molar ratio of Gd to Pr (0.025%Pr, 1.5%Gd), so that the majority of Pr ions are in Pr-Gd pairs.

In Pr-Gd pairs, there are no cross-relaxation processes involving the ³P₁ and ³p₀ states of Pr³⁺ ions. When Pr³⁺ is excited to the ³P₁, or ³P₀ state, relaxation is determined by luminescence from ³P₁, and ³P₀, by ³P₁à ³P₀ multiphonon emission, and by ³P₀à ³P₁ multiphonon absorption. Four coupled differential equations describing the relaxations of ³P₀ and ³P₁ following different excitations were obtained. These equations were used to IV fit the time-dependence of the luminescence from ³P₁ and ³P₀, and thereby to obtain the rate constants for the ³P₁ à ³P₀ multiphonon emission, and by ³P₀à ³P₁ multi-phonon absorption transitions. These rate constants were obtained in the temperature range 8-340 K.

Investigation of the mechanisms and temperature-dependence of the cross[1]relaxation processes in Pr-Pr pairs in CsCdBr₃ following excitation into the ³P₁ state of Pr³⁺. CsCdBr₃ crystals doped with 0.12% Pr was made to study cross-relaxation between Pr ions in Pr-Pr pairs. The excited ³P₁ state of Pr³⁺ was quenched extremely rapidly through a cross-relaxation process involving the neighboring Pr³⁺ in the ³H₄ state. This cross-relaxation process was too fast to measure on the time-scale of our experiments (~20 ns). Reverse energy transfer from the ¹D₂ state of the acceptor ion back to the ³P₁ state of the initial donor ion was also found. The equilibrium between the ³P₁ state of the donor and the ¹D₂ state of the acceptor was proved by the emission spectra and kinetic data of these two excited states.

The initial cross-relaxation from ³P₁ leads to two intermediate excited states, ³H₆ and ¹D₂ in the Pr-Pr pairs. A second step of cross-relaxation between these intermediate states produced one P³⁺ ion in the ³P₀ excited state and one Pr³⁺ ion in ³H₄ ground state. The rate of the second cross-relaxation is moderately fast and increases with temperature. This rate constant was measured from 8 to 170 K.

Energy transfer from Tb^{3 + 5} D₃ to Pr³⁺ in Pr-Tb pairs in CsCdBr₃ and its temperature dependence was studied. CsCdBr₃ crystals co-doped with Pr³⁺ and Tb³⁺ were made for the investigation of cross-relaxation between Pr³⁺ and Tb³⁺. The cross- relaxation of the ⁵D₃ state of Tb³⁺ by Pr³⁺ was evidenced by a strong emission from the ³P₁ state of Pr³⁺ following the selective excitation of the ⁵D₃ state of Tb³⁺. The rate constants of the cross-relaxation were obtained from the time-dependence of the luminescence from both the ⁵D₃ state of Tb³⁺ and the ³P₁state of Pr³⁺.

Two pathways of energy transfer are possible. One is the energy transfer from the 5D3 of Tb3+ ions to excite the Pr3+ ions to the ³P₁ state; the other is to excite the Pr³⁺ to ³ P₂. It is not possible to observe the emission from the 3 P2 due to its fast multiphonon relaxation to the ³P₁. However, the temperature dependence of the cross-relaxation rate constants follow the same trend as that of the total overlap of the emission spectrum of the ⁵D₃ with the excitation spectra of the ³P₁and ³P₂. This indirectly indicates that both the ³P₁ and the ³P₂ states of Pr³⁺ ions are involved in the cross-relaxation of the ⁵D₃ state of Tb³⁺ ions.

Library of Congress Subject Headings

Rare earth metals -- Spectra.
Laser spectroscopy.
Relaxation phenomena.
Energy transfer.

Publisher

South Dakota State University

Recommended Citation

An, Yizhe, "Spectroscopic Investigation Of Electronic Relaxation Processes in Isolated Pairs of Trivalent Lanthanide Ions CsCdBr3, CsMgCl3, and CsMgI3" (2004). Electronic Theses and Dissertations. 1062.
https://openprairie.sdstate.edu/etd2/1062