S. Heer, M. Wermuth, K. Krämer and H. U. Güdel
Sharp ²E upconversion luminescence of Cr³⁺ in Y₃Ga₅O₁₂ codoped with Cr³⁺ and Yb³⁺
Phys. Rev. B 65, 125112/1-10 (2002)                 

Abstract: We report on upconversion luminescence in Y₃Ga₅O₁₂ codoped with Cr³⁺ and Yb³⁺. This is, to our knowledge, the first upconversion process directly involving Cr³⁺. At low temperature excitation around 10 314 cm^–1 leads to relatively strong red luminescence in the title compound. The sharp upconversion luminescence around 14 300 cm^–1 is identified as the ²E ⁴A₂ transition of Cr³⁺. The upconversion excitation spectrum follows he Yb^{3+ 2}F_7/2 ²F_5/2 absorptions in the range 11200 - 10315 cm^–1. From transient measurements the upconversion mechanism is found to be dominated by an energy transfer step: Two excited Yb³⁺ ions simultaneously transfer their excitation energy to the Cr³⁺ ion. In Y₃Ga₅O₁₂:2% Cr³⁺, 1% Yb³⁺ the efficiency of the process at 10 K and for 150 mW laser power is 6% and decreases with increasing temperature due to intrinsic loss mechanisms of the system.

Oliver S. Wenger and Hans U. Güdel
Luminescence spectroscopy of V³⁺-doped Cs₂NaYCl₆ under high pressure
Chem. Phys. Lett. 354, 75-81 (2002)                 

Abstract: The luminescence behavior of 1% V³⁺:Cs₂NaYCl₆ at room temperature is studied as a function of hydrostatic pressure. At ambient pressure the V^{3+ 3}T_2g excited state origin lies 234 cm^–1 above the ¹T_2g first excited state. At room temperature the two states are in thermal equilibrium, and spin-allowed ³T_2g ³T_1g emission is dominant. Pressure increases the energetic separation of ¹T_2g and ³T_2g, leading to a decreasing ³T_2g steady state population at constant temperature. This manifests itself in an increasing relative amount of spin-forbidden ¹T_2g ³T_2g luminescence, as evidenced by the occurrence of sharp luminescence features and an increase in the luminescence decay time at higher pressures.

Rafael Valiente, Oliver S. Wenger and Hans U. Güdel
Upconversion luminescence in Yb³⁺ doped CsMnCl₃: Spectroscopy, dynamics, and mechanisms
J. Chem. Phys. 116, 5196-5204 (2002)                 

Abstract: Single crystals of CsMnCl₃ doped with 0.9% Yb³⁺ were grown from the melt by the Bridgman technique and studied by means of variable temperature optical spectroscopy. At cryogenic temperatures, near-infrared Yb³⁺-excitation around 1 mm leads to intense Mn²⁺ upconversion luminescence in the red spectral region. This very efficient upconversion process is possible because of magnetic Yb³⁺-Mn²⁺ exchange interactions, and a new type of upconversion mechanism is found to be active in this system. The upconversion properties of Yb³⁺:CsMnCl₃ are compared to those of Yb³⁺:RbMnCl₃ and Yb³⁺:CsMnBr₃. The upconversion efficiencies at cryogenic temperatures differ by many orders of magnitude. The bridging geometry between Yb³⁺ and Mn²⁺ is found to be a key factor for the efficiency of the process. The highest efficiency is observed for the title compound, and this is correlated with the most likely linear Yb³⁺-Cl^–-Mn²⁺ arrangement in this crystal. At 15 K the dominant upconversion mechanism in the title compound involves an energy transfer step. By increasing the temperature to 100 K a new and very efficient mechanism involving a sequence of ground state and excited state absorption steps becomes dominant.

Christine Reinhard and Hans U. Güdel
High-Resolution Optical Spectroscopy of Na₃[Ln(dpa)₃]·13H₂O with Ln = Er³⁺, Tm³⁺, Yb³⁺
Inorg. Chem. 41, 1048-1055 (2002)                       

Abstract: The title compounds were synthesized and studied by solution and single-crystal absorption, luminescence, and excitation spectroscopy. The f-f luminescence is induced in the Tm³⁺ and Yb³⁺ complexes in solution by exciting into the ¹P-¹P* absorptions of the ligand in the UV. A single-configurational coordinate model is proposed to rationalize the nonradiative relaxation step from ligand-centered to metal-centered excited states in [Yb(dpa)₃]^3- (dpa = 2,6-pyridinedicarboxylate). Direct f-f excitation is used in crystals of Na₃[Tm(dpa)₃]·13H₂O and Na₃[Yb(dpa)₃]·13H₂O to induce f-f luminescence. From low-temperature, high-resolution absorption, luminescence, and excitation spectra, the ligand-field splittings in the relevant states can be determined. It was impossible to induce NIR to VIS upconversion in any of the complexes. This is mainly due to the fact that nonradiative relaxation among the f-f excited states is highly competitive, even in [Yb(dpa)₃]^3- with an energy gap between ²F_5/2 and ²F_7/2 of about 10000 cm^–1. It can be rationalized on the basis of an adapted energy gap law. No luminescence at all could be detected in Na₃[Er(dpa)₃]·13H₂O.

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