Research Highlights

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Research Highlights 2002

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)      Full Text (PDF)      DOI-Link     

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 cm1 leads to relatively strong red luminescence in the title compound. The sharp upconversion luminescence around 14 300 cm1 is identified as the 2E 4A2 transition of Cr3+. The upconversion excitation spectrum follows he Yb3+ 2F7/2 2F5/2 absorptions in the range 11200 - 10315 cm1. 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)      Full Text (PDF)      DOI-Link     

Abstract: The luminescence behavior of 1% V3+:Cs2NaYCl6 at room temperature is studied as a function of hydrostatic pressure. At ambient pressure the V3+ 3T2g excited state origin lies 234 cm1 above the 1T2g first excited state. At room temperature the two states are in thermal equilibrium, and spin-allowed 3T2g 3T1g emission is dominant. Pressure increases the energetic separation of 1T2g and 3T2g, leading to a decreasing 3T2g steady state population at constant temperature. This manifests itself in an increasing relative amount of spin-forbidden 1T2g 3T2g 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)      Full Text (PDF)      DOI-Link     

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.

Journal Cover Picture, a414cover.jpg (46 kByte) 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)      Full Text (PDF)      DOI-Link      Journal Cover Picture     

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 2F5/2 and 2F7/2 of about 10000 cm1. 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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Last modified: 13.12.11 by Gabriela Frei