Research Highlights

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Research Highlights 2001

Ralph Schenker, Myriam Triest, Christian Reber and Hans U. Güdel
Interference Dips in the Single-Crystal Absorption Spectrum of the Tri-m-fluoro-Bis(trifluorochromate(III)) Complex
Inorg. Chem. 40, 5787-5794 (2001)      Full Text (PDF)      DOI-Link      Supplementary Data (PDF)     

Abstract: The lowest energy spin-allowed crystal field band of (Et4N)3Cr2F9 shows a distinct interference dip at 15000 cm1 with an approximate width of 200 cm1. This spectroscopic feature is due to spin-orbit coupling between the 2E and 4T2 excited states and is analyzed with a set of two coupled potential energy surfaces. The minimum of the 4T2 potential surface is displaced along at least two normal coordinates. The modes involved cannot be directly determined from the unresolved absorption spectrum, but are obtained from Raman spectra and from the well-resolved spin-forbidden crystal field transition to the 2A1 state. The first mode with a frequency of 415 cm1 has predominant Cr-F stretching character; the second mode has a frequency of 90 cm1 and involves the entire Cr2F93- dimer.

E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer and H. U. Güdel
High-energy-resolution scintillator: Ce3+ activated LaBr3
Appl. Phys. Lett. 79, 1573-1575 (2001)      Full Text (PDF)      DOI-Link     

Abstract: The scintillation properties of LaBr3 doped with 0.5% Ce3+ are presented. Under optical and gamma ray excitation, Ce3+ emission is observed to peak at 356 and 387 nm. The scintillation light output is 610005000 photons/MeV at 662 keV. More than 90% is emitted with a decay time of 35 ns. An energy resolution (full width at half maximum over the peak position) of 2.850.05% was observed for the 662 keV full absorption peak. A time resolution of 385 ps was obtained using BaF2 as second scintillator and gamma rays of 60Co.

E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer and H. U. Güdel
Scintillation Properties of LaCl3:Ce3+ Crystals: 
Fast, Efficient and High-Energy Resolution Scintillators

IEEE Trans. Nucl. Sci. 48, 341-345 (2001)      Full Text (PDF)      DOI-Link     

Abstract: The scintillation properties of LaCl3 doped with different Ce3+ concentrations studied by means of optical, X-ray, and g-ray excitation are presented. Under optical and g-ray excitation, Ce3+ emission is observed peaking at 330 and 352 nm. For LaCl3 doped with 2%, 4%, 10%, and 30% Ce3+ and pure CeCl3, we measured a light yield of 460003000 photons per MeV of absorbed g-ray energy. The scintillation decay curve con be described by three decay components: short (t = 25 ns), intermediate (t = 200-800 ns), and long (t = 0.8-14 ms). The contribution of the short decay component to the total light yield increases with Ce3+ concentration: ranging from 10% for LaCl3: 2% Ce3+ to 69% for pure CeCl3. An energy resolution (full-width half-maximum over peak position) for the 662-keV full energy peak of 3.50.4%, 3.50.4%, 3.10.3%, 3.30.3%, and 3.40.3%, respectively, was observed for LaCl3: 2%, 4%, 10%, 30% Ce3+ and pure CeCl3.

Oliver S. Wenger and Hans U. Güdel
Chemical Tuning of the Photon Upconversion Properties in Ti2+-Doped Chloride Host Lattices
Inorg. Chem. 40, 5747-5753 (2001)      Full Text (PDF)      DOI-Link     

Abstract: The photophysical properties of Ti2+-doped NaCl and MgCl2 at 15 K are compared. At this temperature both materials emit luminescence from their respective lowest excited states and from the 3T1g(t2geg) higher excited state. In Ti2+:MgCl2 the ligand field is significantly stronger than in Ti2+:NaCl, leading to 1T2g(t2g2) and 3T2g(t2geg) lowest excited states, respectively, in these materials. The near-infrared to visible upconversion mechanisms of these two materials are identified. The upconversion efficiency calculated for Ti2+:MgCl2 is approximately 2 orders of magnitude higher than for Ti2+:NaCl. This is mainly due to the more efficient energy storage in the intermediate upconversion level in Ti2+:MgCl2, and its higher 3T1g(t2geg) -> 3T1g(t2g2) luminescence quantum yield relative to Ti2+:NaCl.

Oliver S. Wenger, Rafael Valiente and Hans U. Güdel
Influence of hydrostatic pressure on the Jahn-Teller effect in the 4T2g excited state of CrCl63- doped Cs2NaScCl6
J. Chem. Phys. 115, 3819-3826 (2001)      Full Text (PDF)      DOI-Link     

Abstract: The 4T2g 4A2g luminescence of a 4.1% Cr3+ doped Cs2NaScCl6 crystal is studied as a function of hydrostatic pressure at room temperature and 15 K. The vibrational fine structure observed in the low-temperature variable pressure emission spectra is analyzed with a two configurational coordinate approach, involving the totally symmetric a1g and the eg Jahn-Teller normal coordinate. Increasing hydrostatic pressure is found to reduce the tetragonal distortion of the CrCl63- unit in the electronic 4T2g state resulting from the Jahn-Teller effect. Additionally, pressure impedes expansion along the a1g coordinate of the CrCl63- complex upon 4A2g 4T2g photo-excitation, and thus has a greater influence on the 4T2g excited state than on the 4A2g ground state. The absolute Cr3+ - Cl average distance reduction upon increasing pressure is estimated using a simple point charge model.

Ralph Schenker, Høgni Weihe, Hans U. Güdel and Berthold Kersting
Exchange Pathways in Tris-m-thiolato Bridged [Cr2L3](CIO4)2CI·H2O·MeOH (L = 2,6-Bis(aminomethyl)-4-tert-butyl-thiophenolate)
Inorg. Chem. 40, 3355-3362 (2001)      Full Text (PDF)      DOI-Link     

Abstract: From polarized optical absorption and emission spectra of the tris-m-thiolato bridged [Cr2L3](CIO4)2CI·H2O·MeOH (L = 2,6-Bis(aminomethyl)-4-tert-butyl-thiophenolate) 1 in the visible and near UV, the exchange splittings of the 4A2 ground and the 2E and 2T1 excited states are determined. In view of the large Cr-Cr distance of 3.01, the antiferromagnetic ground state splitting with J = 78cm1 (H = J(SA·SB)) is large compared to other triply bridged Cr3+ dimers. This can be rationalized using a model based on a valence bond approach, in which the exchange splittings are derived from configuration interactions between the ground electron configuration and ligand-to-metal (LMCT) as well as metal-to-metal (MMCT) charge-transfer configurations. It allows us to distinguish the interactions via ligand orbitals from the direct interactions between the metal-centered orbitals. We compare our results with those obtained for the tri-m-hydroxo-bridged [Cr2(OH)3(tmtame)2](NO3)3 (tmtame = N,N',N''-trimethyl-1,1,1-tris(aminomethyl)ethane) 2. In the latter the interactions via the ligands are negligible, whereas they play an important role in 1, due to the softness of the sulfur ligand atoms.

Mark Murrie, Helen Stoeckli-Evans and Hans U. Güdel
Assembly of Ni7 and Ni21 Molecular Clusters by Using Citric Acid
Angew. Chem. 113, 2011-2014 (2001)      Full Text (PDF)      DOI-Link     
Angew. Chem. Int. Ed. 40, 1957-1960 (2001)      Full Text (PDF)      DOI-Link     

Abstract: The proligand citric acid has been used to assemble two new anionic NiII clusters, [Ni7(cit)6(H2O)2]10- and [Ni21(cit)12(OH)10(H2O)10]16-, from basic aqueous solution. The former possesses a high-spin S = 7 ground state, while the latter shows predominantly antiferromagnetic exchange.

Rafael Valiente, Oliver S. Wenger and Hans U. Güdel
Near-infrared-to-visible photon upconversion process induced by exchange interactions in Yb3+-doped RbMnCl3
Phys. Rev. B 63, 165102/1-11 (2001)      Full Text (PDF)      DOI-Link     

Abstract: Crystals of RbMnCl3 doped with Yb3+ exhibit a strong orange luminescence under near-infrared Yb3+ excitation at 10 K. The broad band extending from 17200 cm1 to 13000 cm1 is identified as a 4T16A1 transition on Mn2+. Of the two inequivalent Yb3+ sites identified by site-selective high-resolution spectroscopy, only one is able to induce efficient upconversion. Excitation with 10-ns pulses indicates that no energy transfer is involved, and the upconversion process consists of a sequence of ground-state absorption and excited-state absorption steps. One- and two-color excitation spectra confirm the occurrence of these two steps. The efficiency of the upconversion process at 10 K and for an excitation power of 200 mW focused by a 53-mm lens is 2 percent. An upconversion mechanism based on exchange interactions between the Yb3+ and Mn2+ ions is proposed to account for the observed behavior.

N. Cavadini, G. Heigold, W. Henggeler, A. Furrer, H.-U. Güdel, K. Krämer and H. Mutka
Magnetic excitations in the quantum spin system TlCuCl3
Phys. Rev. B 63, 172414/1-4 (2001)      Full Text (PDF)      DOI-Link     

Abstract: The S = 1/2 quantum system TlCuCl3 has a nonmagnetic singlet ground state and a finite-energy gap to triplet excited states. At Hc~6 T it undergoes field-induced three-dimensional magnetic ordering. Its dynamical spin properties in zero field were studied by inelastic neutron scattering on single crystals. The elementary spectrum consists of well-defined triplet waves of dimer origin. From the observed energy dispersion, the underlying spin-exchange coupling scheme is rationalized. Appreciable three-dimensional correlations are reported, in accordance with the high-field phase. An antiferromagnetic Heisenberg model in the strong-coupling approximation satisfactorily accounts for the energy and intensity dependence of the triplet excitations in the whole reciprocal space. TlCuCl3 is characterized as a strongly coupled spin system in the vicinity of a quantum critical point. A comparison with the parent compound KCuCl3 is proposed.

S. Heer, M. Wermuth, K. Krämer and H. U. Güdel
Upconversion excitation of Cr3+ 2E emission in Y3Ga5O12 codoped with Cr3+ and Yb3+
Chem. Phys. Lett. 334, 293-297 (2001)      Full Text (PDF)      DOI-Link     

Abstract: The 2E 4A2 emission of Cr3+ in crystals and glasses is of great importance in science and technology. We report a new excitation mode for this emission in Y3Ga5O12 (YGG) codoped with Cr3+ and Yb3+. It is based on an upconversion mechanism using 2F5/2 of Yb3+ as an intermediate state. Excitation in the near-IR at 969.6 nm leads to visible luminescence around 700 nm at low temperature. In YGG:2%Cr3+,1%Yb3+ at 15 K and for a laser power of 150 mW the efficiency of the process is 5.5%. From its time dependence the upconversion mechanism is found to be dominated by an energy transfer step: Two excited Yb3+ ions simultaneously transfer their 2F5/2 excitation to Cr3+.


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Highlights 2005 Highlights 2004 Highlights 2003 Highlights 2002 Highlights 2001 Highlights 2000

Last modified: 13.12.11 by Gabriela Frei