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)                       

Abstract: The lowest energy spin-allowed crystal field band of (Et₄N)₃Cr₂F₉ shows a distinct interference dip at 15000 cm^–1 with an approximate width of 200 cm^–1. 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 cm^–1 has predominant Cr-F stretching character; the second mode has a frequency of 90 cm^–1 and involves the entire Cr₂F₉^3- 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)                 

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 61000±5000 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 2.85±0.05% 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)                 

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 46000±3000 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 3.5±0.4%, 3.5±0.4%, 3.1±0.3%, 3.3±0.3%, and 3.4±0.3%, 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)                 

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 ³T_1g(t_2ge_g) higher excited state. In Ti²⁺:MgCl₂ the ligand field is significantly stronger than in Ti²⁺:NaCl, leading to ¹T_2g(t_2g²) and ³T_2g(t_2ge_g) 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 ³T_1g(t_2ge_g) -> ³T_1g(t_2g²) 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 ⁴T_2g excited state of CrCl₆^3- doped Cs₂NaScCl₆
J. Chem. Phys. 115, 3819-3826 (2001)                 

Abstract: The ⁴T_2g ⁴A_2g 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 a_1g and the e_g Jahn-Teller normal coordinate. Increasing hydrostatic pressure is found to reduce the tetragonal distortion of the CrCl₆^3- unit in the electronic ⁴T_2g state resulting from the Jahn-Teller effect. Additionally, pressure impedes expansion along the a_1g coordinate of the CrCl₆^3- complex upon ⁴A_2g ⁴T_2g photo-excitation, and thus has a greater influence on the ⁴T_2g excited state than on the ⁴A_2g 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)                 

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 3.01Å, the antiferromagnetic ground state splitting with J = 78cm^–1 (H = J(S_A·S_B)) 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)                 
Angew. Chem. Int. Ed. 40, 1957-1960 (2001)                 

Abstract: The proligand citric acid has been used to assemble two new anionic Ni^II 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)                 

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 cm^–1 to 13000 cm^–1 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)                 

Abstract: The S = 1/2 quantum system TlCuCl₃ has a nonmagnetic singlet ground state and a finite-energy gap to triplet excited states. At H_c~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 Cr^{3+ 2}E emission in Y₃Ga₅O₁₂ codoped with Cr³⁺ and Yb³⁺
Chem. Phys. Lett. 334, 293-297 (2001)                 

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 ²F_5/2 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 ²F_5/2 excitation to Cr³⁺.

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