F. Callens

Ghent University, Gand, Flemish, Belgium

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Publications (186)415.25 Total impact

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    ABSTRACT: The electronic properties of manganese in crystalline germanium have been investigated by means of deep level transient spectroscopy (DLTS). Mn was diffused in the material by a thermal treatment at 700 °C. Next to the deep levels of nickel and copper, which are known contaminants in Ge treated at high temperature, three not previously reported levels were observed. These two hole and one electron traps, with apparent energy level positions at EV +0.136 eV, EV +0.342 eV and EC − 0.363 eV, were assigned to substitutional Mn. The analysis of the carrier capture cross-sections, the absence of field-assisted emission and the observation of the Mn2− electron paramagnetic resonance spectrum in n-type Ge:Mn at low temperature are all compatible with Mn introducing two acceptor and one donor levels in the band gap of Ge.
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    ABSTRACT: Quenching experiments have been performed on both n- and p-type Ge in a dedicated furnace using infrared lamp heating. The capture and emission characteristics of the induced deep-level defects in the quenched samples were investigated by means of deep level transient spectroscopy. For all defect levels, a high impact of capture in the transition region (slow capture) was found. An empirical approach to analyse this effect is presented, which allows to extract reliable capture cross section parameters. The defect parameters thus obtained were compared with previously published data and it was found that some prominent quenching-induced deep levels are related to metal impurities (Cu and Ni), while others may be vacancy-related.
    Semiconductor Science and Technology 11/2014; 29(12):125007. DOI:10.1088/0268-1242/29/12/125007 · 2.21 Impact Factor
  • J. Lauwaert, F. Moens, F. Callens
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    ABSTRACT: 1. Introduction. Mn doped germanium was studied almost 60 years ago by means of Hall effect [1] and photo conductivity [2] measurements. Based on the temperature dependence of the resistivity and carrier concentration, two levels, one at 0.37eV from the conduction band and one at 0.16eV from the valence band have been assigned to Mn impurities[1]. In agreement with the valence bond model and similar observations by counterdoping with donors for Fe, Co and Ni, these impurities have been assigned to double acceptors. Moreover, the presence of a double acceptor level was confirmed by the observation of the Mn2- state with Electron Paramagnetic Resonance (EPR). [3] However it was also noted in the early studies that the presence of contamination with acceptor impurities may cast doubt on the charge transition assignment. Since the availability of Deep Level Transient Spectroscopy (DLTS), which is a more reliable technique to study deep levels in semiconductors, the electronic properties of most of the 3d transition metals have already been studied with a higher energy resolution. Metal in-diffusion was successful to introduce Cu and Ni[4]. More recently the defect levels of Ti, Cr, Fe and Co have been identified with DLTS on metal-implanted Ge [5,6]. The present study reports on results obtained on Mn doped Ge. 2. Experimental conditions. The source material are single crystal germanium wafers, supplied by Umicore, with a shallow dopant concentration of 1014 cm-3 Ga and Sb for p- and n-type, respectively. To introduce Mn impurities, 99.995% pure Mn was thermally evaporated on the Ge surface. Afterwards the specimens were placed in a resistive furnace under Ar atmosphere for 10 minutes at 700°C. Finally, the residual Mn on the surface was removed by etching with a mixture of HNO3 and HF (3:1?). Schottky diodes for DLTS were prepared by evaporation of Au (n-type) or In (p-type) barriers immediately after etching. To identify the contamination introduced during the processing, Schottky diodes were also prepared on parts of the specimens outside the Mn covered areas. Schottky diodes on n-Ge were also prepared by annealing the n-Ge:Mn samples (after indiffusion at 700°C) to 400°C for 20 min in order to form a MnGe metallic alloy contact. 3. Results and discussion. Fig. 1 a shows the DLTS spectrum of n-Ge:Mn with a MnGe Schottky contact. A similar spectrum was recorded on the specimen with an Au-Schottky barrier. Conventional DLTS shows one electron trap (Mn-E1), while injection of minority carriers (Vp>0) reveals two additional levels (Mn-H1 and Mn-H2). Fig. 1 b shows the DLTS spectrum of p-type Ge, with presence of Mn-H1 and Mn-H2 only after Mn diffusion. Figure 1: DLTS spectrum of a) n-Ge/MnGe diode. b) p-type Ge with and without Mn-difussion. The electron trap Mn-E1 (E=0.36eV) and the hole trap Mn-H1 (E=0.14eV) correspond with the two levels reported before [1,2]. The Mn-H2 peak (E=0.34eV) has to our knowledge not been reported before in the context of transition metal impurities. Nonetheless, its appearance only in Mn diffused samples, its almost equal amplitude and concentration profile to Mn-H1 suggests that it is also related to substitutional Mn in Ge. This may indicate that the substitutional Mn defect can adopt 4 charge states in Ge, one of which certainly corresponds with 2-, as identified with EPR. 4. Conclusions. Three Mn related defect levels have been identified with DLTS. The striking similarity with previously published results for two of the observed levels suggest the assignment to substitutional Mn in germanium. The third level was not reported before but can most probably also be assigned to Mn. References [1]H. H. Woodbury and W.W. Tyler, Phys. Rev., 100, p. 659 (1955). [2]R. Newman, H.H. Woodbury and W.W. Tyler, Phys. Rev., 102, p. 613 (1956). [3]G.D. Watkins, Bull. Am. Phys. Soc., 2, 235 (1957). [4]P. Clauws and E. Simoen Mater. Sci. Semicond. Process. 9,546 (2006) [5]J. Lauwaert, J. Van Gheluwe, J. Vanhellemont, E. Simoen and P. Clauws, J. Appl. Phys. 105, 073707 (2009) [6]J. Lauwaert, J. Vanhellemont, E. Simoen, H. Vrielinck and P. Clauws, J. Appl. Phys. 111, 113713 (2012)
    2014 ECS and SMEQ Joint International Meeting; 10/2014
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    ABSTRACT: Sucrose, the main component of table sugar, present in nearly every household and quite radiation sensitive, is considered as an interesting emergency dosemeter. Another application of radiation-induced radicals in sugars is the detection of irradiation in sugar-containing foodstuffs. The complexity of electron paramagnetic resonance (EPR) spectra of radicals in these materials, as a result of many hyperfine interactions and the multi-compositeness of the spectra of individual sugars, complicate dose assessment and the improvement of protocols for control and identification of irradiated sugar-containing foodstuffs using EPR. A thorough understanding of the EPR spectrum of individual irradiated sugars is desirable when one wants to reliably use them in a wide variety of dosimetric applications. Recently, the dominant room temperature stable radicals in irradiated sucrose have been thoroughly characterised using EPR, electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. These radicals were structurally identified by comparing their proton hyperfine and g-tensors with the results of Density Functional Theory calculations for test radical structures. In this paper, the authors use the spin Hamiltonian parameters determined in these studies to simulate powder EPR spectra at the standard X-band (9.5 GHz), commonly used in applications, and at higher frequencies, up to J-band (285 GHz), rendering spectra with higher resolution. A few pitfalls in the simulation process are highlighted. The results indicate that the major part of the dosimetric spectrum can be understood in terms of three dominant radicals, but as-yet unidentified radicals also contribute in a non-negligible way.
    Radiation Protection Dosimetry 05/2014; 159(1-4). DOI:10.1093/rpd/ncu168 · 0.86 Impact Factor
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    ABSTRACT: In the past, decennia radiation-induced radicals were successfully identified by electron magnetic resonance (EMR) in several solid-state amino acids and sugars. The authors present a room temperature (RT) EMR study of the stable radicals produced by X-ray-irradiation in the amino acid l-threonine (CH3CH(OH)CH(NH3(+))COO(-)). Its chemical structure is similar to that of the well-known dosimetric material l-alanine (CH3CH(NH3(+))COO(-)), and radiation defects in l-threonine may straightforwardly be compared with the extensively studied l-alanine radicals. The hyperfine coupling tensors of three different radicals were determined at RT using electron nuclear double resonance. These results indicate that the two most abundant radicals share the same basic structure CH3(•)C(OH)CH(NH3(+))COO(-), obtained by H-abstraction, but are stabilised in slightly different conformations. The third radical is most probably obtained by deamination (CH3CH(OH)(•)CHCOO(-)), similar in structure to the stable alanine radical.
    Radiation Protection Dosimetry 04/2014; 159(1-4). DOI:10.1093/rpd/ncu123 · 0.86 Impact Factor
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    ABSTRACT: In the deep-level transient spectroscopy (DLTS) spectra of the 3d-transition metals cobalt and chromium in p-type germanium, evidence is obtained that hole emission from defect levels can occur by two parallel paths. Besides classical thermal emission, we observed a second, slower and temperature-independent emission. We show that this extra emission component allows determining unambiguously whether or not multiple DLTS peaks arise from the same defect. Despite similar characteristics, we demonstrate that the origin of the non-thermal emission is not tunnelling but photoionization related to black-body radiation from an insufficiently shielded part of the cryostat.
    Journal of Physics D Applied Physics 10/2013; 46(42):5101P-. DOI:10.1088/0022-3727/46/42/425101 · 2.52 Impact Factor
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    ABSTRACT: Ionizing radiation induces a composite, multi-line electron paramagnetic resonance (EPR) spectrum in sucrose, that is stable at room temperature and whose intensity is indicative of the radiation dose. Recently, the three radicals which dominate this spectrum were identified and their proton hyperfine tensors were accurately determined. Understanding the powder EPR spectrum of irradiated sucrose, however, also requires an accurate knowledge of the g tensors of these radicals. We extracted these tensors from angular dependent electron nuclear double resonance-induced EPR measurements at 110 K and 34 GHz. Powder spectrum simulations using this completed set of spin Hamiltonian parameters are in good agreement with experimentally recorded spectra in a wide temperature and frequency range. However, as-yet non-identified radicals also contribute to the EPR spectra of irradiated sucrose in a non-negligible way.
    The Journal of Physical Chemistry B 05/2013; 117(24). DOI:10.1021/jp400053h · 3.38 Impact Factor
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    ABSTRACT: Vacuum-deposited CsBr needle plates doped with Eu(2+) have been investigated with Q (similar to 34 GHz) and W (similar to 95 GHz) band electron paramagnetic resonance in a large temperature interval (4 K: room temperature). At low temperatures (< 35 K), two Eu-related centers were found with different symmetry (tetragonal and orthorhombic), while in earlier studies at room temperature only one center was found with tetragonal symmetry. Possible models for the three defects are investigated, taking into account their temperature behavior, their symmetry, and the atypical values of their zero-field splitting parameters.
    Physical review. B, Condensed matter 04/2013; 79. DOI:10.1103/PhysRevB.79.174102 · 3.66 Impact Factor
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    ABSTRACT: Radicals generated in trehalose single crystals by X radiation at room temperature were investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR measurements, together with periodic density functional theory calculations. In the first days after irradiation, three radical species (I1, I2 and I3) were detected, two of which (I1 and I2) dominate the EPR spectrum and could be identified as H-abstracted species centered at C3' (I1) and C2 (I2), the latter with additional formation of a carbonyl group at C3. Annealing the sample at 40°C for 3 days or storing it in ambient conditions for three months resulted in another, more stable EPR spectrum. Two major species could be characterized in this stage (S1 and S2), only one of which was tentatively identified as an H-abstracted, C2-centered species (S1). Our findings disagree with a previous EPR study [Gräslund and Löfroth (23)] on several accounts. This work stresses the need for caution when interpreting composite EPR spectra and thermally induced spectral changes of radiation-induced species, even in these relatively simple carbohydrates. It also provides further evidence that the pathways for radiation damage critically depend on the specific conformation of a molecule and its environment, but also that carbonyl group formation is a common process in the radiation chemistry of sugars and related compounds.
    Radiation Research 01/2013; DOI:10.1667/RR3179.1 · 2.45 Impact Factor
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    ABSTRACT: In this paper, we describe the post-functionalization of a V-containing Metal-organic framework with TiO(acac)2 to create a bimetallic oxidation catalyst. The catalytic performance of this V/Ti-MOF was examined for the oxidation of cyclohexene using molecular oxygen as oxidant in combination with cyclohexanecarboxaldehyde as co-oxidant. A significantly higher cyclohexene conversion was observed for the bimetallic catalyst compared to the non-functionalized material. Moreover, the catalyst could be recycled at least 3 times without loss of activity and stability. No detectable leaching of V or Ti was noted. Electron paramagnetic resonance measurements were performed to monitor the fraction of V-ions in the catalyst in the +IV valence state. A reduction of this fraction by ∼17% after oxidation catalysis is observed, in agreement with the generally accepted mechanism for this type of reaction.
    Catalysis Today 12/2012; 208. DOI:10.1016/j.cattod.2012.09.037 · 3.31 Impact Factor
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    ABSTRACT: Electron paramagnetic resonance (EPR) measurements were performed on poly-ε-caprolactone (PCL) films at different stages of the postplasma-grafting process. PCL films prepared by solvent casting (SC) or electrospinning (ESP) yield very similar EPR spectra after Ar-plasma treatment and subsequent exposure to air, but the EPR signal is much stronger in the PCL-ESP films. The free radicals appear to be mainly, and possibly exclusively, oxygen centered. The radicals generated by UV irradiation in PCL-ESP films were studied in situ with EPR, using a UV-LED (λ = (285 ± 5) nm). Their EPR spectrum is distinctly different from the plasma-induced signal, indicative of carbon-centered radicals, and appears to be independent of the plasma pretreatment. UV-induced homolytic splitting of (hydro)peroxide bonds was not observed. Both the plasma- and UV-induced radicals decay at room temperature (RT), even in an inert atmosphere. This study demonstrates the potential of electrospun films and UV-LEDs for the study of plasma- and UV-generated free radicals with EPR in polyesters, and raises questions with respect to the validity of some generally accepted molecular mechanisms underpinning the postplasma grafting technique for polyesters. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012
    Journal of Polymer Science Part A Polymer Chemistry 06/2012; 50(11):2142-2149. DOI:10.1002/pola.26007 · 3.54 Impact Factor
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    ABSTRACT: The structure of Eu2+ monomer centers in CsBr single crystals is investigated using electron paramagnetic resonance (EPR) spectroscopy. These centers are produced by heating the melt-grown crystals above 600 K in vacuum followed by a rapid quench to room temperature (RT) or 77 K. The angular dependence of their EPR spectrum demonstrates that these centers have cubic symmetry. At RT the EPR spectrum decays by aggregation of the Eu2+ ions. This strongly contrasts with the situation for CsBr:Eu needle image plates synthesized by physical vapor deposition, where the Eu2+-related EPR spectrum was observed to exhibit long-term stability at RT.
    Physical review. B, Condensed matter 04/2012; 85(14). DOI:10.1103/PhysRevB.85.144119 · 3.66 Impact Factor
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    ABSTRACT: In this work, we investigate the chemistry for an aqueous acetate-triethanolamine-ammonia based YBa(2)Cu(3)O(7-δ) (YBCO) precursor system. These precursor solutions are suited for the chemical solution deposition of superconducting YBCO layers on top of single crystal SrTiO(3) or buffered NiW tapes. The development of this kind of precursor inks often involves trial-and-error experimenting and thus is very time-consuming. To reduce labwork to the minimum, the theoretical prediction of pH stability limits and the complexation behaviour of the different metal ions and complexants in the inks are very important. For this purpose, we simulated, based on literature values, the complexation behaviour of Cu(2+) in the aqueous precursor solutions as a function of pH. To validate the used model, we performed potentiometric pH titrations for solutions with similar composition and checked the correctness of fit between experiment and model. The generated complexometric results are coupled with X-band EPR spectra to further confirm the results. EPR spectra for fully prepared precursor solutions as well as for Cu(2+) reference solutions containing only one type of ligand (acetate, triethanolamine or ammonia) were investigated as a function of pH. We find that, in line with speciation simulation, only acetates are actively complexing the Cu(2+) ions at pH values below 7, while when reaching higher pH levels mainly triethanolamine complexes are formed. Over the entire pH range, no trace of free Cu(2+)or Cu(OH)(2), possibly creating precipitation during gelation and thus complicating further processing, could be found.
    Dalton Transactions 03/2012; 41(12):3574-82. DOI:10.1039/c2dt11639a · 4.10 Impact Factor
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    ABSTRACT: The radicals obtained in trehalose dihydrate single crystals after 77 K X-irradiation have been investigated at the same temperature using X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) techniques. Five proton hyperfine coupling tensors were unambiguously determined from the ENDOR measurements and assigned to three carbon-centered radical species (T1, T1*, and T2) based on the EIE spectra. EPR angular variations revealed the presence of four additional alkoxy radical species (T3 to T6) and allowed determination of their g tensors. Using periodic density functional theory (DFT) calculations, T1/T1*, T2, and T3 were identified as H-loss species centered at C4, C1', and O2', respectively. The T4 radical is proposed to have the unpaired electron at O4, but considerable discrepancies between experimental and calculated HFC values indicate it is not simply the (net) H-loss species. No suitable models were found for T5 and T6. These exhibit a markedly larger g anisotropy than T3 and T4, which were not reproduced by any of our DFT calculations.
    The Journal of Physical Chemistry A 03/2012; 116(13):3377-87. DOI:10.1021/jp300979g · 2.78 Impact Factor
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    ABSTRACT: A Metal Organic Framework, containing coordinatively saturated V+IV sites linked together by terephthalic linkers (V-MIL-47), is evaluated as a catalyst in the epoxidation of cyclohexene. Different solvents and conditions are tested and compared. If the oxidant TBHP is dissolved in water, a significant leaching of V-species into the solution is observed, and also radical pathways are prominently operative leading to the formation of an adduct between the peroxide and cyclohexene. If, however, the oxidant is dissolved in decane, leaching is negligible and the structural integrity of the V-MIL-47 is maintained during successive runs. The selectivity toward the epoxide is very high in these circumstances. Extensive computational modeling is performed to show that several reaction cycles are possible. EPR and NMR measurements confirm that at least two parallel catalytic cycles are co-existing: one with V+IV sites and one with pre-oxidized V+V sites, and this is in complete agreement with the theoretical predictions.
    Journal of Catalysis 01/2012; 285(1). DOI:10.1016/j.jcat.2011.09.014 · 6.07 Impact Factor
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    Phosphoros 2011; 09/2011
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    ABSTRACT: Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e. carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT) calculations for simulating experimental data suggests that R1 and R2 are the hydrogen-abstracted alkyl species centered at C5' and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4'. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation. Comparison of these results with those obtained from sucrose single crystals X-irradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH(2)OH hydroxymethyl group is a better radical trapping site than other positions.
    Physical Chemistry Chemical Physics 06/2011; 13(23):11294-302. DOI:10.1039/c0cp02616f · 4.20 Impact Factor
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    ABSTRACT: A trigonal Cr3+ (3d3) defect in sillenite type Bi12SiO20 crystals co-doped with chromium and phosphorus has been identified by means of multi-frequency X-band (9.4 GHz), Q-band (35 GHz) and W-band (94 GHz) electron paramagnetic resonance (EPR). A consistent analysis of the observed spectra and their angular dependence was reached using an S = 3/2 spin Hamiltonian with axial symmetry around a 〈111〉 crystallographic direction, an isotropic g-value g = 1.983, and a zero field splitting parameter B20 = 0.1950 cm−1. The spectra are attributed to a chromium ion in the unusual Cr3+ valence replacing a substitutional Si4+ in tetrahedral oxygen coordination. Evidence is found that the symmetry lowering from tetrahedral to trigonal is not spontaneous but induced by an associated defect, for which a P5+ ion in a nearest-neighbor Si4+ site is the most plausible candidate. Optical excitation results in a reversible charge transfer process directly correlated with a photochromic effect: near-UV light leads to a strong reduction of the Cr3+ EPR signals, and induces a broad band at 675 nm in the absorption spectrum, and both effects are reversed under red light excitation.
    Journal of Applied Physics 04/2011; 109(8):083506-083506-7. DOI:10.1063/1.3561503 · 2.19 Impact Factor
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    ABSTRACT: Rare-earth related centres have been investigated in K(2)YF(5):Tb(3+) crystals, exhibiting thermoluminescence (TL) below and above room temperature (RT), using electron paramagnetic resonance (EPR) spectroscopy at Q(34 GHz) and W-band (94 GHz). The spectra have been studied prior to irradiation, after exposure in the kGy range to X-rays at 77 K and subsequent pulse annealing up to 570 K. In addition to Gd(3+), previously studied in detail, we identified Er(3+) and Yb(3+) centres as accidental impurities in as-grown crystals and determined their effective g tensors. The EPR spectra of irradiated and annealed crystals provide evidence for the production of at least three distinct Tb-related trapped hole centres, two of which could definitely be identified as Tb(4+). Hence, we prove that the Tb(3+) activator ions also act as hole traps in K(2)YF(5). Pulse annealing experiments indicate that the TL above RT results from thermal release of electrons, recombining at these Tb4(+) ions. (C) 2011 Elsevier B.V. All rights reserved.
    Optical Materials 04/2011; 33(6):865-871. DOI:10.1016/j.optmat.2011.01.011 · 2.08 Impact Factor
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    ABSTRACT: In order to find a reliable and efficient calculation scheme for electron paramagnetic resonance (EPR) spectroscopic parameters for transition metal complexes in ionic solids from first principles, periodic and finite cluster-in-vacuo density functional theory (DFT) simulations are performed for g tensors, ligand hyperfine tensors (A), and quadrupole tensors (Q) for Rh(2+)-related centers in NaCl. EPR experiments on NaCl:Rh single crystals identified three Rh(2+) monomer centers, only differing in the number of charge compensating vacancies in their local environment, and one dimer center. Periodic and cluster calculations, both based on periodically optimized structures, are able to reproduce experimentally observed trends in the ligand A and Q tensors and render very satisfactory numerical agreement with experiment. Taking also computation time into account as a criterion, a full periodic approach emerges as most appropriate for these parameters.The g tensor calculations, on the other hand, prove to be insufficiently accurate for model assessment. The calculations also reveal parameters of the complexes which are not directly accessible through experiments, in particular related to their geometry.
    The Journal of Physical Chemistry A 02/2011; 115(9):1721-33. DOI:10.1021/jp109517g · 2.78 Impact Factor

Publication Stats

2k Citations
415.25 Total Impact Points


  • 1987–2014
    • Ghent University
      • • Department of Solid-state Physics
      • • Department of Inorganic and Physical Chemistry
      • • Center for Molecular Modeling
      • • Faculty of Sciences
      • • Department of Analytical Chemistry
      Gand, Flemish, Belgium
  • 2004
    • University of Oslo
      Kristiania (historical), Oslo, Norway
  • 2002
    • Vrije Universiteit Brussel
      Bruxelles, Brussels Capital, Belgium
    • University of Antwerp
      • Department of Physics
      Antwerpen, Flemish, Belgium
  • 1994
    • Catholic University of Louvain
      Лувен-ла-Нев, Walloon, Belgium