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1
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Vibronic Effects in EPR Spectra of (NH_{4})_{2}Cu(BeF_{4})_{2}·6H_{2}0 Single Crystal

100%
Hoffmann S., Gomółka-Marciniak M.
Acta Physica Polonica A
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1993
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vol. 83
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issue 6
817-830
EN
Angular variations of the g-factors and linewidth value were recorded at X-band, at room temperature, and at 77 K. The temperature dependence of the g-factors was measured along the principal g-tensor axes in the temperature range from 4.2 K to 300 K. The rigid lattice g-factors at 4.2 K are: g_{z} = 2.428, g_{y} = 2.138, and g_{x} = 2.058. The g_{x} value very slowly increases linearly on heating, whereas g_{z} and g_{y} nonlinearly tend towards their average value. This is a typical behaviour of Cu(II) EPR in Tutton salt crystals, however the g(T) dependence is much less pronounced in (NH_{4})_{2}Cu(BeF_{4})_{2}·6H_{2}0 as compared to the Cu(II) doped salts. The results are described in terms of the two vibronic coupling models of the strong Jahn-Teller effect. It is shown that the models are not adequate to describe the vibronic g-factor behaviour in paramagnetic Cu(II) crystals which is attributed to the cooperative Jahn-Teller effect existing in these crystals.
2
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Effects of Vibronic Dynamics of Cu(H_2O)_6 Complexes and Electron Spin Relaxation in Temperature Dependence of EPR Linewidth in Diamagnetic Tutton Salt Single Crystals

100%
Hoffmann S., Goslar J.
Acta Physica Polonica A
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2006
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vol. 110
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issue 6
807-816
EN
EPR linewidth of Cu^{2+} in the Tutton salt crystals weakly depends on temperature at low temperatures and then it rapidly grows above 60 K. We present detailed results of measurements and analysis for ^{63}Cu^{2+} in K_2Zn(SO_4)_2·6 H_2O, K_2 Zn(SO_4)_2·6D_2O, (NH_4)_2Mg(SO_4)_2·6H_2O and Cs_2Zn (SO_4)_2·6H_2O in a temperature range of 4.2-300 K and compare them with already published electron spin-lattice relaxation data. The relaxation contributes weakly to the linewidth which is dominated by molecular dynamics and grows exponentially with temperature. To describe this we are discussing the influence of jumps between two sites of Cu^{2+} complexes in a slow motion region where the sites are differently thermally populated. This case has not been considered so far. We have derived appropriate expressions describing the contribution of jumps to the linewidth which allows the determination of the jump rate and energy difference δ_{A,B} between the two sites being two Jahn-Teller distorted configurations of the vibronic Cu(H_2O)_6 complexes. The jump rate 1/τ strongly depends on temperature and reaches 10^9 s^{-1} at room temperature, whereas theδ_{A,B} varies from 117 cm^{-1} for K_2Zn(SO_4)_2·6D_2O to 422 cm^{-1} for Cs_2Zn(SO_4)_2·6 H_2O. The comparison with vibronic level splitting, which varies in the range of 67-102 cm^{-1}, indicates that the reorientation mechanism involves phonon induced tunnelling via excited vibronic levels. These reorientations do not contribute, however, to the spin-lattice relaxation which is governed by ordinary two-phonon relaxation processes in the whole temperature range. Thus, the reorientations and spin relaxation are two independent phenomena contributing to the total linewidth.
3
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Applications of the Transport Integrals in Solid-State Physics and in Electron Spin Relaxation

81%
Radczyk T., Hoffmann S., Goslar J.
Acta Physica Polonica A
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2003
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vol. 104
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issue 5
469-477
EN
Transport integrals I_n(Θ_D/T) are reviewed with their applications in solid-state physics, molecular dynamics, and electron spin-lattice relaxation. Analytical approximations of I_n for n=2-8 are proposed as applicable for computer fitting procedures in the range of the variable x=Θ_D/T from 0.1 to 40. The results are applied for description of the spin-lattice relaxation data collected for Cu^{2+} ions in triglycine sulphate and are compared with relaxation data for Cu^{2+} and Mn^{2+} in (NH_4)_2Mg(SO_4)_2·6H_2O.
4
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The Role of Ferric(III) Oxide in Poly-(1,4-Phenylene Sulphide) Curing. EPR and X-ray Studies

81%
Wejchan-Judek M., Hoffmann S., Goslar J., Garbarczyk J.
Acta Physica Polonica A
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2005
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vol. 108
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issue 1
107-112
EN
Commercially available poly-(1,4-phenylene sulfide) = PPS with trade names TEDUR and RYTON contain a great amount of Fe(III) (order of 10^{20} Fe/gram) as shown by electron spin resonance spectra. The curing of the pure and Fe-doped (0.2% Fe_2O_3) PPS polymers proves that Fe(III) can be used for improvement of the PPS strength by increasing the polymer cross-linking and a decrease in the polymer crystallinity degree as shown by X-ray diffraction.
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Adsorption Properties of the Cu(115) Surface: Basic Interfaces

81%
Godowski P., Groso A., Hoffmann S., Onsgaard J.
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issue 6
928-935
EN
The interfaces: K/Cu(115) and CO/Cu(115) have been characterized using surface sensitive techniques, including low energy electron diffraction and photoelectron spectroscopy. K adatoms show tendency to occupy the sites close to the step edges. At low temperature (near 125 K), after completion of two layers, potassium grows in 3D islands (the Stranski-Krastanov mode). At higher temperature, e.g. at room temperature, potassium introduces reconstruction of the substrate even at low coverages. Calibration of the alkali coverage, up to completion of the first layer, using the work function changes curve has been confirmed as a very convenient and precise procedure. The adsorbed state of CO at 130 K has been identified by registration of core levels obtained by the use synchrotron radiation photoelectron spectroscopy. The characteristics of the main 1s and satellite peaks have been analyzed in context of substrate geometry and compared with the ones of other copper planes. There are no indications of dissociative adsorption of CO, only residual carbon and oxygen were found after adsorbate desorption around 220 K. CO molecules show a strong tendency to "on top" adsorption in sites far from the step edges of the Cu(115) surface.
6
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Electron Spin Relaxation of Cu(II) Ions in ZnGeF_6·6H_2O Crystal with Strong Jahn-Teller Effect

81%
Idziak S., Hoffmann S., Goslar J.
Acta Physica Polonica A
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2005
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vol. 108
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issue 1
177-185
EN
Cu^{2+} ions doped to ZnGeF_6·6H_2O substitute the host Zn^{2+} ions and undergo a strong Jahn-Teller effect producing nearly axial elongation of the Cu(H_2O)_6 octahedra with equal population of the three possible deformations at low temperatures as shown by the EPR spectra. Reorientations between these distorted configurations are observed as a continuous shift of EPR lines leading to averaging of the g- and A-tensors. The full averaging is observed at the phase transition temperature 200 K. Electron spin relaxation was measured up to 45 K only, where the electron spin echo signal was detectable. Electron spin-lattice relaxation is governed by the Raman two-phonon process allowing to determine the Debye temperature asΘ_D=99 K. There is no contribution of the Jahn-Teller dynamics to the spin-lattice relaxation rate. Electron spin echo decay is strongly modulated by dipolar coupling to the ^1H and ^{19}F nuclei. The phase memory time is governed by instantaneous diffusion at helium temperatures and then by spin-lattice relaxation processes and excitation to the first vibronic level of energyΔ=151 cm^{-1}.
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Restricted Validity of the Two-State Model Describing a Vibronic EPR g-Factors Averaging in Cs_{2}Zn(SO_{4})_{2}·6H_{2}O Tutton Salt Crystals Doped with Cu^{2+} Ions

81%
Hoffmann S., Kaszyński R., Augustyniak M., Hilczer W.
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vol. 96
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issue 6
733-740
EN
Single crystals and powder EPR spectra of Cu^{2+} ions in Cs_{2}Zn(SO_{4})_{6}·6H_{2}O were recorded in the temperature range of 4.2-300 K and the g-factor temperature variations were determined. The g_{z} and g_{y} have the values of 2.443 and 2.134, respectively, at the rigid lattice limit below 20 K, and then continuously tend to an average value on heating. This vibronic averaging produced by reorientations of Cu(H_{2}O)_{6}-complexes between Jahn-Teller distorted octahedral configurations is described in terms of a two-state Silver-Getz model which is known as a good model for diamagnetic non-ammonium Tutton salts. We found, however, that this model is only a crude approximation in Cs_{2}Zn(SO_{4})_{6}·6H_{2}O below 150 K. Above this temperature the model works better and describes the vibronic dynamics between the two lowest energy potential wells in the adiabatic potential surface differing in the energy of δ_{12}=318(9) cm^{-1}=3.7 kJ/mol.
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Photoemission Study of CO_{2} adsorbed on K/Cu(110). Analysis of Adsorbate Induced Structures

81%
Godowski P., Onsgaard J., Hoffmann S., Nerlov J.
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vol. 95
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issue 3
423-431
EN
Using photoemission spectroscopy, adsorption and reaction of CO_{2} on potassium modified Cu(110) were studied. In agreement with published results of thermally programmed desorption, apart from carbonate and carbon monoxide as the disproportionation reaction products, a linear CO_{2} molecule and a bent active CO_{2}(-) species were identified. The reaction paths are independent on the potassium precoverage but the number of CO_{3} molecules increases with the number of potassium adatoms. The presence of the CO_{2}(-), stable up to 200 K, suitable for the reactivity of the interface in respect of the methanol synthesis, could be confirmed in the complex valence band spectra by occurrence of the characteristic peak at binding energy of 6.8 eV.
9
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EPR Dating of Hydroxyapatite from Fossil Bones. Transient Effects after γ and UV Irradiation

81%
Wencka M., Hoffmann S., Hercman H.
Acta Physica Polonica A
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2005
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vol. 108
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issue 2
331-337
EN
ESR measurements at room temperature have been performed on fossil bones of cave bear (from Magurska Cave of the Tatra Mountains) and mammoth (from Dniestr River). Various paramagnetic centres related to defects in hydroxyapatite were identified. The ESR spectrum is dominated by Mn^{2+} lines and free radical centres CO_2^- and CO_3^-. Suitability of various EPR lines for the dating was evaluated. The spectra are affected by γ-ray and UV irradiation which produce new relatively unstable (transient) radical centres. From their decay curves we determined the delay period (waiting time) between irradiation and ESR dating measurements. For γ-rays the period is about 15 days and depends on the chosen ESR line. UV generates mainly surface O^- centres which decay practically after a few days. The ESR dating of the bear bones using an additive dose method given the age t = 21.7 kiloyears (ky), whereas from radionuclide Th/U method t= 34 ky and from radiocarbon ^{14}C method t > 33.1 ky.
10
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Binding of the Trace Elements: Cu(II) and Fe(III) to the Native and Modified Nutritive Potato Starches Studied by EPR

81%
Śmigielska H., Lewandowicz G., Goslar J., Hoffmann S.
Acta Physica Polonica A
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2005
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vol. 108
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issue 2
303-310
EN
The Cu(II) and Fe(III) ions have been adsorbed by four potato starches of different degrees of oxidation (different numbers of COOH groups replacing host CH_2OH groups): native (no oxidized), white (pudding) with oxidation degree of 0.04%, gelating (0.1%), and LUBOX starch (0.5%). Concentration of the ions in starches was determined from atomic absorption and EPR spectrum intensity. For small concentration of the adsorbed ions (below 4 mg/g) nearly all ions are adsorbed from the solution. EPR shows that adsorbed copper(II) ions are chemically bonded to the starch molecules (preferably) at COOH sites and uniformly dispersed in the starch structure. The complexes are typical of octahedral or square-quadratic coordination with spin-Hamiltonian parameters g_ǁ=2.373, g_⊥= 2.080, A_ǁ=12.1 mT, A_⊥=1.0 mT. For higher concentrations the Cu(II) displays a tendency to clustering. Iron(III) ions are introduced into starch in a form of clusters mainly, even for the smallest concentration. The highest concentrations of both Cu(II) and Fe(III) were observed in LUBOX starch having the highest degree of oxidation.
11
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Electron Spin-Lattice Relaxation in Polymers and Crystals Related to Disorder and Structure Defects

81%
Hoffmann S., Hilczer W., Radczyk T., Polus I.
Acta Physica Polonica A
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2003
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vol. 103
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issue 4
373-385
EN
Temperature dependences (4-300 K) of the electron spin-lattice relaxation have been determined by electron spin echo technique for free radicals in two polymeric systems: phenol-formaldehyde resin and polyhydrazodisulphide. The dependences are described in terms of dynamics specific for amorphous systems involving two-level tunnelling states at low temperatures, exchange-coupled clusters of paramagnetic centres and local oscillators. Some universal temperature behaviour of the spin-lattice relaxation of amorphous systems is suggested, with a strong increase in relaxation rate with temperature at low temperatures and much weaker increase above 50 K with characteristic linear temperature dependence in a broad temperature range and cosech(Δ/kT) -type behaviour. It is also shown that the amorphous-type behaviour appears in low temperatures relaxation studies of single crystals but it is due to a non-uniform distribution of doped paramagnetic ions. Such behaviour we have found in Tutton salt crystals doped with Cu^{2+}, as well as for free radical centres produced by ionising irradiation in (NH_4)_3H(SeO_4)_2 and Li(N_2H_5)SO_4 single crystals, where extended phonons are suppressed around radiation damage centres suggesting a local amorphisation of the crystal structure.
12
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Temperature Effects in ESR Spectra of Radical Centres in Dripstone Calcite Samples Used for ESR Dating

81%
Wencka M., Hoffmann S., Krzyminiewski R., Mielcarek S.
Acta Physica Polonica A
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2005
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vol. 108
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issue 3
491-503
EN
Natural calcite samples were collected from the dripstone layers of the Black Cave of the Tatra Mountains. ESR spectra, thermal analysis, and thermoluminescence data were analysed to obtain a consistent picture of the calcite properties. The ESR spectra were analysed using computer enhanced resolution spectra and an influence of the thermal annealing on intensity of individual ESR lines was measured. These results together with the effects of γ-irradiation in ESR spectra allowed one to discriminate between ESR lines from various paramagnetic centres. A general conclusion of the paper is that in the ESR dating the outer ESR lines of CO^-_3 (g=2.0125) and CO^-_2 (g=1.9980) radicals should be used instead of the commonly used multicomponent central line.
13
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Electron Spin Echo Envelope Modulation Analysis of SeO¯_{3} Radical in (NH_{4})_{3}H(SeO_{4})_{2} Single Crystal

71%
Tritt-Goc J., Goslar J., Hilczer W., Hoffmann S., Augustyniak M.
Acta Physica Polonica A
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1993
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vol. 84
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issue 6
1131-1141
EN
A detailed computer analysis of the electron spin echo envelope modulations of SeO_{3} radical in a suitable orientation of (NH_{4})_{3}H(SeO_{4})_{2} single crystal is presented. It was found that the modulations are due to a weak dipolar coupling with nitrogens and protons of the only two neighbouring NH_{4} groups among the five NH_{4} groups surrounding SeO¯_{3} center. Isotropic dipolar coupling constant is 1.7 MHz for nitrogens and 0.9 MHz for protons. It was shown that thermal reorientations of NH_{4} groups observed by NMR have a negligible effect on the electron spin echo envelope modulation pattern but can be responsible for the same value of a^{H}_{iso} for all protons in a NH_{4} group. A good fit obtained between experimental spectra and theoretical calculations assuming nondisturbed crystal geometry indicates a small damage of the crystal by X-rays during the radical center formation.
14
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EPR and Spectral Studies of a Molecular and Crystal Structure of Cu (3,5-dimethylpyridine)_{3}(NO_{3})_{2}

71%
Hoffmann S., Goher M., Hilczer W., Goslar J., Hafez A.
Acta Physica Polonica A
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1994
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vol. 85
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issue 3
517-530
EN
X-band single crystal EPR, IR and UV-VIS data are used to deduce a molecular and crystal structure of a new compound with a chemical formula Cu(3,5-dimethylpyridine)_{3}(NO_{3})_{2}. EPR shows that four magnetically inequivalent molecules exist in the crystal unit cell of C_{4v} symmetry. IR spectra suggested that both monodentate and bidentate NO_{3}-groups exist in the crystal. A most probable Cu(II) complex geometry with d_{x²-y²} ground state is a square pyramid with three nitrogens from 3,5-dimethylpyridine molecules and one oxygen from monodentate nitrato group in a basal plane and an oxygen from NO_{3}-group in apical position. There exists a chemical contact between complexes as it is indicated by the exchange interaction with |J| ≤ 1 cm^{-1}. It suggests that square pyramid complexes are arranged in polymeric chains with semi-coordination of bidentate NO_{3}-group to the adjacent Cu(II) ion. An extended analysis of an exchange averaging effect between EPR lines from two and four Cu(II) sites is presented with equations relating molecular and crystal g-factors.
15
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Influence of the Grain Size on Polaron Transport in Nanostructured La_{2/3}Ca_{1/3}MnO_3 Evidenced by EPR

62%
Raita O., Grecu M., Filip X., Toloman D., Giurgiu L., Idziak S., Hoffmann S.
Acta Physica Polonica A
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2005
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vol. 108
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issue 1
113-118
EN
We report an electron paramagnetic resonance investigation of the grain size effects on the polaron transport in the paramagnetic regime of nanosized La_{2/3} Ca_{1/3}MnO_3 manganites. The temperature dependences of the EPR integral intensity were analyzed in terms of spin-spin exchange interaction and small polaron hopping scenarios. The polaron activation energy decreases with the reduction of the grain size. A discussion is given concerning the factors which could explain the observed change.
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