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1
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Investigations of the EPR and Optical Spectra for VO^{2+} in C_3H_7NO_2 Powders

100%
Wei Q.
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|
issue 6
962-964
EN
The EPR and optical spectra for VO^{2+} in C_3 H_7 NO_2 powders are calculated from complete diagonalization method and perturbation theory method, respectively. The calculated results are in good agreement with observed values. The negative signs of hyperfine structure constants A_∥ and A_⊥ for VO^{2+} in C_3 H_7 NO_2 powders are also suggested from the calculations.
2
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Luminescence and Photoconductivity of Cerium Compounds

100%
Yen W., Basun S., Happek U., Raukas M.
Acta Physica Polonica A
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1996
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vol. 90
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issue 2
257-266
EN
The study of the luminescence properties of rare earth doped systems historically focuses on systems which exhibit strong luminescence. More recently, extensive studies on materials with high quantum efficiency are in part motivated by the search for new phosphor and scintillator materials. However, a thorough study of certain systems which show very low quantum yield will certainly lead to a better understanding of phosphor materials and rare earth systems in general. As an example of recent studies which address both the fundamental question of relaxation processes in rare earth doped systems and phosphor applications we present studies on cerium-doped lutetium oxide crystals which are characterized by a complete quenching of the 5d-4f luminescence and compare its optical properties to that of very efficient cerium doped phosphor material, lutetium oxyorthosilicate. To find the mechanisms which lead to the different quantum efficiency in these systems, extensive absorption, photoexcitation and photoconductivity studies were performed on single crystals. We demonstrate that the radically different emission properties of the investigated systems originate in small but crucial differences in the location of the emitting 5d level of the cerium ion with respect to the conduction band of the host - a general result which can be applied to a broad range of materials.
3
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Investigation of the Active Ions Distribution in Laser Crystals Using High Resolution Spectroscopy

100%
Pelletier-Allard N., Pelletier R.
Acta Physica Polonica A
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1996
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vol. 90
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issue 1
143-150
EN
High resolution spectroscopy techniques are used to investigate the distribution of rare-earth ions inside crystal hosts and their transfer properties. Examples concerning two typical laser materials are treated.
4
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Multiphonon Absorption and Photon Avalanche Criterion in Erbium Doped Materials

100%
Auzel F.
Acta Physica Polonica A
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1996
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vol. 90
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issue 1
7-19
EN
The photon avalanche effect has been recently observed at room temperature in erbium doped fluoride glasses and crystals, in a particularly clear way, displaying simultaneously all the typical features of this effect: (i) existence of a threshold for transmission at the excitation wavelength, (ii) up-conversion excitation threshold, (iii) long delay for threshold establishment. Analysing the first step of the avalanche effect as an anti-Stokes multiphonon absorption, it can be shown why this effect is so clearly observed at room temperature in our erbium studies. Our results are compared with other cases of avalanche, with cases which have been called "looping effects" and "quasi-avalanche" ones.
5
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Spectroscopic Study of Rare Earth Chromates: Relation to the Structure

100%
Antic-Fidancev E., Lemaitre-Blaise M., Parada C.
Acta Physica Polonica A
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1996
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vol. 90
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issue 1
33-44
EN
The luminescence spectra of the trivalent europium ion embedded in various rare earth chromates were analyzed. The spectroscopic data in most of chromates are in agreement with the structural determination but for some others the discrepancy between two methods is underlined. Energy level schemes were deduced from the experimental emission spectra and the crystal field simulation has been performed. The maximum splitting of the ^{7}F_{1} manifold of the Eu^{3+} ion as a function of N_{v}, the so-called crystal field strength parameter, is given. This allows us to classify the compounds according to their crystal field extent.
6
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Comparison Between Standard and Modified Judd-Ofelt Theories in a Pr^{3+}-Doped Fluoride Glass

100%
Goldner P., Auzel F.
Acta Physica Polonica A
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1996
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vol. 90
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issue 1
191-196
EN
A method which takes into account normalized oscillator strengths is detailed for the calculation of parameters in Judd-Ofelt theory (B.R. Judd, Phys. Rev. 127, 750 (1962), G.S. Ofelt, J. Chem. Phys. 37, 511 (1962)). In the case of a Pr^{3+} -doped fluorozirconate glass, the Judd-Ofelt parameters obtained in this way do not depend strongly on the transitions included in the fit. Particularly, it is no longer necessary to exclude the ^{3}H_{4} → ^{3}P_{2} transition from the analysis. Three modified theories (F. Auzel, S. Hubert, P. Delamoye, J. Lumin. 26, 251 (1982), A.A. Kornienko, A.A. Kaminskii, E.B. Dunina, Phys. Status Solidi B 157, 267 (1990)) are also considered but do not improve the calculated intensities when the energy of the 5d level is set to its experimentally determined value. Finally, in connection with 1.3 μ amplification, the 1.3 μ reabsorption (^{1}G_{4} → ^{1}D_{2}) oscillator strength is computed from the various models as well as the 1.3 μ emission branching ratio (^{1}G_{4} → ^{3}H_{5}/^{1}G_{4} → ^{3}H_{6}). The best agreement with experiment is obtained with the standard Judd-Ofelt theory.
7
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Steady State Analysis of Photon Avalanche Effect

100%
Pellé F., Goldner P.
Acta Physica Polonica A
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1996
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vol. 90
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issue 1
197-205
EN
An analysis of the three-level model for avalanche up-conversion in a steady state case is presented. Until now, no quantitative criterion has been defined to distinguish avalanche from nonlinear processes giving rise to anti-Stokes emissions. From our model, we drive a quantitative limit for the ratio between non resonant and resonant absorption cross-section to observe an avalanche process. Application of these calculations to practical cases demonstrates the ability of our model to predict an avalanche behavior. The interest of avalanche to pump up-conversion lasers is discussed by introducing a term in the rate equations due to the stimulated emission.
8
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Intra-Shallow-Donor Photoconductivity in Semi-Insulating GaAs

100%
Karpierz K., Sadowski M.
Acta Physica Polonica A
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1991
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vol. 79
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issue 1
121-124
EN
The far-infrared photoconductivity of a semi-insulating GaAs sample was measured at 4.2 K as a function of magnetic field up to 7 T. Apart from a peak corresponding to the 1s - 2p_{+1} transition of the hydrogen-like shallow donor, a well-pronounced structure was observed which does not appear in n-type GaAs.
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