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61.
Jigang Yin Yin Hang Lianhan Zhang Jing Xiong Pengchao Hu 《Journal of luminescence》2010,130(8):1338-1342
The electronic structures of LiYF4 (YLF) crystals containing F color center (YLF-F) and Yb doped YLF crystals (Yb3+:YLF, Yb2+:YLF) are systematically studied within the framework of the density functional theory. The calculated results indicate that the 330 nm absorption band originates from the F center in YLF crystals. Thus the doping of Yb3+ can weaken the 330 nm absorption band by competing with F vacancies in capturing free electrons arising after γ-irradiation and change to Yb2. By analyzing the lattice relaxation and the electronic structure of YLF containing Yb2+, we can reasonably believe that once Yb2+ is formed in YLF crystal, its compensating hole will turn out to be shared by two F− nearest to Yb2+ forming a diatomic fluoride molecular ion () perturbed by Yb2+, or to say VF color center. According to the molecular-orbital linear combination of atomic orbital (MO-LCAOs) theory, compared to the alkali halides, e.g. LiF, the in VF center in LiYF4 peaks at about 340 nm, which is in agreement with the experimental results. 相似文献
62.
Irradiation effects on emission centers were studied with photo and X-ray luminescence investigations of alkali chromate crystals. The X-rays and powerful laser irradiations were used for this investigation. The irradiation effects were found to depend on a type and structure of emission centers. In the alkali Chromate crystals complex emission centers of a molecular type are formed by two weight centers: the Chromate oxyanion and the defect. The bond between weight centers can be broken under irradiation action. As established in our experiment, some types of the emission centers can decay or transform into other types under action of irradiation. 相似文献
63.
Abstract Results are reported of measurements of the properties of diodes formed by ion implantation, and for comparison boron diffused P+N diodes of similar area close by on the same chip. The four group III acceptor impurities were implanted separately to a dose of 5 × 1015 ions/cm2 at room temperature into similar samples of suitably masked silicon. Boron ions were also implanted at liquid nitrogen temperature and 450°C. Annealing was limited to a maximum temperatare of 550 °C. Measurements have been made of sheet resistance, forward and reverse I-V characteristics (from 10?9 amps/cm2), reverse breakdown voltage, noise, minority carrier storage time and junction series resistance. The bulk properties of boron implanted diodes were found to be reproducible. The introduction of recombination centres by implantation is the major factor influencing variation in these properties between one implantation condition and another. Changes in surface oxide conditions probably affect reverse leakage currents and breakdown voltages. The properties of boron implanted diodes are considered suitable for applications such as the MOSFET, and are superior to those of the AI, Ga and In implanted diodes. 相似文献
64.
P. W. Levy 《辐射效应与固体损伤》2013,168(1-4):259-264
The properties of thermoluminescence (TL) glow curves have been studied in systems, containing one or more glow peaks, not restricted to the assumptions used to derive the usual first and second order glow peak kinetic expressions. If retrapping is negligible first order glow peaks are obtained whose peak temperature and shape are independent of other factors such as initial trapped charge concentrations. If retrapping occurs, glow peaks are obtained that are, in most cases, only approximated by first or second order kinetics. Also, in these cases the peak temperature, shape, relative intensity and other glow curve characteristics depend strongly on initial trapped charge concentrations, recombination and retrapping cross sections and other factors. 相似文献
65.
Dr. Dinar Abdullin Nico Fleck Christoph Klein Philipp Brehm Sebastian Spicher Prof. Dr. Arne Lützen Prof. Dr. Stefan Grimme Prof. Dr. Olav Schiemann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(10):2586-2596
Iron(III) porphyrins have the propensity to form μ2-oxo-dimers, the structures of which resemble two wheels on an axle. Whereas their crystal structure is known, their solution structure and internal dynamics is not. In the present work, the structure and dynamics of such dimers were studied by means of electron paramagnetic resonance (EPR) spectroscopy and quantum chemistry based molecular dynamics (MD) simulations by using the semiempirical tight-binding method (GFN-xTB). To enable EPR investigation of the dimers, a nitroxide was attached to each of the tetraphenylporphyrin cores through a linear and a bent linker. The inter-nitroxide distance distributions within the dimers were determined by continuous-wave (cw)-EPR and pulsed electron–electron double resonance (PELDOR or DEER) experiments and, with the help of MD, interpreted in terms of the rotation of the porphyrin planes with respect to each other around the Fe–O–Fe axis. It was found that such rotation is restricted to the four registers defined by the phenyl substituents. Within the registers, the rotation angle swings between 30° and 60° in the proximal and between 125° and 145° in the distal register. With EPR, all four angles were found to be equally populated, whereas the 30° and 145° angles are strongly favored to the expense of the 60° and 125° angles in the MD simulation. In either case, the internal dynamics of these dimers thus resemble the motion of a step motor. 相似文献
66.
Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans 下载免费PDF全文
Dr. Sunggi Lee Dr. Han Yong Bae Prof. Dr. Benjamin List 《Angewandte Chemie (International ed. in English)》2018,57(37):12162-12166
O‐heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo‐ and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five‐ and six‐membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5‐trisubstituted and 2,2,5,5‐tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three‐step synthesis of the side chain of anhydroharringtonine. 相似文献
67.
Maulik N. Mungalpara Jingjing Wang Martyn P. Coles Paul G. Plieger Gareth J. Rowlands 《Tetrahedron》2018,74(38):5519-5527
A planar chiral secondary phosphine oxide based on [2.2]paracyclophane was synthesized and its chemistry investigated; it was shown to be a competent pre-ligand in palladium(0)-mediated reactions, and displayed promising activity in gold(I)-catalysed cyclisations. The secondary phosphine oxide could be transformed into a collection of P-stereogenic tertiary phosphine oxides. These are rare examples of the planar chirality of [2.2]paracyclophane being combined with a P-stereogenic centre. Unfortunately, epimerisation of the phosphorus stereocentre during reduction limits the use of this chemistry. 相似文献
68.
Summary Samples of Nasicon compounds (Na super ionic conductor) with different silicon and phosphorus contents were investigated by
performing radioluminescence experiments. In all samples examined, the Frenkel pairs, originated by the displacements of sodium
ions in lacunar sites, were found to be the dominant defects. Some useful information on sodium ion mobility was obtained
from the analysis of radioluminescence spectra. 相似文献
69.
The layered-step interior-point algorithm was introduced by Vavasis and Ye. The algorithm accelerates the path following interior-point algorithm and its arithmetic complexity depends only on the coefficient matrixA. The main drawback of the algorithm is the use of an unknown big constant
in computing the search direction and to initiate the algorithm. We propose a modified layered-step interior-point algorithm which does not use the big constant in computing the search direction. The constant is required only for initialization when a well-centered feasible solution is not available, and it is not required if an upper bound on the norm of a primal—dual optimal solution is known in advance. The complexity of the simplified algorithm is the same as that of Vavasis and Ye. © 1998 The Mathematical Programming Society, Inc. Published by Elsevier Science B.V.Research supported in part by ONR contract N00014-94-C-0007 and the Grant-in-Aid for Scientific Research (C) 08680478 and the Grant-in-Aid for Encouragement of Young Scientists (A) 08780227 of the Ministry of Science, Education and Culture of Japan. This research was partially done while S. Mizuno and T. Tsuchiya were visiting IBM Almaden Research Center in the summer of 1995. 相似文献
70.
C. Bernard-Henriet J. Jaud A. Baldy J. M. Mattalia M. Chanon 《Journal of chemical crystallography》1997,27(8):485-488
The crystal of 5-phenyl-2-endo-(propane-2-sulfonyl)-bicyclo[2-2-1]hept-5-ene-2-carbonitrile1, a new radical-clock is described. The structure has confirmed the configuration for the positions of the phenyl and sulfonyl
groups. In the crystal, the radical precursor carbon atom of the propane group is not neighbouring with the double bond of
the norbornene. Nevertheless, in solution, it may be expected an ideal conformation with a short intergroup distance leading
to a cyclization process. Crystal data: C17H19NO2S, triclinic, space group P−1,a=6.567 (3),b=10.407(5),c=12.610(6), α=110.48(2), β=95.88(2), λ=102.67(3). 相似文献