共查询到20条相似文献,搜索用时 12 毫秒
1.
N K Porwal R M Kadam Y Babu M D Sastry M D Aggarwal Putcha Venkateswarlu 《Pramana》1997,48(4):929-935
Photo-induced charge transfer and its kinetics were investigated in Bi12SiO20 in 10–300 K temperature range, using EPR of Fe3+ centre, underin situ illumination with copper vapour laser (CVL). The decay kinetics was found to follow double exponential behaviour. Relaxation
of the photo-induced electron transfer to the preillumination condition occurred even at 10 K. Shallow traps were, therefore,
associated with the electron trapping, leading to a better understanding of the fast photorefractive response of BSO. 相似文献
2.
Electron paramagnetic resonance (EPR) evidence is presented for the radiation stabilization of pentavalent uranium in CaO
matrix. From the theoretical predictions ofg value for U5+ in axial symmetries, it was concluded that U5+ at Ca2+ site is associated with a second neighbour charge compensating Ca2+ vacancy. EPR measurements also revealed the presence of Mn2+, Mn4+ and Cu2+ impurities in the samples. The thermal stability of U5+ was investigated using EPR and thermally stimulated luminescence (TSL) techniques. The TSL and EPR studies on gamma irradiated
uranium doped calcium oxide samples had shown that the intense glow peak at 540 K is associated with the reduction in the
intensity of EPR signal of U5+ ion around this temperature. This peak is associated with the process U5++hole→U6+*→U6++hv. The activation energy for this process was determined to be 1.4eV. 相似文献
3.
The electron paramagnetic resonance of Mn2+ and Gd3+ doped in Pr2Zn3(NO3)12.24H2O single crystals has been studied at X-band. Mn2+ substitutes for two Zn2+ sites, while Gd3+ substitutes for single type of Pr3+ sites. The spin-Hamiltonian analysis of the EPR spectra is presented at 298 K as well as 77 K. 相似文献
4.
The electron paramagnetic resonance parameters, zero-field splittings (ZFSs) b20, b40, b44, b60, b64 and the g factors for Gd3+ on the tetragonal Y3+ site in KY3F10 are theoretically studied from the superposition model for the ZFSs and the approximation formula for the g factor containing the admixture of the ground 8S7/2 and the excited 6L7/2 (L=P, D, F, G) states via the spin-orbit coupling interactions, respectively. By analysing the above ZFSs, the local structure
information for the impurity Gd3+ is obtained, i.e., the impurity-ligand bonding angles related to the four-fold (C4) axis for the impurity Gd3+ center are found to be about 0.6° larger than those for the host Y3+ site in KY3F10. The calculated ZFSs based on the above angular distortion as well as the g factors are in reasonable agreement with the observed values. The present studies on the ZFSs and the local structure would
be helpful to understand the optical and magnetic properties of this material with Gd dopants.
相似文献
5.
The efficiency of acoustooptic (AO) interaction in YZ-cut proton exchanged (PE) LiNbO3 waveguides is theoretically analysed by determining the overlap between the optical and acoustic field distributions. The
present analysis takes into account the perturbed SAW field distribution due to the presence of the PE layer on the LiNbO3 substrate determined by the rigorous layered medium approach. The overlap is found to be significant upto very high acoustic
frequencies of the order of 5 GHz, whereas in the earlier analysis by vonHelmolt and Schaffer [6] for diffused waveguides,
it was shown that the overlap integral rolls down to nearly zero at this high frequency range. 相似文献
6.
H. G. Demirkhanyan G. G. Demirkhanyan V. G. Babajanyan R. B. Kostanyan E. P. Kokanyan 《Journal of Contemporary Physics (Armenian Academy of Sciences)》2008,43(1):13-18
Optical spectra of absorption and luminescence of LiNbO3:Yb3+ crystals are investigated experimentally and theoretically. Within the framework of the point charge approximation of crystal field, the Stark problem is solved: crystal field parameters and wave functions of Stark states are determined. Line strengths due to indirect electric-dipole and magnetic-dipole inter-Stark transitions are calculated and values of the Judd-Ofelt parameters are determined. Values of the branching ratios of luminescence. Einstein coefficients and, lifetime of the excited sublevel are calculated. A satisfactory agreement of calculated results with experimental data is obtained. 相似文献
7.
Photo-EPR measurements carried out on Nd3+ -doped polyvinyl alcohol (PVA) films have shown that nearly 100% reduction occurs in the intensity of EPR of Nd3+ under in situ copper vapor laser (CVL) illumination (510.5 nm and 578.2 nm). The kinetics of decay and recovery were investigated. Photoacoustic
(PA) spectra, observed under CVL pump condition had shown that the CVL induced changes were not due to photoinduced valence
change, and that the CVL pumping creates highly favorable conditions for non-equilibrium population distribution in the excited
electronic states. The complete disappearance of EPR under CVL pumping is attributed either to the possible equalization of
population of |+〈 and |−〈 Zeeman components, through the decay of many excited states in the presence of magnetic field or
configurational changes around Nd3+ shifting the resonance frequency. The former appears less probable in view of the relatively slower recovery of EPR signal. 相似文献
8.
Electron spin resonance studies have been carried out on Cu2+ ion doped in single crystals of cadmium maleate dihydrate at 303 and 77K. It has been found that Cu2+ enters this lattice interstitially. The spin Hamiltonian parameters have been evaluated and the ground state wave function
is found to be predominantly |X
2 −Y
2〉 with a slight admixture of |3Z
2 −r
2〉. 相似文献
9.
Room temperature EPR spectra of (NH4)2SO4 doped K2SO4 monocrystals irradiated with x-rays show the presence of NH3
+ radicals. The EPR parameters areg ‖=2.0037 andg ⊥ = 2.0068;14NA
XX=13.75;A
YY=24.5;A
ZZ=25.5 gauss;1HA
XX=A
YY=22 andA
ZZ=25 gauss. From the14N and1H coupling constants it has been inferred that at room temperature the planar NH3
+ radical undergoes rotation about theC
3 axis which corroborates with the equivalence of the protons, but the radical itself is in an asymmetric crystal field environment.
The 77K spectra indicate a considerable reduction in the motion of the radical with the free motion almost completely stopped.
Part of Ph.D. work of the second author 相似文献
10.
The EPR g factors g// and g⊥ for Ti3+ ions at the trigonal octahedral Li+ sites of LiNbO3 and LiTaO3 crystals are calculated from the third-order perturbation formulas of g factors for 3d1 ion in trigonal symmetry. In the calculations, the crystal-field parameters are obtained from the structural data by using the superposition model. The calculated values are in reasonable agreement with the observed values. The results are discussed. 相似文献
11.
The Ce3+ ions incorporation inside lutetium oxyorthosilicate (Lu2SiO5) single crystals was studied by electron paramagnetic resonance. Already known Ce1 and Ce2 centers originating from the lattice peculiarity allowing two lutetium sites coordinated by different number of the oxygen ions were detected. Remarkably, for the Ce2 center, the determined g2 tensor is asymmetric and could not be diagonalized as compared to the Ce1 center, for which the three principal values and corresponding axes orientation have been determined and reported previously. Besides, the much weaker resonance lines found in spectra close to those coming from the Ce1 and Ce2, and following them under crystal rotation with respect to the direction of an external magnetic field, have been revealed as well. They were classified as doublets produced by the exchange coupled Ce3+ ions, creating the Ce1–Ce1, Ce2–Ce2 and Ce1–Ce2-like dimers. The corresponding spin–spin coupling constants were estimated. They are in the range 0.04–0.4 cm−1. The Ce1, Ce2 and total dimer centers populations were calculated as 89%, 4.5% and 6.5%, comparing integral intensities of corresponding resonance lines. 相似文献
12.
The EPR parameters (g factors g
‖, g
⊥ and zero-field splitting D) of Mn4+ ion in h-BaTiO3 crystal are calculated from the complete high-order perturbation formulas based on a two-mechanism model for the EPR parameters
of 3d
3 ions in trigonal symmetry. In the model, not only the widely used crystal-field mechanism, but also the charge-transfer mechanism
(which is not considered in crystal-field theory) are included. The calculated results are in reasonable agreement with the
experimental values. The relative importance of charge-transfer mechanism to EPR parameters and the defect structure of Mn4+ centre in h-BaTiO3 crystal obtained from the calculations are discussed.
相似文献
13.
With a view to understanding the role of photo-induced valence changes of impurities in BaTiO3 in the phenomena of photorefraction, EPR experiments were conducted under in situ He-Ne laser illumination. These experiments gave evidence for photoinduced valence change of Fe in BaTiO3 at room temperature. The EPR signal due to trivalent iron was found to reduce in intensity with laser illumination The kinetics
of the valence change has been investigated. Under large fringe width condition, the time constant of the decay is identified
as the dielectric relaxation time τd. The changes in line shape on laser illumination to Dysonian form, appeared most predominantly in mechanically poled crystal
compared to electrically poled single domain crystals. This demonstrated the possible role of domain walls and the defects
there, as source or sinks of charge carriers on photo excitation. It is observed, that there is transient growth of Fe3+, when the laser illumination was put on, before its decay. This was attributed to charge transfer between electrons in oxygen
vacancies and Fe4+. This predicted the growth of a transient grating under depleted pump condition in a two beam coupling experiment. This was
experimentally proved by following the diffracted signal of the reading beam under the depleted pump condition. 相似文献
14.
I. Stefaniuk 《Opto-Electronics Review》2018,26(2):81-91
In this topic review the results of the X-band electron paramagnetic resonance (EPR) measurements of Mn, Co, Cr, Fe ions in YAlO3 (YAP) crystals and Fe ions in LiNbO3 (LNO) crystals and of chromium doped Bi12GeO20 (BGO) and Ca4GdO(BO3)3 single crystals, are presented. It is well known that the oxide crystals (for example:YAP, LNO, BGO) are one of the most widely used host materials for different optoelectronic applications. The nature of point defect of impurities and produced in the oxide crystal after irradiation by bismuth ions and after irradiation by the 235U ions with energy 9.47 MeV/u and fluency 5?×?1011?cm?1 is discussed. The latter is important for applications of these oxide crystal as laser materials. 相似文献
15.
An impurity mediated mechanism of photorefractive effect in BaTiO3 is proposed. The photoinduced changes in the relative concentration of Fe3+ in BaTiO3 results in an electro-optic coupling through a combination of the Sangster and piezoelectric effects. This is based on the
examination of the extensive results on the EPR of Fe3+ in the BaTiO3 lattice. This model explains the improved photorefractive behavior of BaTiO3 on doping with Co2+. 相似文献
16.
Zheng Wen-Chen 《辐射效应与固体损伤》2013,168(3-4):323-328
Abstract The zero-field splitting D, the anisotropic g-factors g ∥, Δg(=g ∥ ? g ⊥) and the first excited state splitting Δ(2 E) for the trigonal Cr3+–VK center in KMgF3: Cr3+ crystals have been studied from Macfarlane's high-order perturbation formulas. From the studies, the local structure of the trigonal center is obtained. The local lattice distortions (i.e., the displacement directions of the ions in the center) are consistent with the expectation based on the electrostatic interaction. 相似文献
17.
In order to understand the mechanism of optical image storage in photorefractive polyvinyl alcohol (PVA) films, photo-EPR
and photoacoustic spectral investigations were carried out on PVA films doped with Cr3+ and VO2+. The EPR spectrum of Cr3+ has shown reduction in intensity onin situ illumination with copper vapor laser (CVL). The decay and recovery of Cr3+ signal, with and without CVL illumination respectively, was monitored at different temperatures in 10–300 K region. These
were found to obey a double exponential, with one time constant independent of temperature, and the other showing significant
temperature dependence. From Τ(T), activation energy for the charge carrier transport in one of the processes was estimated to be 0.016 eV. The PA spectra
showed shift towards lower wavelength side on consecutive runs. On the other hand, VO2+ doped PVA film has not shown any significant changes in intensity on laser illumination. These observations suggest (i) interaction
of PVA matrix with excited Cr3+ and (ii) predominant non-radiative relaxation in VO2+: PVA system with no change in the oxidation state. 相似文献
18.
The spin Hamiltonian parameters (g factors, hyperfine structure constants and zero-field splittings D and E) and local structures for Mn2+ and Ni2+ in [Zn(en)3](NO3)2 single crystal are theoretically investigated from the perturbation calculations for trigonally distorted 3d5 and trigonally (or orthorhombically) distorted 3d8 cluster. The trigonal Mn2+ and Ni2+ centres are found to undergo the moderate angular variations Δβ of 4.5° and 5.2°, respectively, related to host Zn2+ site due to size mismatch. The orthorhombic Ni2+ centre shows the relative axial elongation ratio ρ (≈ 2.5%) and the relative perpendicular bond length variation ratio τ (≈0.2%). For Mn2+ centre, the contributions to g-shifts ΔgCT (or hyperfine structure constants ACT and zero-field splitting DCT) from charge-transfer (CT) mechanism are opposite in sign and five times (or 5% and 8%) in magnitude compared with those from crystal-field (CF) mechanism. For the trigonal Ni2+ centre, ΔgCT (or DCT) are the same (or opposite) in sign and 17% (or 2%) in magnitude related to those from CF mechanism. For the orthorhombic Ni2+ centre, ΔgCT and ECT (or DCT) are same (or opposite) in sign and 16% and 48% (or 442%) in magnitude with respect to those from the CF mechanism. The signs and magnitudes of the trigonal distortion angles δβ (≈ ?0.3 and 0.4°) related to an ideal octahedron and the local angular variations Δβ related to the host bond angle are suitably illustrated by those of the axial distortion degree (ADD) and the angular variation degree (AVD) of the systems, respectively. 相似文献
19.
20.
Ping SuWen-Chen Zheng 《Physica B: Condensed Matter》2011,406(23):4429-4431
Seven crystal field energy levels (obtained from the optical spectra) and three g factors gx, gy and gz (obtained from electron paramagnetic resonance (EPR) spectra) for Ce3+ ion in Y3Ga5O12 crystal are calculated together by diagonalizing a complete energy matrix. The Hamiltonian of this energy matrix includes all the interactions for 4f1 ion Ce3+ in rhombic crystal field and under an external magnetic field, and so the optical and EPR data can be studied in a unified way. The calculated crystal field energy levels are in better agreement with the experimental values than the calculated values in the previous paper, and the g factors (which have not been calculated previously) are explained reasonably. The results are discussed. 相似文献