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1.
Low energy (1 keV) electron bombardment of LiNbO 3 single crystals causes the emission of O + ions with a kinetic energy of 20 eV. The emission process can be identified as a Knotek-Feibelman mechanism, whereby the creation of Nb 4p core-holes is followed by an interatomic Auger process which leaves oxygen in a highly repulsive state. This oxygen subsequently desorbs. 相似文献
2.
The dependence between structural-defect generation and the growth kinetics of germanium single crystals pulled from melts is investigated via selective chemical etching and the optical, atomic-force and scanning electron microscopy techniques. It is ascertained that the surface microrelief of germanium crystals grown from melts by means of directional crystallization contains protrusions and cavities with spatial periodicities of 5 and 50 μm. The values of the kinetic coefficients are estimated. It is demonstrated that the main part of the crystals is formed according to the normal mechanism with the kinetic coefficients β k = 2 × 10 ?5 m s ?1 K ?1. 相似文献
3.
采用飞行时间质谱计测量了纳秒激光诱导C 60分子碎裂中轻碎片离子C +n( n≤11)的初始平均动能,结果显示轻碎片离子具有相同的初始平均动能(约为0.34eV),并且该动能在一定范围内不随激光通量的变化而明显改变.结合前人的实验结果,对纳秒激光诱导C 60分子碎裂中轻碎片离子C +n( n<30)的主要产生模式作了新的阐述,即C<
关键词:
飞行时间质谱计
轻碎片离子
笼形结构塌陷
初始动能 相似文献
4.
Trigonal and monoclinic Gd 3+ centers with close initial splittings that have no analogs in crystals without yttrium are detected in the electron spin resonance spectrum of Y 0.03Cd 0.97F 2.03: Gd 3+ single crystals. The centers observed are attributed to gadolinium ions localized in tetrahedral yttrium and yttrium-cadmium clusters. The values of the parameters of the second-rank spin Hamiltonian are explained (in order of magnitude) in terms of a superposition model that uses the tetrahedral-cluster structure obtained by minimizing the energy of a lattice with a rare-earth cluster. 相似文献
5.
In this topic review the results of the X-band electron paramagnetic resonance (EPR) measurements of Mn, Co, Cr, Fe ions in YAlO 3 (YAP) crystals and Fe ions in LiNbO 3 (LNO) crystals and of chromium doped Bi 12GeO 20 (BGO) and Ca 4GdO(BO 3) 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?×?10 11?cm ?1 is discussed. The latter is important for applications of these oxide crystal as laser materials. 相似文献
6.
Photochemical properties of Ce 3+:SrAlF 5 and Ce 3+,Yb 3+:SrAlF 5 single crystals together with spectroscopic and kinetic characteristics of several optically nonequivalent impurity centers and energy transfer between them are described. It is shown that co-activation by Yb 3+ ions effectively suppresses color centers in Ce,Yb:SAF crystals. It was found out that in Ce,Yb:SAF crystals Yb ions exist simultaneously in 2+ and 3+ valent state. Three types of optically nonequivalent luminescent centers corresponding to the doublets in luminescence spectrum centered at 290, 305 and 370 nm (Ce I, Ce II, Ce III, respectively) have been observed. Analysis of luminescence spectra and decays leads to the conclusion that there is no energy transfer between either cerium centers or from Ce 3+ to Yb 2+ apart from the Ce III center which luminescence is slightly quenched by Yb 2+. 相似文献
7.
Optical and electron paramagnetic resonance study have been carried out on BaY 2F 8 single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY 2F 8 single crystals doped with Yb due to the 2F 7/2→ 2F 5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb 2+ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10 5 Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb 3+ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb 3+ and Er 3+, where Er 3+ ions are unintentional dopants. The EPR spectra of BaY 2F 8:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure. 相似文献
8.
The electron paramagnetic resonance (EPR) spectra of gamma-irradiated single crystals of phenidone (fenidon C 9H 10N 2O) have been studied for different orientations of crystals in a magnetic field. Phenidone single crystals have been irradiated with 60Co-γ rays at room temperature. The EPR spectra have been investigated at temperatures between 125 and 450 K. The spectra have been found to be temperature independent. The spin-Hamiltonian parameters have been obtained from the single-crystal EPR analysis. The principal values of the hyperfine coupling tensor of the unpaired electron and the principal values of the g-tensor have been determined. 相似文献
9.
We have measured fragment kinetic energies in electron induced fission of 232Th for electron energies in the range 7 MeV ≦ Ee ≦ 66 MeV. The relative contribution of the distribution peak associated with high fragment kinetic energies decreases continuously with electron energy. This is interpreted as a relative increase of the symmetric fission yield as compared to the asymmetric fission yield; this fact in turn indicates a non-negligible increase in the average excitation of the fissioning nucleus, with the energy of the bombarding electrons, even above the giant dipole resonance. 相似文献
10.
Spectral optical investigations of two low-dimensional organic molecular conductors with differently oriented conducting layers of ethylenedithiotetrathiafulvalene (EDT-TTF) molecules, namely, the (EDT-TTF)3Hg2Br6 and (EDT-TTF)3Hg(SCN)3I0.5(PhCl)0.5 single crystals, have been carried out. The polarized reflectance spectra of the single crystals have been measured in the frequency range 700–6500 cm−1 (0.087–0.810 eV) at temperatures from 300 to 15 K. The optical conductivity spectra have been obtained using the Kramers-Kronig relations, and their quantitative analysis has been performed in terms of a theoretical model that takes into account electron-electron correlations in the approximation of the Hubbard Hamiltonian for trimerized stacks, the vibronic coupling, and the influence of the counterion on the electronic states in the trimer. A satisfactory agreement between the theoretical and experimental spectra for both crystals made it possible to estimate the parameters of the electronic structure of the crystals in the conducting plane: the integral t of the electron transfer between the EDT-TTF molecules in the trimer, the energy U of the Coulomb repulsion between two electrons (holes) in one EDT-TTF molecule, the electron transfer damping constant γ
e
, the energy shift Δ of the molecular orbital under the influence of the anions and vibronic coupling, the vibronic coupling constant g
n
, and the binding energy E
p
of the molecular polaron. It has been found that there are large differences in the anisotropies of the optical properties and the obtained Hubbard parameters of the electronic structure for the studied crystals. 相似文献
11.
The luminescence of Ca 2GeO 4: Cr 4+ single crystals at wavelengths in the range of 1.3 μm upon excitation with a 1-μ m semiconductor laser is investigated in the temperature range up to 573 K. At T<110 K, the Ca 2GeO 4: Cr 4+ crystals are characterized by the electron paramagnetic resonance, which is attributed to the Cr 4+ ions substituted for Ge 4+ ions. The components of the g tensor and its principal axes are determined. It is revealed that the Cr 4+ impurity centers in calcium germanate affect the crystal symmetry to a lesser degree compared to Cr 4+ ions in forsterite. The observed deviation of the temperature dependence of the electron paramagnetic resonance from the Curie law is explained by the transition to the excited state with a low activation energy, as is the case in impurity 3 d ions in diamond-like semiconductors. The inference is made that the giant effective degeneracy multiplicity of the excited state is associated with the initiation of soft phonon modes in the crystal upon excitation of the defect. 相似文献
12.
Experimental results of a mass-spectro metric analysis of photodesorption from ZnO single crystals at different temperatures are reported. They provide direct evidence that CO 2 is the only photodesorbed species from both the single crystal and powder samples studied. The CO 2 photodesorption occurs only when the incident photon energy exceeds the ZnO band gap energy. Excellent agreement between the illumination time dependence of the CO 2 photodesorption and surface conductivity data in both single crystals and powder samples strongly suggests a substrate dependent mechanism in which photodesorption occurs by the neutralization of chemisorbed CO 2? “ion-molecules” by photogenerated holes. In addition, measurements of the chemisorption kinetics of oxygen on ZnO single crystal and powder surfaces are reported. The results are compared with CO 2 and CO chemisorptiun experiments to show that, of these gases, only oxygen chemisorbs from the gas phase. Auger analysis of oxygen saturated and photodesorbed surfaces of ZnO show a significant relation between the carbon content and the photo-desorptive and conductive activities of those surfaces. These observations indicate that impurity carbon atoms on ZnO surfaces can be oxidized by electron capture to produce chemisorbed CO 2? “ion-molecules’ which will then readily photodesorb by bandgap radiation. This proposed process is discussed together with further supporting evidence. 相似文献
13.
This paper reports on luminescence studies of lithium borate Li 6Gd(BO 3) 3 doped with Eu 3+ and Ce 3+ and Li 6Eu(BO 3) 3 crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd 3+ → Ce 3+ and Gd 3+ → Eu 3+ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce 3+-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed. 相似文献
14.
Up-conversion blue emissions of trivalent thulium ions in monoclinic KGd(WO 4) 2 single crystals at 454 and 479 nm are reported for a single pump laser source at 688 nm. We grew thulium-doped KGd(WO 4) 2 single crystals at several concentrations from 0.1% to 10%. We recorded a polarized optical absorption spectrum for the 3F 2+ 3F 3 energy levels of thulium at room temperature and low temperature (6 K). From the low temperature emission spectra we determined the splitting of the 3H 6 ground state. The blue emissions are characterized as a function of the dopant concentration and temperature from 10 K to room temperature. To our knowledge, this is the first time that sequential two-photon excitation process (STEP) generated blue emissions in thulium-doped single crystals with a single excitation wavelength. 相似文献
15.
The using of sonochemically prepared components for growth of SbI 3·3S 8 single crystals from the vapor phase is presented for the first time. The good optical quality of the obtained crystals is important because this material is valuable for optoelectronics due to its non-linear optical properties. The products were characterized by using techniques such as X-ray crystallography, powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, high-resolution transmission electron microscopy, selected area electron diffraction, optical diffuse reflection spectroscopy and optical transmittance spectroscopy. The direct and indirect forbidden energy gaps of SbI 3·3S 8 illuminated with plane polarized light with electric field parallel and perpendicular to the c-axis of the crystal have been determined. The second harmonic generation of light in the grown crystals was observed. 相似文献
16.
The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3 d and carbon 2 p states at ca. 3–4-eV binding energy, and a single carbon 2 s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2 p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3 d and C 2 p states. The C 2 p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW 5–9, MAPW 10 and EPM 11 band structure calculations predict a flat band of p-character between the symmetry points X′ 4 and K3, most likely responsible for the majority of C 2 p excitations observed. The C 2 s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations 5–11, 13 that predict a combined bonding of covalent, ionic and metallic nature. 相似文献
17.
The bivalent chromium impurity centers in CdF 2 and CaF 2 crystals are investigated using electron paramagnetic resonance (EPR) in the frequency range 9.3–300 GHz. It is found that Cr 2+ ions in the lattices of these crystals occupy cation positions and form [CrF 4F 4] 6? clusters whose magnetic properties at low temperatures are characterized by orthorhombic symmetry. The parameters of the electron Zeeman and ligand interactions of the Cr 2+ ion with four fluorine ions in the nearest environment are determined. The initial splittings in the system of spin energy levels of the cluster are measured. 相似文献
18.
In ScF 3 single crystals (pure and doped) as well as in Rb 2KScF 6 and Rb 2KDyF 6 crystals with a perovskite-like structure, point nanodefects (vacancy in place of trivalent cations) have been found and studied. Electron paramagnetic resonance has been used to investigate local paramagnetic centers that are not detected using X-ray diffraction. The angular dependence of the spectra indicates a local distortion of the cubic symmetry of the crystals. An additional hyperfine structure in the observed spectra is due to the delocalization of electrons over six F ? ions forming the first coordination polyhedron around the vacancy. The crystals studied are characterized by a high electron mobility and a high electron velocity, which depends on the impurity. The high mobility of electrons of the cation center can be indirectly responsible for the structural phase transition occurring in the ScF 3 crystal under uniaxial pressure. 相似文献
19.
Cu 2+-doped Cs 2CO 3 and CsHCO 3 single crystals were investigated by electron paramagnetic resonance between 113–273 and 173–313 K, respectively. For both single crystals, two sites were observed for the Cu 2+ at ambient temperature for arbitrary orientations of the single crystals in the magnetic field. However, when the temperature is varied, the spectra indicate the equivalence of the two sites at 225 and 240 K for the single crystals, respectively, to the above order. Below and above these temperatures two sites for Cu 2+ appear, and below 133 and 173 K the signals do not vary and two sites were always observed. This is attributed to the transition of the dynamic Jahn–Teller effect to a static situation at lower temperatures. Cu 2+ seems to replace Cs + and the charge compensation is fulfilled by another Cs +. Spin-Hamiltonian parameters for both single crystals at ambient temperature are reported and discussed. 相似文献
20.
The electron paramagnetic resonance of Mn 2+ and Gd 3+ doped in Pr 2Zn 3(NO 3) 12.24H 2O single crystals has been studied at X-band. Mn 2+ substitutes for two Zn 2+ sites, while Gd 3+ substitutes for single type of Pr 3+ sites. The spin-Hamiltonian analysis of the EPR spectra is presented at 298 K as well as 77 K. 相似文献
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