稀土在无机晶体中的发光

前言一、晶体中稀土离子的电子跃迁和发光光谱 1.f—f电子跃迁 2.f—d电子跃迁二、基质晶格对稀土离子以及稀土离子之间的相互作用 1.基质晶格对稀土离子发光的影响     

稀土离子掺杂Ⅲ-Ⅴ族半导体的发光

稀土离子掺杂Ⅲ-Ⅴ族半导体的发光是近年来开展起来的新的研究领域.在GaAs,InP和GaP等材料中,用离子注入、LPE、MBE、MOCVD以及单晶生长等方法掺杂Er、Yb和Nd等稀土离子,得到了尖锐稀土离子的特征发光.这些发光来自占据正常立方格点和非立方格点的稀土离子的内部4f能级跃迁,其发光行为与掺杂条件关系很大.这种稀土一半导体材料已开始用来制备具有稳定发射波长的发光和激光器件.     

Judd—Ofelt模型及其应用

稀土离子激活的发光晶体在现代科学技术中有着广泛的应用.许多种发光粉、上转换材料及激光材料就是掺稀土离子的.三价自由稀土离子的4fN组态的电子波函数具有相同的宇称,没有电偶极辐射和吸收.当三价稀土离子掺入晶体后,看到了发光,对此早有一些定性的解释.Judd和Ofelt由其中一种解释即静态晶场引起相反宇称的组态混杂出发,推导了跃迁几率表达式,后来被称为J—O理论或J—O模型,用于对实际稀土发光体进行定量计算.     

Yb:YAG及Cr,Yb:YAG晶体的发光特性

研究了室温下YbYAG的上转换荧光光谱,此荧光归因于Yb3+离子的"合作"发光和Yb3+离子到稀土杂质离子的能量转移.测试了YbYAG晶体的X射线荧光,发光峰对应于电荷迁移态到Yb3+离子的基态、激光态间的跃迁.研究了Cr,YbYAG晶体的荧光光谱,讨论了Cr4+激光输出的可能性.     

CaF2:Yb3+/Mn2+体系中Mn2+的上转换发光

稀土离子的上转换发光通常具有发射带宽窄及峰位难以调节等问题。为了获得近红外光激发下的宽带上转换发光,我们对Yb~(3+)-Mn~(2+)共掺杂CaF_2材料的上转换发光性质进行了研究。将稀土离子Yb~(3+)及过渡族金属离子Mn~(2+)掺入到CaF_2材料中作为发光中心,利用高温固相反应法制备了Yb~(3+)单独掺杂及不同浓度Yb~(3+)及Mn~(2+)离子共掺杂的CaF_2体相材料。在980 nm近红外光激发下对不同样品的上转换发光进行了比较研究。实验结果表明,与单独掺杂Yb~(3+)离子的材料相比,CaF_2∶Yb~(3+)/Mn~(2+)材料在980 nm激光激发下出现了一个位于620nm附近的宽带发光,我们认为这个发光来自于Yb~(3+)团簇向Mn~(2+)离子的合作敏化,对应于Mn~(2+)离子的~4T_1→~6A_1跃迁。因此,CaF_2体系中存在Yb~(3+)离子二聚体向Mn~(2+)离子的合作能量传递过程。     

发光学讲座 第五讲 稀土发光

本文通过对稀土元素电子结构的特点、稀土离子在晶体中的能级跃迁及其对应的发光光谱的分析讨论，阐述了稀土发光的几个基本问题，介绍了稀土发光在科学技术、国民经济中的主要应用情况．     

协同发光效应及机理探讨

在研究共存离子对稀土配合物荧光强度影响时发现,许多稀土和非稀土离子在一定浓度范围内可对配合物的荧光产生增强效应.这种现象与协同萃取类似,我们将其命名为“协同发光效应”.本文采用激光诱导荧光光谱法,研究了十四种稀土离子对Eu、Sm-TTA-Phen-Triton X-100体系和Eu、Sm-DBM-CPB体系的协同发光效应.并从稀土离子的化学性质,光谱能级和所形成配合物的化学结构和空间结构等方面综合考虑,提出协同发光机理.拟定出用于痕量铕和钐测定的薪的超高灵敏度分析方法.     

固体中过渡金属离子占位、价态及光谱性质的第一性原理研究EI北大核心CSCD

过渡金属(TM)激活离子由于在近红外发光、红外激光、荧光转化的白光LED、荧光温度计、余辉发光等方面的优异性能和应用潜力而被广泛研究。TM离子在固体中可占据八配位、六配位、四配位等多种配位格位,可呈现多种价态且光跃迁性质强烈依赖于晶体环境,因此TM离子在固体中的发光中心确定、发光机理理解和性能预测存在困难。本文通过第一性原理计算探索固体中TM离子的热力学和光跃迁性质。内容包括:通过对各种化学氛围下形成能的计算结果,分析基质的本征缺陷以及TM离子占位、价态、分布和浓度;理解不同晶体环境中TM离子的各激发态和能级结构;构建位形坐标图分析激发、弛豫、发射及猝灭过程;提出通过合成氛围、共存条件和离子共掺等方式调控或优化TM离子的占位、价态和光跃迁的方案。本文以若干具有代表性的体系为依托,展示了如何运用第一性原理计算手段进行掺TM离子固体发光材料的研究。具体所涉及的代表体系和研究内容为:Ti:Al_(2)O_(3)激光晶体中红外残余吸收机理及其减弱或尽可能消除的方法,典型尖晶石和石榴石基质中Mn^(2+)、Mn^(3+)、Mn^(4+)的占位和激发、弛豫、发射等光跃迁性质,固溶氧化物基质中铬离子的占位、价态及相应的光跃迁性质等,表明第一性原理计算可用于发光材料的机理研究、理性设计和优化。     

稀土离子的多格位占据及能量传递

多格位占据及其能量传递普遍存在于稀土发光材料的各个领域,且具有重要的科学及现实意义。Ce³⁺的4f-5d和Eu³⁺的4f-4f跃迁代表了稀土离子两种典型的电子跃迁类型。我们以Ce³⁺或Eu³⁺分别激活的E(δ)-Gd₂Si₂O₇(Pnma)、G-La₂Si₂O₇(P2₁/n)和F-Eu₂Si₂O₇(■)为代表案例,讨论和思考稀土离子的多格位占据及其非辐射能量传递,重点阐述了晶体结构、样品相纯度、掺杂浓度、光谱测试、格位间能量传递和光谱分辨率等需关注的事项。在稀土离子激活的发光材料研究中,我们既需考量材料晶体结构与光谱的自洽,也要从光谱及发光衰减动力学对格位间能量传递(包括传出能量和接收能量)方面进行分析。希望这些内容在实验及表征上对刚涉足相关领域的读者有所启发。     

温度对晶体中稀土离子(E)-型能级电磁感应透明的影响

运用产经典理论，在Pr^3 ：YSO晶体Pr离子E－型三级级系统中，求解了探测光吸收在不同光跃迁驰豫速率与自旋跃迁驰豫速率下的瞬态及稳态解，得到了电磁感应透明现象，分析了两种驰豫对电磁感应透明的,人而揭示了掺稀土离子晶体中电磁感应透明对温度的依赖性。     

Selection rules for electric multipole transition of triatomic molecule in scattering experiments

In the electron or x-ray scattering experiment,the measured spectra at larger momentum transfer are dominated by the electric dipole-forbidden transitions,while the corresponding selection rules for triatomic molecules have not been clearly elucidated.In this work,based on the molecular point group,the selection rules for the electric multipolarities of the electronic transitions of triatomic molecules are derived and summarized into several tables with the variation of molecular geometry in the transition process being considered.Based on the summarized selection rules,the electron energy loss spectra of H_2O,CO_2,and N_2O are identified,and the momentum transfer dependence behaviors of their valence-shell excitations are explained.     

Charge transfer-transitions and X-ray photoelectron spectroscopy of copper(II) complexes

Satellite structure in the X-ray photoelectron (Cu 2p) spectra of some squareplanar cupric complexes has been compared to electron paramagnetic resonance and electronic spectra results on the electronic structure of these complexes. It is shown that these satellites can be assigned to various ligand → metal 3d or ligand → ligand* transitions within the monopole selection rules of the sudden approximation.     

奇异粒子非轻子衰变的选择定则

本文用SU₃和G₂羣讨论了奇异粒子非轻子衰变的弱作用的变换性质。若强作用是SU₃对称的,可引进类似于同位旋(T旋)的V旋和U旋,和T旋一起来表示奇子非轻子衰变的选择定则。求得了简单选择定则情况下各个衰变过程的关系,其中包含了前人的结果而避免引用过多的假设。若强作用是G₂对称的,可引进包含同位旋在内的六种“旋”来给出选择定则。根据这些定则求得若干衰变过程的关系。证明了弱作用不能按G₂羣的七维或十四维表示变换。     

Majorana mass term,Dirac neutrinos and selective neutrino oscillations

In a theory of neutrino mixing via a Majorana mass term involving only the left-handed neutrinos there exist selection rules for neutrino oscillations if true Dirac and/or exactly zero mass eigenstates are present. In the case of three neutrino flavours no oscillation is allowed if the mass spectrum contains one Dirac and one nondegenerate Majorana massive neutrino. The origin of these selection rules and their implications are discussed and the number of possible CP-violating phases in the lepton mixing matrix when Dirac and Majorana mass eigenstates coexist is given.     

ZnO晶体的偏振拉曼散射的深入研究

利用拉曼选择定则,设计了ZnO单晶的直角偏振几何配置。在室温下测量了ZnO单晶的各种振动模式的偏振拉曼散射光谱。与原先的文献相比较,初步讨论了各种振动模式的线宽和强度的变化原因。除ZnO晶体中包括非极性和极性拉曼基频振动,准横光学和准纵光学模式和振动属性被指认外,它们的高阶拉曼散射模式首先被确定。本研究结果为深入了解ZnO晶体和薄膜的宏观性质和微观结构提供了依据。     

晶体管施密特电路的开关过程

本文研究了施密特电路的开关过程。推导出了开关过程的具体解析表达式。根据这个表达式对两种常见的晶体管施密特电路进行了数值计算。最后用实验方法验证了计算结果。实验结果与计算值总的来说相当吻合。从这个研究中得到了挑选晶体管和电路的一些原则。计算结果可以作为设计施密特电路时估计开关速度的一个依据。     

The rotational resonance Raman spectrum of nitrogen dioxide and the determination of electronic state symmetry from resonance Raman selection rules

The pure rotational Raman spectrum of nitrogen dioxide has been observed and shown to be consistent with existing determinations of molecular parameters. Upon observation at 600 Torr pressure and 0.4 cm⁻¹ resolution a well-defined rotational spectrum is obtained. This spectrum is overlaid with a number of fluorescence lines. The fluorescence lines are separated from the Raman spectrum by a comparison of Stokes and anti-Stokes branches of the rotational spectrum. Out of seven strong fluorescence lines seen with 5145 Å excitation, five probably are identifiable with vibration-rotation fluorescence progressions observed by Abe.The most striking feature of these observations is the potential use of the resonance Raman effect for the analysis of complicated electronic spectra. When this rotational spectrum is observed with excitation by 5309 Å or 5145 Å excitation, the Raman spectrum follows a-axis selection rules and the Q-branches are in the noise level or barely out of it. However, at 4880 Å the ΔK = 2Q-branches become a major feature of a spectrum, indicating that an appreciable part of the absorption at this wavelength is occurring through the operation of b- or c-axis selection rules. These findings are consistent with present notions of a ²B₂ excited state dominating absorption at longer wavelengths, while at shorter wavelengths a ²B₁ excited state becomes important. Given a tunable laser, one could map the relative importance of these two possible selection rules for NO₂ without any theoretical analysis more sophisticated than that presented in this paper.A simplified statement of the selection rules for resonance rotational Raman spectra of asymmetric tops has been developed in the course of this investigation. No attempt has been made to refine the rotational parameters of NO₂ since all of the lines seen areunresolved multiplets. Our data should be regarded as a search spectrum preliminary to investigation on a high resolution instrument.     

Light absorption involving longitudinal optical phonons in semiconductor quantum dots

A theory of single-photon interband transitions involving optical phonons in semiconductor quantum dots (QDs) has been developed. This theory assumes that the electron subsystem of QDs with infinite potential walls is in strong confinement, and its energy spectrum can be described according to the two-band semiconductor model. Longitudinal optical phonons are considered to be related to the QD electron subsystem via polar (Fröhlich) electronphonon interaction. It is shown that, in these approximations, only the off-diagonal part of electron-phonon interaction leads to the generation of electron-hole pairs with the participation of phonons; the selection rules for these transitions differ from those for zero-phonon transitions. Analytical expressions for the light-absorption coefficients of ensembles of identical and size-distributed QDs have been obtained.     

Semiconductor nanocylindrical layer in a strong electric field: Spectrum of carriers and intraband transitions

The single-electron states in a quantized cylindrical layer in the presence of a strong homogeneous electric field have been considered in the isotropic effective mass approximation. The energy spectrum and the envelope wave functions of charge carriers in the layer have been obtained in the explicit form. It has been shown that a strong external electric field leads to an additional localization of carriers in their angular motion. The corresponding selection rules have been derived, and the absorption band of intraband-intersubband optical transitions in the layer has been calculated.     

Long-lived states in solution NMR: selection rules for intramolecular dipolar relaxation in low magnetic fields

Recent work has shown that singlet states of two-spin systems in low magnetic fields can have lifetimes up to an order of magnitude longer than the usual spin-lattice relaxation time. This result may enable new applications of NMR, and in particular hyperpolarized NMR via parahydrogen-induced polarization, to the study of slow processes that take place over previously inaccessible timescales. At present it is unclear whether similar results apply to multi-spin systems, or if these long lifetimes are a peculiarity of the two-spin case. Moderately long-lived states have been observed in systems containing more than two spins, although the mechanisms that prolong their lifetimes are not well understood. Here we present formalism for the study of relaxation in multi-spin systems in low magnetic fields. This approach is used to derive a family of quantum-mechanical selection rules governing intramolecular dipolar relaxation at low field that may account for the extended lifetimes observed in multi-spin systems.