稀土发光和光谱理论的研究进展

简述了近20多年以来我们在稀土发光和光谱理论方面所做的主要工作、取得的进展和成果。包括:(1)关于(4f)^N组态内单光子跃迁强度:推广Judd-Ofelt三参量公式,在其中计入"J混合"(偶宇称晶场成分造成的)导致的修正;澄清了国际上关于Judd-Ofelt全参量公式中"静力学项"和"动力学项"相对正负号的争论(仍为负号)。(2)关于(4f)^N组态内双光子过程强度:提出直接计算强度方法(放弃缔合近似,利用Ce³⁺的5d和左邻RE³⁺的(4f)^N能级-波函资料,近似模拟出最重要的虚中间组态(4f)^N-15d的能级-波函结构)来讨论和改进Judd-Ofelt缔合近似。(3)固体中声子辅助的稀土离子间能量传输理论:发现HLO理论关于单声子过程中的"干涉相消"实际只适合于对角过程,而非对角过程则是"干涉相加",因而,后者首要的是单声子而非双声子过程;同时,用"晶体格波模"取代"Debye模"描述能量传输中的声子,给出了新模型下计算对角和非对角过程速率等物理量的整套公式。并由之看出:波矢q=0和对应色散曲线扁平段的光学声子对单声子非对角过程速率有完全不能忽略的贡献,这些都不同于或超出于HLO理论。(4)提出和发展了分析指认4f-5d跃迁光谱的简单理论模型,用能量和跃迁强度同时进行近似拟合,成功地分析指认了从(4f)³到(4f)¹³离子在几种基质中的4f→5d吸收(激发)谱,也成功用于若干5d→4f辐射谱。(5)对跃迁的初末晶场态同时引用有效自旋哈密顿描述稀土离子间超交换作用,成功地解释了低温10 K时立方结构的纯稀土冰晶石晶体Cs₂NaLnCl₆(Ln=Er,Yb)等的吸收谱中0声子线(量级为几个cm^-1)的精细分裂。

Research Progress on Theory of Rare-earth Luminescence and Spectroscopy

In this paper we briefly review our research progress in the past twenty more years on the theory of rare-earth luminescence and spectroscopy,which can be enumerated as follows:1.On one-photon transition intensities:A.Extending the Judd-Ofelt 3-parameter formula to contain the effect of J-mixing caused by even-parity component of crystal field Hamiltonian.The extended formula still uses 3 parameters while introduces J-mixing coefficients which can be obtained from crystal-field splitting data of a few terms.B.Conforming the relative sign between the "dynamic term" and the "static term" contained in the Judd-Ofelt all-parameter formula is still negative by finding another sign error in the original paper.2.On two-photon processes intensities:Presenting a direct calculation scheme of intensities of this sort of processes by simulating energy-wavefunction structure of the intermediate configuration(4f)^N-15d based on the(4f)^N-1 energy-wavefunction deta of the left labour RE³⁺and the 5d deta of Ce³⁺.This scheme was used to Tm³⁺ compared to the Judd-Ofelt approximation with some discussion.3.On theory of phonon-assistant process of energy transfer(ET) between RE ions in solids:It is found that the coherent cancellations occurring in the HLO theory of one-phonon-assisted ET process only holds for diagonal process in which one phonon emits or absorbs from a definite electronic state of the donor or acceptor.For nondiagonal processes in which one phonon emits or absorbs accompanying the electronic electric-dipole transition at the donor or acceptor site,the coherence is enhanced that the most important is one-phonon but two-phonon assisted process.In addition,instead of Debye model used in HLO theory,the running lattice wave is introduced for the first time into the ET theory to describe the phonon involved in the above diagonal and nondiagonal processes.The formulae calculating transition rates etc of both the above processes based on the lattice wave model are given.And it is noticed that contrary to and out of the HLO theory,the contributions to the one-phonon nondiagonal process from optical phonons with wave-vector q=0 and corresponding to flat parts of dispersion curves are important.4.On theory of 4f-5d transition spectra:A simple model to calculate approximate term energies of the configuration(4f)^N-15d and the 4f-5d transition intensities based on 3 energy parameters and the quantum numbers of the initial and the final states was presented and developed in the past few years.Therefore,the 4f-5d transition spectra can be analyzed and assigned by executing at the same time energy and intensity fitting.There has been successful examples of RE ions from(4f)³ to(4f)¹³ doped in a few hosts.5.On interaction between RE ions:The fine structure of zero phonon lines in the low temperature electronic absorption spectra of the elpasolite crystals(like Cs₂NaLnCl₆ etc.where Ln=Er,Yb) is interpreted by an effective spin hamiltonian model describing superexchange interaction between RE ions.