Site selective excitation spectroscopy of CsCdBr3∶U3+

第19卷第3期原子与分子物理学报Vol.19，(.3 )**)年+月,-./0102345/6738693:.,6/;:370,4765<-=1.,12>l.，)**)

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Article ID：1000?0364（2002）03?0379?03

Site selective excitation spectroscopy of CsCdBr3:U3+*

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CLC number：3561.3Document code：6

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*收稿日期：)**)?*)?)*

基金项目：教育部《跨世纪优秀人才培养计划》基金和国家自然科学基金（批准号19++]*^)）资助项目。

作者简介：尹民（19_*‘），男，安徽省亳州市人，中国科技大学物理系教授，博士。主要从事固体光学和激发态物理等方面的研究工作。

Lecroy model9350M oscilloscope，using pulsed radiation （pulse length：10ns and repetition rate：30Hz）from the dye laser as the excitation source.

Fig.1shows the emission spectra of CsCdBr3：U3+ in the spectral region400～700nm under355nm excitation.At room temperature the spectrum consists of a strong broad band centered at416nm and a small peak located at683.8nm.The former is presumably attributed to the transition from the5f26d band to the ground state 4I

9/2

of U3+ions，while the latter is for4F9/2→4I9/2［8］. When the temperature is decreased to12K，the spectrum changes a lot.The683.8nm peak becomes very sharp and strong.The allowed transition band5f26d→4I9/2 divided into three ones.In addition，some weak sharp peaks also appear at about602nm and 694nm.

Al low temperature，the resolution of the spectrum is high.Accordingly，more peaks will be recorded.This explains why there are three peaks for5f26d→4I9/2of U3+at12K instead of only one peak at room https://www.doczj.com/doc/763677608.html,pared to lanthanide ions，the luminescent properties of actinides are much more sensitive to the temperature.It is attributed to the grater extension of the5f radial wave function.The larger radius induces a larger crystal Stark splitting and a stronger coupling to the lattice.As the temperature decreases，the non-radiative relaxation rate becomes small，resulting in a much stronger fluorescence from

4F

9/2

at about684nm.

Under selective excitation of4G9/2+2G7/2at602.4 nm（16600cm-1），the emission spectrum of CsCdBr3：U3+in the range of630～670nm（Fig.2）consists of two groups of lines，with the peak683.8nm strongest，which is similar to that of U3+ions in LaCl3［9］.The

transitions of the two groups can be easily assigned to 2H

11/2

→4I9/2and4F9/2→4I9/2respectively.In the lower part of the figure，the excitation spectrum for4F9/2 emission at683.8nm is presented.There are three peaks in the figure，giving three Stark levels of4G9/2+

2G

7/2

at16646cm-1，16636cm-1and16600cm-1（602.4nm

）respectively.

Fig.3shows the fluorescence decay curve of4F9/2→4I9/2transitions under selective excitation of4G9/2+

2G

7/2

levels at602.4nm.From the energy level structure of U3+ions［8］，we know that there is only one emitting level2H11/2between4G9/2+2G7/2and4F9/2 and the level4G9/2+2G7/2does not give emission.By fitting the decay curve with a double exponential function a rise time0.35μs and a decay time1.9μs were

173

第19卷第3期YIN Min，et al：Site selective excitation spectroscopy of CsCdBr

3

:U3+

obtained.It is easy to know that the decay time1.9μs corresponds to the lifetime of4F9/2level and0.35μs is for2H

11/2

.They are much shorter than the same levels in LaCl3［10］.The lifetime of an excited state is determined by both radiative and nonrediative depopulation processes.The latter depends on the energies of the accepting phonon models，and the so-called energy gap law can be used to express the multiphonon-relaxation rate constantω0=βe-αβ［11］.Hereα，βare parameters characteristic of the material.p=ΔE/hωis the number of phonons of energy hωrequired to bridge the energy gap ΔE.The highest-energy phonons are the most efficient accepting https://www.doczj.com/doc/763677608.html,ually，multiphonon relaxation is dominant for the process of p<5.The highest phonon energy in chlorides is～260cm-1.For CsCdBr3a value of163cm-1was determined by Raman spectoscopy［12］. we can see that［8］in LaCl3，5phonons are needed to bridge the energy gap between4F9/2and its nearest4G7/2 level.While in CsCdBr3，about8phonons are required. But the measured lifetime of4F9/2level in CsCdBr3is much shorter than that in LaCl3.One reasonable reason for this is the formation of U3+-Cd vacancy-U3+pairs. The interaction and energy transfer between U3+ion in pairs pronouncedly decreases the lifetime of4F9/2level.

In conclusion，new crystal CsCdBr3：U3+was grown by the Bridgman https://www.doczj.com/doc/763677608.html,pared to room temperature，the crystal presents a much stronger emission at low temperature.Under selective excitation at12K，fluorescence from2H11/2and4F9/2levels was recorded，with a quite intense emission at683.8nm.The lifetimes of2H11/2and4F9/2levels were obtained and discussed.

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273原子与分子物理学报2002年