Mechanism of thermoluminescence in RbI: Eu2+

Journal of Luminescence 24/25 (1981) 123—126 North-holland Publishing Company

123

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S. SAPRU

Department of Physics Indian Institute of Technology Liaclra s—600 036 India

Department of Physics S.F. College Srinagar—190 001 India

Thermo~r:iinescence

1

(TI

)

a~ optical

studies

on

3 RbI:Eu indicate that Eu ions convert to Eu ions on irradiation and gain the electron hack on thermal or optical treatment. The integrated light intonsity is two orders of magnitude higher than the undoped samples. TL emission spectrum shows two emission bands at 2.48 oV and 2.86 cv. The origin of these bands is suggested. Ii ftROLUCTION

2~ ions room temperature colorotion Alkali halides doped with Eu[1—3]. have been of considerable interest observed that the rare earth ions,

are known to exhibit hioher TL studies on these systems [1,4]. Nerz and Pershan in CaP 2 lattice, on irradia2+ ions, tion act as acceptors and on thermal troatrrunt release these when electrons [4]. Lately has asbeen reported that 0U doped in alkali halides,it act donors on irradiation thereby converting to ~u3~ ions ~l—3J. As a continuation of the work reported on Eu~ doped rubidium chloride and bromide systems[l,2], we report here some of our results on undoped and 0u2+ doped rubidium iodide. EXPDRINENTAL Crystal growing technique, the exp~rimental set ups and the methods used for the analysis of TL glow curves and TO emission spectra are the same as reported earlier [1,2]. ODSULTS AND DISCUSSION Figure 1 shows the TO glow curves of undoped RbI and DbI:Eu2+ samples recorded with a heating rate of 120 K/mm. Curve ‘a’ is magnified by a factor of twenty. The 5u2+ doped samples show the following features in contrast to the undoped ones 1) The integrated light intensity is two orders of magnitude higher. ii) Two additional glow peaks appear at 442 K and 506 K, whereaz the 405 K glow peak observed in the undoped sample shifts to lower temperature i.e., 395 K. iii) As the dose of irradiation is increased, initially the 395 K and 442 K glow peaks increase in intensity reaching a 0 022—2313/81/0000—0000/S02.75 © North-Holland

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saturation after 20 and 60 minutes of irr~diation respectively. The 506 K ploy; p;ak does not s~ov: any significant changes in its intensity on increusod irradiation. The intensity variation of various pbs; peaks with the doye of irradiation is sio’;n in the Inset of figure 1. From those results ye can conclude that the 4d2 .1 and P06 K dow peaks are due to the impurities whereas the 395 0 glow eak is duo to the host lattice. Initially the exciting onorgy is used in the growth of 395 K plo’; peak which is identified as due to the recorhination processes involving F and V type centres. After about 20 minutes of irradiation the excitincj enersy is used in the growth of 4d2 K plow peak Which iS identified as due to the Ouropjun involvenent i.e., Eu~ ions formed on irradiation act as reconbination centres when tho irradiated samples are subjected to optical or thermal treatments. dfter shout 60 mutes of irradiation both these glow peaks fall in intensity c is due to the detraepinn of shallo;or trans an~ the Iii line er;;’ • The io ~ o~anei eoeal;y ;ignificant changes in its intensity with the dose of irradiation

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Figur~ 1 TL glow curves of ObI and RhI:Eu + recorded with a heating rate of 120 K/mm. (a) Undoped RbI after 60 minutes of T—~.rradiat1on (The scale is magnified twenty times). (b,c,d) RbI:Eu~ after 10, 20 and 60 minutes of y—irradiation respectively. Inset shows the intensity variations of various glow peaks with the dose of irradiation.

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S. Sapru, S.B.S. Sastri’ /Mechanism of thermoluminescence in RbI:Eu

and is attributed to the background trace impurities. The assignment of various glon peaks is supported by the quenching and F—light bleaching experiments performed on those samples. In the unquenched samples the 442 K and 506 K gloat peaks do not appear. Aggregation of impurities can lead ~o such an effect. Similar results were obtained in the iihCl:Eu + system [1]. Optical bleaching (F—light) results in the fall of both 395 K and 442 K glow peaks. Optical absorption results shos; that the characteristic Eu2+ absorption bands fall in intensity on irradiation, implying their transmutation in valence state. In view of the F—hand enhancement and e~lier results on similar systems it is concluded that Eu ions act as donors on irradiation [1,2].

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Figure 2 TL emission spectrum of RbI:Eu~ system. The broken lines show the analysis. Inset shows the TL emission spectrum of undoped RbI.

125

S. Stzpns. LBS. Sast,.r IMechanism ofthenndumlnescence In RhI:Ew?’

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TL emission spectrum recorded on the uncloped sample shows a single emission band around 2.5 cv and two emission ban’~sat 2.48 cv and 2.86 cv in the doped samples (Figure 2). The 2.48 cv band is attributed to the rocotbination processes involving F and V type centres. This fits the energy band diagram. The band gsp is taken to be 64 cv in view or the first exciton peak reported in RbI system and the v—cvntre absorption occurs at 3.5 cv in this system. The 2.86 cv £miscion bgnd corresponds to the fluorescent esission of EuCt ions (lJ. 2 ions loose an electron on~rradiation and convert to Eu~ If ionsEuthem on thernq arcatmont Eu ions shall capture an electron, become Eu + (excited ion) and while coning down to the ground state emits 2.86 cv energy. au2+_

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(s.s) is thankful to the Physics Department of Indian Institute of Technology, Madras for providing the necessary laboratory facilities.

One of the authors

REFcKkPCES (1] Sastry, S.B.S., and Sapru, 5., Phys. Stat. Sol. (b). 103 (1981) 185. (2] Sastry, S.&.S., and Gapru, U., To appear in 3. Lum. (1981). 13J Chowdari, b.V.R., and Itoh, IL, Ehys. Stat. Lol. (b). 46 (1971) 549. [4)Merz, J.L., and }‘ershan, 1.5., Ihys. Rev. 162 (1967) 217 and 235.

(5] /~usin,v and Alvarez Rivas, 3., 3. Phys. C.: Solid State Phys. 5 U9n) 82. 1

Present Address: Instt. de09Physique USTA, DMa ELBPN UiIDlt ALGER, ALG2HIA.