Photoacoustic spectrum of BaF2:Ce3+ phosphor

Physica B 217 (1996) 149-152

ELSEVIER

Photoacoustic spectrum of BaF2" C e

3+

phosphor

Y u g e n g Z h a n g a'*, Yi D o n b Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, China bDepartment of Physics, University of Science and Technology of China, Hefei, Anhui, 230026, China

Received 24 January 1995; revised 23 June 1995

Abstract

The photoacoustic (PA) technique was used to study the properties of BaF2:Ce 3÷ phosphorous material. The PA spectrum and the excitation spectrum are the exact complements of the absorption spectrum. The emission of this phosphorous material observed at 32 787 and 31 056 cm- x are due to the radiative deexcitation process from e9(5d 1) to 2FT/2(4fl) and this emission band is excited by the excitation process: 2Fs/2(4fl ) ~ e9. The electronic configurations of C e 3 + are split in the BaF2 crystal field and all the energy levels of Ce 3÷ in this crystal are given by the fine structure of the PA spectrum.

1. Introduction

Prompt and delayed luminescence from inorganic phosphors as a result of X-ray irradiation has been studied for many decades [1-3]. As a new type phosphor material, BaF2:Ce 3÷ crystal may be excited by UV easily and its lifetime is only about 50 ns [4]. For this reason, it has been widely studied [5-7]. However, most research work has concerned the luminescence properties and the decay time of BaF2:Ce 3+ exciton [8-10]. Visser [11] has reported the Ce "÷ energy levels in alkaline-earth fluoride calculated by the molecular F o c k - D i r a c method. The detailed spectral properties and the experimental energy levels have not been extensively studied. As a new technique, the PA spectrum is used in this work. The fine structure of energy levels of Ce 3 ÷ in BaF2 crystal appears in PA spectrum. The relationship among the spectra *Corresponding author.

of photoacoustic, absorption, excitation and emission is also studied. The PA spectra of BaX2 : Eu 2 + (X = F, C1, Br) have been previously studied by us [12]. Recently, the photoacoustic measurement have been widely used to investigate the physical and chemical properties of many kinds of samples [13,14]. The PA spectroscopy enable obtaining spectra on any type of solid, whether it be crystalline, powder or gel, and it is a direct monitor of energy gap and the non-radiative deexcitation process, the complement of absorption and photoluminescence spectroscopy [15]. The PA effect is selectively sensitive only to the heat-producing deexcitation processes that take place in the sample after the absorption of modulated light. In general, apart from the thermal deexcitation channel, several other deexcitation channels, such as fluorescence, photochemistry, photoconductivity, etc., are simultaneously competing with each other [16]. Denoting by E ° the

0921-4526/96/$15.00 © 1996 ElsevierScienceB.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 4 5 6 - 4

Y. Zhang, Y. Don/Physica B 217 (1996) 149 152

150

energy absorbed by a given system, the heat produced can be expressed as Q = E°(1-

~7i),

(1)

where the 7i are the conversion efficiencies of the several non-thermal deexcitation channels. Since the PA signal is proportional to Q, it can, in general, be written as S = S°(1 - ~ 7 i ) ,

(2)

where S O represents the PA signal if only the thermal deexcitation channel is active. Eq. (2) tells us that the PA signal is complementary to the other photoinduced energy conversion processes. That is, when an optically excited energy level of a given system decays by means, say, of fluorescence or undergoes a photochemical reaction, then little or no acoustic signal is produced. Following this idea, the optical properties, the excited state energy levels and the deexcitation processes are studied on BaFz : C e 3 ÷ materials.

2. Experimental The materials BaF2:Ce 3+ were prepared by mixing BaF2 with C e 2 0 3 and grinding, then sintering in a quartz tube furnace in CO gas atmosphere. The heating process from room temperature to sintering temperature lasted 20 min and then sintering for 50 min. In the measurement of PA spectra, the excitation source was a 500 W xenon lamp and the optical system was a CT-30F monochromator combined with an appropriate absorption filter to eliminate multiple order effects. The light source was modulated by a variable-speed mechanical chopper at a certain frequency, 12 Hz. The acoustic signal was detected with the sample placed in a locally built photoacoustic cell fitted with an ERM-10 electret microphone. After preamplification, the output of the microphone was fed to a lock-inamplifier (LI-574A) to which a reference signal was input from the chopper. The output signal was normalized for changes in lamp intensity using a carbon-black reference.

3. Results and discussion The excitation (EXS) and emission spectrum (EMS), absorption spectrum (ABS) and the photoacoustic spectrum (PAS) of BaF2:Ce 3÷ material are shown in Fig. 1. It is seen that there are several absorption bands in the absorption spectrum, in which one is observed at 38 700 cm- l with a middle intensity, one is observed at 34 843 cm- 1 with very strong intensity and another two are observed at 32679cm -1, 31181cm 1 with weak intensity. There is a peak observed at 34482cm ~ with very strong intensity in the excitation spectrum of the Ce 3 ÷ luminescence. Then, there are several weak peaks and middle intensity peaks lying in the range of 42 194-37 560 cm- t and another several very weak absorption peaks in the range of

EMS

l

/L I PAS

L 300

400

nm

Fig. 1. The PA spectrum and the absorption, excitation, emission spectra of BaF2 : Ce 3 ÷ phosphor.

Y. Zhang, E Don/Physica B 217 (1996) 149- 152

151

Table 1 The spectra and related energy levels of B a F z : C e 3+ (cm 1) Relation energy levels

PAS

ABS

2Fs/2Fu-t2g E 2Fs/eF,, t2gA 2F5/2 Ezu-tzg E ZFs/2 E2u-t2g A ZFv/2Fu leg E 2Fv/2Fu t2g A 2Fv/zE2u-t2g E 2Fv/2 E2u--t2g A

42 194 w 41 500 40000 39370 38 198 37 560

w w w m w

2Fs/2Fu- % 2Fs/zE2u-eg 2FT/zEu eg 2FT/2E2u--eg

34843 34 130 32678 30581

w w w w

EXS

EMS

38700 m

34482 vs

34482 vs

32679 w 31 181 w

32 787 s 31 056 s

Note: s: strong; m: middle; w: weak; vs: very strong.

34 843-30 581 cm- 1 in the PA spectrum, see Table 1. Comparing with the absorption spectrum, the very strong peak observed at 34 843 c m - ~ of ABS is not shown in PAS. And, it is the excitation spectrum that a very strong peak are observed at 34 843 cm- 1. It is seen to be that the excitation spectrum and the PA spectrum are corresponding to the absorption spectrum. As seen in Eq. (1), 7i is the conversion efficiency of the fluorescence deexcitation channel in this material, and it is sensitive in excitation spectrum. The PA effect is selectively sensitive only to the non-radiative deexcitation processes and PA signal is written as Eq. (2). So, the PA spectrum and the excitation spectrum are the complement of the absorption spectrum. As seen the emission spectrum, there exist two strong emission peaks observed at 32 787 and 31 056 c m - 1. This means that, as the material BaF2:Ce excited by UV (34482 cm t), the emission peaks occur at 32787 and 31056cm ~. This is why the very strong 34 482 c m - t peak is not seen in the PA spectrum. It is known that the Ce ion doped in BaF2 crystal is in a cubic site. Then, the excited state of electronic configuration 4f ~ is 5d 1 and it will be split into tzg(ZDs/z) and %(2D3/2 ) states in this cubic lattice [-17]. In the BaF2 crystal, the cerium will be in different centers surrounding cerium chargeuncompensated or accompanied by a charge-compensating interstitial fluorine ion at the (1 1 1) next-nearest-neighbor position or the (1 0 0) nearestneighbor position [11]. Considering these different

!

l

II

Ii II

II 11 Ii

il

li i

Fig. 2. Energy term scheme of BaFz:Ce3+: ~ the excitation process; non-radiative process; ~ radiative process.

centers and the crystal field effect, the C e 3+ 4 f 1 configuration will be split into ZFv/2(E2u, Fu), zFs/2 (Ezu , F u ) and the excited state of Ce 3+ 5d 1 configuration t2g will also have some small splitting (Ezg , Fg) [11], see Fig. 2. For this split energy level, a complex absorption peak shows in the PA spectrum of BaF2:Ce phosphor. As shown in Fig. 1, there are a series of peaks observed in the PA spectrum. And the fine structure of energy levels of BaF2 : Ce 3 ÷ is apparently seen in the PA spectrum, see Table 1 and Fig. 2. However, the peaks observed at 42 194 and 41 500 cm-~ are due to the transition 2F5/z (F., E2u ) ~ t2g and that observed at

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Y. Zhang, Y. Don/Physica B 217 (1996) 149 152

40000 and 39370 cm- 1 are due to the transition 2F7/2 (Fu) ~ t2g (A, E) and 38 198, 37 560 c m - 1 may be assigned as 2F7/2 (E2u) ---* t2g (A, E) with a middle intensity which is conformed to the absorption spectrum. The peaks observed between 34 843 and 30581 cm -1 are due to the transitions 2F5/2 - . % and 2F7/2 ~ %, and they are observed in several very weak bands for the strong emission effects from eg to 2F~/2 which is excited from 2F5/2. There, the excited state of excitation process and emission process is the same one, eg. This is confirmed by the experimental fact: first, if the energy level of excitation processes is not the eg state and with higher energy than e,, the process of deexciting it to the eg state must include some non-radiative deexcitation process and that will be shown in the PA spectrum with a strong band in the relevant range. But the very weak PA absorption in this range indicates that there is hardly any non-radiative deexcitation process; second, the fact of the very short lifetime of the emission process (50 ns) [4] also agrees with this conclusion. It means that the deexcitation process of excited state t2g is mostly by the non-radiative process that will be seen in the PA spectrum. The deexcitation process of the excited eg state is dominantly by the radiative process which cannot appear in the PA spectrum, but will be observed in the emission spectrum. However, the luminescence efficiency of the eg state is very high. According to the fine structure of PA spectrum, the relative energy levels of Ce 3 + doped in BaF2 crystal are calculated and shown in Table 2. There, AE of the splitting of 2F5/2 is 694 cm- 1 and that of 2F7,.2 is 1804 cm -1. The energy interval between 2F5/2 and 2F7/2 is 2959 cm 1. The splitting of t2g energy level is 634 cm-1 and that between eg and t2g for the cubic field effect is 7351 c m - 1. Then, the energy interval between electronic configuration 4f 1 and 5d ~ is 37 424 cm-1. All these experimental energy levels are in good agreement with the calculated results by Visser [11], see Table 2.

Table 2 The split energy levels of Ce 3 + doped in BaF 2 crystal (cm i) Energy levels

Relative energy value Results of Visser [11]

2F5/2 F u 2F5/2 E2u 2F-rj2 F u 2F7/2 E2u eg tz~ A t2g E

0 694 2104 3907 34 843 41 770 42407

0 579 2229 3600 34326 43 900

Acknowledgements We thank the Youth Science Fund of USTC of China for supporting this work. References [1] F. Seitz, Trans. Faraday Soc. 35 (1939) 74. [2] G.F.J. Garlick and A.F. Gibson, Proc. Phys. Soc. 60 (1948) 574. [3] H. Degenhardt, Electromedica 51 (1983) 155. [4] S.A. Smirnova, L.I. Kazakova, A.A. Fyodorov and N.V. Korzhik, J. Lumin. 60/61 (1994) 960. [5] W. Hages et al., IEEE Trans. Nucl. Sci. 34 (1987) 272. [6] R. Visser et al., IEEE Trans. Nucl. Sci. 38 (1991) 178. [7] Chao-Shu Shi et al., J. Lumin. 48/49 (1991) 597. [8] R. Visser, P. Dorenbos, C.W.E. van Eijk and H.W. den Hartog, J. Phys.: Condens. Matter 4 (1992) 8801. [9] W.J. Manthey, Phys. Rev. 8 (1973) 4086. [10] M. Laval, M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odry and J. Vacher, Nucl. Instr. Methods 206 (1983) 169. [11] R. Visser, J. Andriessen, P. Dorenbos and C.W.E. van Eijk, J. Phys.: Condens. Matter 5 (1993) 5887. [12] Zhang Yugeng, Phys. Rev. B 51 (1995). [13] H. Vargas and L.C.M. Miranda, Phys. Rev. 161 (1988)43. [14] Zhang Yugeng, Li Jianmin, Su Qinde and Zhao Guiwen, Spectrochimica Acta 48A (1992) 175. [15] T. Ikari, H. Yokoyama, S. Shigetomi and K. Futagami, Jpn J. Appl. Phys. 29 (1990) 887. [16] A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980). [ 17] H.L. Schlafer and G. Gliemann, Basic Principles of Ligand Field Theory (Wiley-lnterscience, London, 1969) p. 35