NOTES

Temperature Dependence of the Eu 3+ Ion Luminescence Lifetime Exhibited by Anhydrous EuCI3*

N A T H A N A A R O N S T U M P , G A N G C H E N , R I C H A R D G E O F F R E Y H A I R E , a n d J O S E P H R I C H A R D P E T E R S O N t Department of Physical Sciences, Winston-Salem State University, Winston-Salem, North Carolina 27110 (N.A.S.); Department of Chemistry, University of Ten- nessee, Knoxville, Tennessee 37996-1600 (G.C., J.R.P.) ; and Transuranium Research Laboratory (Chemical and A nalytical Sciences Division), Oak Ridge NationalLab- oratory, P.O. Box 2008, Oak Ridge, Tennessee 37831- 6375 (R.G.H., J.R.P.)

Index Headings: Anhydrous EuCI3; Eu 3÷ ion luminescence lifetime; Temperature effect upon luminescence lifetime.

INTRODUCTION

Recently, Chen et al. 1 have discussed the effects of slight variations in temperature on the luminescence lifetime of the Eu 3+ ion in anhydrous EuCI3. In that work, the luminescence lifetime exhibited a continuous exponential decrease with a linear increase in temperature. However, a limited number of data points were employed, and the range of temperatures was limited to between 293 and 368 K. Attempts to expand the study of anhydrous EuCI3 to temperatures above 368 K resulted in irreversible al- terations in the sample (e.g., chemical reaction with its container or structural phase transition), which precluded studies of the sample at higher temperatures. Investiga- tions of the luminescence lifetime below 293 K had not been carried out previously because it was expected that the exponential trend would continue. This assumption appeared to be well founded, since compositional and/or

Received 3 February 1994; accepted 20 June 1994. * Research sponsored by the Division of Chemical Sciences, Office of

Basic Energy Sciences, U.S. Department of Energy under Grant DE- FG05-88ER13865 to the University of Tennessee, Knoxville, and Contract DE-AC05-84OR21400 with Martin Marietta Energy Sys- tems, Inc.

t Author to whom correspondence should be sent.

structural variations in anhydrous EuC13 have not been reported in this temperature range. We have now deter- mined that the previously established trend has not con- tinued and report here the extended thermal dependence of the Eu 3+ ion luminescence lifetime in anhydrous EuC13 at temperatures below 293 K.

EXPERIMENTAL

Our sample preparation and structural confirmation methods paralleled those employed in the earlier work.1 Samples of anhydrous EuC13 were sealed in quartz cap- illaries under less than 1 atm of helium. X-ray powder diffraction and Raman and luminescence spectroscopy were employed to confirm the identity and structure of the samples. The sample chamber of the spectrophotom- eter was modified for both high and low temperatures. Temperatures greater than ambient were obtained with a simple tube furnace modified for optical studies. Lower temperatures were obtained either by immersion of the sample capillary in an optically transparent Dewar filled with various coolants (e.g., liquid nitrogen, ice/acetone, ice/water) or by directing a stream of liquid nitrogen- cooled dry air onto the sample capillary. The temperature was measured via a calibrated Chromel-Alumel ther- mocouple immediately adjacent to the sample. The meth- ods of sample excitation, collection of spectral data, and lifetime determination were identical to those reported by Chen et al.t

RESULTS AND DISCUSSION

Three separately prepared samples of anhydrous EuCl3 were studied multiple times to determine the reversibility of the temperature dependencies of the Eu 3+ ion lumi- nescence. With the exception of samples heated to tem- peratures greater than ~ 368 K (which irreversibly altered their composition), the temperature dependence of the lifetime was independent of the method, direction, or number of times the sample was heated or cooled.

The Eu 3+ ion luminescence lifetimes determined from each of the various samples in the present and earlier studies I are plotted in Fig. 1. At 77 K the luminescence lifetime is approximately 330 #s. The lifetime slowly de- creases as the temperature is increased from 77 K. This trend continues until about 215 K, where the lifetime begins to rise slowly, approaching a maximum of just over 300 #s at approximately 275 K. At temperatures above 275 K, the temperature dependence of the lifetime

1174 Volume 48, Number 9, 1994 0003-702S/94/4809-117452.00/0 APPLIED SPECTROSCOPY © 1994 Society for Applied Spectroscopy

° ~-"1

~D

(D O

O

° w - , 4

6 0 0

400

200

0 40

B

A

B

I I I I I I I I I I I I I I I I

120 200 280 360

Sample Temperature (K) FIG. 1. The Eu 3+ ion luminescence lifetime in anhydrous EuC13 as a function of sample temperature [465.8-nm excitation (TFo --+ 5D2); 615.8- nm transition monitored (7F2 ~ SD0) ]. Solid line A comes from the best exponential fit to the data points at temperatures up to 200 K, and solid line B from that to the data points at temperatures above 270 K.

follows the exponential relationship reported by Chen et al.' (see Fig. 1).

Two exponential functions can be fit to the experi- mental data in Fig. 1 (see solid lines A and B). With the use of the two-level model proposed by Chen et al.' [1/r

(1/re)*e - a a r / k T ], both AE (the energy difference between the involved levels) and r e (the intrinsic lifetime of the transition between these levels) can be calculated. A fit of the data from temperatures above 270 K (solid line B in Fig. 1) yielded values of 1337 _+ 69 cm -I for AE and 0.29 _+ 0.03 us for rg these are in reasonable agreement with the corresponding values of 1320 cm- ' and 0.37 us reported in the original work.' An exponential fit of the data recorded from 77 to 200 K (solid line A in Fig. 1) yields values of 37 _+ 5 cm- ' for AE and 189 ± 12 us for re. Thus, the experimental data can be approximated by the equation 1/r ~ %A*[(1/O.29),e -mT/kT] + %B*[(1/ 189)*e- 37/kT ], in which the °/oA and %B variables designate the fraction of Eu 3÷ ions which de-excite in accord with the first or the second exponential function, respectively. A discussion of the change in the magnitudes of %A and %B as a function of temperature is given later in this report.

An obvious explanation for the observed bimodal be- havior would be the formation of a new structural or compositional phase at or below ~ 275 K. Corresponding plots of temperature vs. luminescence lifetime have been obtained in thermal decomposition studies of neat Eu 3+

I

298K

t ! 100 200 300

AWAVENUMBER (CM -1) FiG. 2. Phonon Raman spectra exhibited by anhydrous EuCI3 at 77 and 298 K.

and Eu3+-doped sesquioxalates. 2-4 In thermal decompo- sition processes, such variations in lifetime with temper- ature imply a change in the composition or structure of the sample through the formation of a decomposition product or intermediate. 4 However, the variation in life- times always coincided with a distinct, though often mi- nor, change in the Eu 3+ ion's emission and/or phonon Raman spectra recorded at the same temperature. 3,4

The Raman and emission spectra of anhydrous EuC13 at 77 and 298 K are given in Figs. 2 and 3, respectively. No significant difference in either the emission or Raman spectrum is evident at these two temperatures. Therefore, we have concluded that the variation in the temperature dependence of the luminescence lifetime of the Eu 3+ ion in anhydrous EuC13 must result from some cause other than a thermally dependent compositional or structural change.

Another viable explanation of our experimental ob- servations is that the Eu 3+ ion in anhydrous EuCI3 has two competing de-excitation pathways available. The first pathway, A, is followed almost exclusively at tempera- tures below 215 K (i.e., %B ~ 0), while the second path- way, B, dominates overwhelmingly at temperatures above 280 K (i.e., %A ~ 0). De-excitation via pathway B initially becomes apparent at temperatures above 215 K. Between 215 and 250 K, de-excitation occurs primarily via path- way A with some de-excitation via pathway B (i.e., %A > %B). At temperatures between 250 and 280 K, pathway B predominates but with some de-excitation via pathway A (i.e., %B > %A).

APPLIED SPECTROSCOPY 1175

Z W F- Z

298K L

It [l

77 K

I I I

17000 16000 15000 14000

Wovenumbers (cm -1) ,FIG. 3. The Eu 3+ ion luminescence spectra exhibited by anhydrous EuCI3 at 77 and 298 K.

An observed loss o f the exponential character o f the luminescence decay curve in the in termedia te t empera - ture region between 215 and 280 K would suppor t the assumpt ion that two compet ing decay processes are ac- tive. However , our exper imenta l data showed no such var ia t ions in this region where the compet i t ion between, and combina t ion of, the two functions should be mos t evident . In fact, l ifet ime measu remen t s in this t empera - ture region produced excellent decay curves which were easily fit by exponential functions. The combina t ion ap- pears to be homogeneous and difficult to part i t ion into separate exponential relationships.

We specifically refrain here f rom speculating on the exact nature o f the A and B de-exci tat ion pathways. How- ever, it has been suggested 5 that the partial exclusion o f pa thway B f rom the de-exci tat ion process at t empera tures below ~ 2 7 3 K could result f rom exponential depopula- t ion o f the phonon levels with temperature , as modeled by the Bol tzmann distr ibution. Phonon-ass is ted inter- level crossing is a wel l -known m e c h a n i s m for the exci- ta t ion and de-exci ta t ion o f lanthanide compounds . 6 As the c o m p o u n d is cooled, the avai labi l i ty o f these phonon- assisted routes would be decreased. According to this interpretat ion, the B pa thway would follow a phonon- assisted de-exci ta t ion pa thway which becomes unavai l- able at tempera tures below ~275 K.

C O N C L U S I O N

Two exponential curves are suggested to fit the exper- imenta l data in plots o f the t empera ture dependence o f

the Eu 3+ ion luminescence lifetime in anhydrous EuC13. One curve applies to tempera tures below ~ 2 1 5 K; the second to tempera tures greater than ~ 2 8 0 K. I t is be- l ieved that these two marked ly different t empera tu re de- pendencies result f rom two compet ing de-exci tat ion path- ways avai lable in anhydrous EuC13 ra ther than being due to a t empera tu re -dependen t compos i t iona l or structural variat ion.

ACKNOWLEDGMENTS

The authors express their appreciation to Drs. K. L. Bray of the University of Wisconsin-Madison, E. L. Wehry and G. K. Schweitzer of the University of Tennessee-Knoxville, and G. M. Murray of the University of Maryland-Baltimore for their ideas, suggestions, and as- sistance in the completion of this research.

1. G. Chen, N. A. Stump, R. G. Haire, and J. R. Peterson, Appl. Spectrosc. 46, 1198 (1992).

2. P. K. Gallagher, F. Schrey, and B. Prescott, Inorg. Chem. 9, 215 (1970).

3. J. K. Gibson and N. A. Stump, Thermochim. Acta 226, 301 (1993). 4. C. L. Kay, MS Thesis, The University of Tennessee, Knoxville (1993). 5. G. M. Murray, University of Maryland-Baltimore, personal com-

munication, 1993. 6. W. D. Partlow and H. W. Moos, Phys. Rev. 157, 252 (1967).

Spectral Measurement of Sodium Decanoate Aqueous Solution by Microscope Fourier Transform Infrared Spectrometry with the Electrophoresis/Attenuated Total Reflection Method

T O M O K O M A T S U I , * C H I H I R O JIN, F U M I K O K A N E U C H I , K A Z U H I R O KAWASAKI, and N O R I Y U K I W A T A N A B E Faculty of Engineering, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan (T.M., N.W.); and JASCO Corporation, 2967-5, Ishikawa-Cho Hachioji-shi, Tokyo 192, Japan (C.J., F.K., K.K.)

Index Headings: IR; ATR; Electrophoresis; Aqueous solution; Micro- FT-IR.

I N T R O D U C T I O N

It has always been difficult to pe r fo rm analysis and character izat ion o f substances in aqueous solution by us- ing infrared (IR) spectrometry . We propose a new me thod in which ions are concentra ted on the a t tenuated total reflection (ATR) surface o f a g e r m a n i u m pr i sm by using an electrophoret ic means. The m e t h o d m a y provide in-

Received 2 September 1993; accepted 2 June 1994. * Auther to whom correspondence should be sent.

1176 Volume 48, Number 9, 1994 0003-702s/94/4809-117652.00/0 APPLIED S P E C T R O S C O P Y © 1994 S o c i e t y for Applied S p e c t r o s c o p y