[ SPECTROSCOPIC TECHNIQUES Energy Transfer by Ion-Ion Cross-Relaxation in Cs2NaTmCI6 P. A. TANNER,* T. K. CHOI, and K. HOFFMANt Department of Applied Science, City Polytechnic of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong (PoA.T., T.K.C); and Department of Physics and As- tronomy, University of Georgia, Athens, Georgia 30605, U.S.A. (K.H.) Index Headings: Luminescence; Absorption; Concentration quenching; Lanthanide ion; Spectroscopic techniques. INTRODUCTION In this study we present experimental results from absorption, excitation, and luminescence spectroscopy which enable us to account for the concentration quench- ing of near-ultraviolet emission in Cs2NaTmC16. The thu- lium ions are situated at sites of octahedral symmetry1 in this compound, forming a model system for the in- vestigation of energy transfer phenomena. 2 EXPERIMENTAL Neat Cs2NaTmC16 and TmCI~-, doped (0.01-10 tool %) into the colorless, cubic-host Cs2NaGdC16, were pre- pared as single crystals by the Morss method. 3 Absorp- tion spectra of polished single crystals were recorded between 300 and 10 K with the use of a Bio-Rad FTS- 60A wide-range spectrometer equipped with DTGS, InSb, PbSe, Si, blue and solar-blind photomultiplier detectors. Spectral calibrations with neon, cadmium, and mercury lamps indicate that the wavenumber error in the assign- ment of the energy levels is similar to the resolution employed (1-2 cm-1). The sample was held in an Oxford Instruments closed-cycle-cooler cryostat. Excitation spectra were recorded at 300-85 K with an SLM 4800C spectrofluorometer. Selective excitation was provided by a xenon lamp. The luminescence and emission decay measurements (between 300 and 2 K) utilized the 476.5- Received 10 December 1992; revision received 10 February 1993. * Author to whom correspondence should be sent. t Present address: Division of Basic Sciences and Mathematics, Whit- man College, Walla Walla, WA 99362. nm excitation (into the vibronic sideband of the 1G 4 level of Tm 3+ ) from an argon-ion laser, the emission being dispersed by a 0.25-m Jobin-Yvon monochromator and detected by a photomultiplier. Decay measurements were made by chopping the beam and also involved the use of a waveform digitizer, a Digital Pro 300 computer, and an X-Y plotter. RESULTS AND DISCUSSION Excitation Spectra. With the use of selective excitation into the tG4 vibronic sideband, for dilute Cs2NaGdC16: TmCI~-, emission is observed from the 1G 4 level (near 20,850 cm -1) to the ground-state term (3H 6) and the first excited terms (3F 4 and 3H5), within the spectral region of our detectors. However, for neat Cs2NaTmC16, no emis- sion is observed from the 1G 4 level, but instead, emission is observed from the 3H 4level, situated near 12,500 cm -1. The excitation spectrum of the 3H 4 emission in the neat compound shows that this level is populated directly from 1G4, and that the intermediate levels 3F 2 and 3F 3 are bypassed. The concentration quenching of the ~G4 emission is therefore due to an ion-ion cross-relaxation between Tm 3+ neighbors at sites A and B, of the form: 1G 4 (site A/B) + 3H 6 (site B/A) -~ 3H 4 (site A/B) + X(site B/A). Absorption Spectra. The energy levels of Tm 3+ in TmCI~- are labeled according to representations of oc- tahedral point group symmetry. From previous studies, 4 the locations of the crystal field components of the 1G4, 3H4, and 3H~ levels have been determined, and the rele- vant energy levels are shown in Fig. 1. The term multiplet X is therefore situated near 8000 cm -~ and thus corre- sponds to 3H5. We have investigated the crystal field energy levels of this term by absorption spectroscopy (Fig. 2). Most of the intensity of the 3H 5 ~- 3H 6 transition arises from the magnetic dipole (MD) mechanism. The oscillator strengths, P(calc), of the individual transitions j ~- i between crystal field components were calculated with the use of the eigenvectors4 from a six-parameter crystal field model: 5 87r2mc P(calc) = 3he2g------- ~ PijniySij where gi is the degeneracy of level i; Pii and n~j are the transition wavenumber and the refractive index, respec- tively; and S~j is the transition line strength. The cal- culated oscillator strengths are in good agreement with 1084 Volume 47, Number 7, 1993 ooo3-7o2s/93/47o7-~o~,2.oo/o APPLIED SPECTROSCOPY © 1993 Societyfor AppliedSpectroscopy