Electronic Spectra and Configuration Interaction of Tm 3+ in TmCl 6 3- Miche ` le D. Faucher, † Peter A. Tanner,* ,‡ and Chris S. K. Mak ‡ 88 AVenue Jean Jaure ` s, 92140 Clamart, France, and Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong SAR. P.R. China ReceiVed: February 5, 2004; In Final Form: April 13, 2004 Low-temperature electronic absorption and emission data are reported for Tm 3+ at the octahedral site in crystals of Cs 2 NaTmCl 6 . Thirty-seven crystal field levels (total degeneracy 88) of the f 12 configuration (total degeneracy 91) have been assigned, and in several cases the levels are split due to electron-phonon coupling interactions. The fitting of the energy levels, using the conventional f 12 analysis with 12 variable parameters, gives a mean deviation of 53.3 cm -1 . This is reduced to 9.3 cm -1 by including the 4f 12 np 6 /4f 13 np 5 configuration interaction, using 16 variable parameters. The results indicate a tendency for the early members in the series of Ln 3+ ions to interact with the p-electron, and the later members with the p-hole, configurations, following the redox properties of the ions. The interacting configuration is charge-transfer (n ) 3) rather than metal ion (n ) 5), and the mixing of ligand wave functions with those of the metal ion may be responsible for the unusually strong electron-phonon coupling identified for several electronic states of Tm 3+ in TmCl 6 3- . Introduction The energy levels of lanthanide ions in the crystalline hexachloroelpasolite system 1 are simpler than for Ln 3+ diluted into other crystals because the octahedral site symmetry gives rise to high degeneracies of electronic levels. Several system- atic energy level parametrizations have been carried out for the entire series of lanthanide ions with general overall success, but with some notable discrepancies. 2-4 Although the SL-term- dependence of the crystal field parameters has been recognized for some time, attempts to explain “anomalous” multiplet splittings using two-electron operators have not proved to be conclusive. 5,6 More recent parametrizations have utilized larger datasets, including energy levels deduced from two-photon spectroscopy. Evidence was reported for electron correlation induced by the crystal field, 7 yet until recently the comparisons of experimental versus calculated energy level listings have been those from calculations involving one-electron crystal field operators. 8-12 A recent paper by Thorne et al. 13 shows that improvement can be obtained by adding the spin-correlated crystal field to the normal crystal field. Some discrepancies remain, however, and a detailed account of correlation crystal field analysis for Cs 2 NaTbBr 6 is given in ref 14. From our previous studies, we have found that the inclusion of 4fmp 1 configuration interaction into the parametrization of the 4f 2 energy level scheme decreases the mean deviation of the fit using the single configuration 4f 2 alone by a factor of 2.9 (i.e., from 32.7 to 11.6 cm -1 ). 15a The interaction is most marked for the 1 G 4 and 1 D 2 multiplets of Pr 3+ , since large crystal field off- diagonal matrix elements between these terms and certain singlet terms of the 4fmp 1 configuration perturb the crystal field levels of these 4f 2 multiplet terms. Using the same method, for another elpasolite compound Cs 2 NaErCl 6 , the mean deviation for 75 levels (with a total degeneracy of 130) was reduced from 21.4 to 10.5 cm -1 . 15b The objective of the present study is to see if the interaction with excited configurations is equally important for the f 12 ion Tm 3+ . Crude point charge calculations predict a decrease in crystalline field across the lanthanide series. 2 In the cubic elpasolites Cs 2 NaLnCl 6 , the lattice parameters are 1091 and 1069 pm for Ln ) Pr and Ln ) Tm, respectively, 16 so that the Tm- Cl distance is shorter. However the radial integrals 〈r k 〉 are smaller for Ln ) Tm so that overall the crystal field experienced by Tm 3+ is weaker. Slater parameters are expected to scale linearly with atomic number, so that the effects of higher metal ion configurations may be less important in perturbing 4f 12 levels than 4f 2 levels. However, the nature of the interacting config- uration mp M has not been clear in previous cases because its energy seems to be anomalously low for metal-centered configurations. 15a,b,17 Evidence from the energy level datafit of Er 3+ strongly suggests that the equiparity interacting configu- ration is a charge-transfer configuration. 15b Several previous studies have been concerned with the electronic spectra of Tm 3+ in octahedral symmetry. The absorption and magnetic circular dichroism spectra were reported by Schwartz et al., 18 who assigned 12 crystal field levels. Subsequent reports of the absorption and emission spectra 19,20 were not in agreement and remained unresolved. 21 An alternative relativistic calculation approach was equally unsatisfactory. 22 One of the problematic terms was the electronic ground state, 3 H 6 , where an apparent discrepancy between experiment and calculation of ca. 100 cm -1 existed for the Γ 2 (A 2g ) crystal field level. (Since all of the electronic levels of the f 12 configuration are of even parity, we omit the crystal field level irrep label g throughout). Amberger et al. 23 had previously commented on the unusual behavior in the electronic Raman spectrum of Cs 2 NaTmCl 6 , and this was subsequently reinves- tigated by Tanner et al. 24 Two energy levels (at 108 and 148 cm -1 at 10 K) were found to be derived from the 3 H 6 aΓ 5 (T 2g ) crystal field level, through the electron-phonon coupling interaction of the vibronic level 3 H 6 Γ 1 + ν 5 (τ 2g ) and the electronic level 3 H 6 aΓ 5 . From the observation of a hot electronic Raman transition, the Γ 2 (A 2g ) level was reassigned near the * To whom correspondence should be addressed. E-mail: (P.A.T.) bhtan@cityu.edu.hk, (M.D.F.) faucher.michele@free.fr. † 88 Avenue Jean Jaure `s. ‡ City University of Hong Kong. 5278 J. Phys. Chem. A 2004, 108, 5278-5287 10.1021/jp049471o CCC: $27.50 © 2004 American Chemical Society Published on Web 05/20/2004