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The High Resolution 4f-5d Absorption Spectrum of Divalent Dysprosium (Dy ) in Strontium Chloride Host SrCl - Fine Structure and Zero-phonon Transitions Revealed 2+

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Miroslaw Karbowiak, Czeslaw Rudowicz, and Jakub Cichos J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08620 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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The Journal of Physical Chemistry

The High Resolution 4f-5d Absorption Spectrum of Divalent Dysprosium (Dy2+) in Strontium Chloride Host SrCl2 - Fine Structure and Zero-phonon Transitions Revealed Mirosław Karbowiak*,†, Czesław Rudowicz‡ and Jakub Cichos† †

Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland E-mail: [email protected] ‡ Visiting Professor: Faculty of Chemistry, A. Mickiewicz University, Umultowska 89B, 61-614 Poznań, Poland; On leave of absence from: Institute of Physics, West Pomeranian University of Technology Szczecin, Al. Piastów 17, 70-310 Szczecin, Poland

Abstract The virtual lack of information on electronic spectra of divalent lanthanide elements (Ln2+) other than Sm2+, Eu2+, Tm2+ and Yb2+ has prompted us to set for synthesis and characterization of novel Ln2+ systems. First successful attempt concerned SrCl2:Nd2+ single crystals. Here, we report stabilization of divalent dysprosium in a chloride host. Importantly this has been accomplished with Dy ions introduced in a divalent state during synthesis, unlike by γ-irradiation of Dy3+ systems employed previously. This synthesis method yields good quality SrCl2:Dy2+ single crystals. The electronic absorption spectra of Dy2+ doped in SrCl2 have been recorded with high resolution at liquid helium temperature (4.2 K). Identification of the absorption bands occurring in the spectral range of 5000 - 45000 cm-1 is achieved. Based on theoretical calculations using semi-empirical Hamiltonian model, assignment of bands and determination of the Hamiltonian parameters for Dy2+(4f95d1) configuration is carried out. The experimental and theoretical studies reveal fine structure and zero-phonon transitions and thus enable high resolution assignment of spectral lines. It is shown that spin-forbidden transitions gain relatively high intensity due to significant admixing of low-spin character to nominally high-spin states. I. Introduction Alongside with Sm2+ ion, Dy2+ ion was one of the first divalent lanthanide (Ln2+) ions intensively studied already in the early 1960's. Dy2+ ion in the CaF2 and SrF2 matrices exhibits a strong emission associated with the 4f-4f transition from the excited level 5I7 to the ground level 5I8. The main drive for this pioneering research was to obtain a stimulated emission at approximately 2.4 µm.1 For this reason, the then research focused on f-f transitions, and even to this day relatively little is known about the spectroscopic properties of this ion in the spectral range of the f-d transitions. Nevertheless, in recent years there has been a significant increase in interest in spectroscopic properties of Ln2+ ions, which have been studied experimentally2,3,4,5,6,7,8 as well as theoretically.9,10,11,12,13,14 To the best of our knowledge, based on extensive literature database searches, the only available spectra for the Dy2+ ion are those published in the 1960's. McClure and Kiss15 and Kiss16 have presented absorption spectra of Dy2+ in CaF2, SrF2, and BaF2 crystals. The selection of these crystals was due to the fact that divalent lanthanide ions could be obtained in the fluoride matrices by photoreduction, e.g. by gamma irradiation. Although spectra of Dy2+ in CaF2, SrF2, and BaF2 were recorded at 4.2 K, only wide bands were observed. Presumably this is due to the way the samples were synthesized, namely, first crystals doped with Dy3+ were obtained, which were then reduced by gamma irradiation. When substituting in MF2 crystals the M2+ ion by Ln3+, e.g. Dy3+, charge compensation is required, which may lead to the presence of Dy3+ ions at various local crystallographic positions. For example, in the case of CaF2:Er3+ it has been found that cubic, tetragonal, and trigonal sites are present, and moreover various defect clusters can be created. Therefore, the Dy2+ ions resulting due to photoreduction presumably also occupy more than one site, thus leading to widening of the bands and lack of subtle structure. In the absorption spectra of CaF2, SrF2, and BaF2 measured in the range of 1000-300 nm,15,16 only a few wide bands are discernible. Above 300 nm the f-d bands are overshadowed by the host band absorption. Spectra for all these three matrices have a similar structure, only shifting of the bands towards the higher energies is observed when passing from CaF2 to BaF2. In fact, the only specific information that can be obtained from these spectra is the location of the first spin-allowed f-d transition, which is observed at 918 nm (10893 cm-1), 861 nm (11614 cm-1), and 835 nm (11976 cm-1) for CaF2, SrF2, and BaF2,16,17 respectively. Weakliem et al.18 have presented the linear and circular magnetic dichroism spectra for the Dy2+ ion in CaF2. Their spectra are practically identical with those shown in Ref. 16. The only difference is the appearance of ACS Paragon Plus Environment

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a relatively sharp band at about 800 nm, which, however, most likely originates from the Dy3+ f-f transitions. Also, in this case, the Dy impurity ions were introduced via admixture of DyF3 compound containing trivalent Dy3+ ions. The so-obtained crystals were then reduced by heating them in the presence of alkaline earth metal contained in a sealed quartz ampoule. To the best of our knowledge, no study is available that would present the absorption spectrum recorded for a crystal in which the Dy2+ ion was introduced in the form of a divalent compound during synthesis. In this report the electronic absorption spectra of Dy2+ doped in SrCl2 single crystal are presented. The divalent dysprosium was stabilized in a chloride host. Importantly, dysprosium ions were introduced in a divalent state and not, as in previous studies,15,16,18 by post-synthesis reduction of Dy3+ ions. A good quality of the synthesized SrCl2:Dy2+ single crystals has enabled recording of high resolution absorption spectrum at liquid helium temperature (4.2 K) and identification of absorption bands in the 5000 - 45000 cm1. To resolve the bands observed in the spectra, we performed theoretical calculations utilizing the semi-empirical model Hamiltonian.19 The comprehensive experimental and theoretical studies reveal fine structure and zerophonon transitions and thus enable high resolution assignment of spectral lines arising from the divalent state of another (Dy2+) lanthanide ion. The organization of this paper is as follows. Experimental details are provided in Section II. In Section III the results and discussion are presented. Summary and conclusions are given in Section IV. II. Experimental SrCl2 single crystals doped with 0.11 at.% of divalent dysprosium ions (as determined with ICP-OES spectroscopy) were grown by the Bridgman method. So-obtained pure SrCl2 single crystal was crushed and mixed with appropriate amount of DyCl2, then placed in a vitreous carbon crucible, which was put into silica ampoule and heated for several hours at 720 K under high dynamic vacuum. After sealing under vacuum the ampoule was lowered through the vertical furnace at 1195 K at a rate of 5 mm/h. DyCl2 used in this procedure was earlier synthesized via comproportionation route20 in reaction of anhydrous DyCl3 with Dy powder at 1100 K in a sealed niobium tube. Growing crystal in quartz ampoule or tantalum tube has lead to SrCl2 single crystals containing mostly Dy3+ ions. Successful synthesis of SrCl2:Dy2+ using glassy graphite crucible indicates that vitreous carbon ensures more reduction-conducive conditions necessary for stabilization of Dy2+ ions during crystal growth. Absorption spectra were recorded in the 2500 - 200 nm range at 4.2 K on a Cary-5000 UV-Vis-NIR spectrophotometer, equipped with an Oxford Instrument model CF1204 cryostat. Emission and excitation spectra were recorded Emission and excitation spectra were recorded at 77 K on an Edinburgh Instruments FLSP 920 spectrofluorimeter using the Optistat DN liquid nitrogen cryostat (Oxford Instrument). III. Results and discussion The presence of Dy2+ ions in the synthesized crystal can be easily verified visually. SrCl2:Dy2+ (0.11 at.% w/w) crystals are characterized by intense dark green color, while SrCl2:Dy3+ crystals containing the similar amount of dysprosium ions are practically colorless.

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The Journal of Physical Chemistry Figure 1. Survey absorption spectrum of SrCl2:Dy2+ (0.11 at.% w/w) recorded at 298 K (a) and 4.2 K (b); for comparison the 4.2 K absorption spectrum of SrCl2:Dy3+ (0.16 at.% w/w) is shown at the bottom in blue (color online) (c). The region in which the fingerprint transitions of Dy2+ occur is marked with a darker background.

Figure 1a shows the absorption spectrum recorded at room temperature. A number of intense and broad bands extending from NIR to UV are observed in the spectrum. Since Dy3+ ions do not have absorption transitions in the range of 14000 - 20000 cm-1, the bands observed in this region (marked in Figure 1) can be considered as the most characteristic of Dy2+ ions and thus treated as fingerprint transitions. Above the 45000 cm-1 the edge of the absorption of the host matrix appears. In the spectrum measured at 4.2 K (Figure 1b), an additional fine structure of the bands is revealed, particularly well visible for bands located at lower energies. For comparison, Figure also shows the spectrum of Dy3+ in SrCl2. The differences in the spectral features observed for divalent and trivalent Dy ion are evident. For Dy3+ one observes the typical atomiclike absorption spectrum arising from the intraconfigurational f-f transitions, while broad bands for Dy2+ arise from the interconfigurational f-d transitions. The absence in the SrCl2:Dy2+ spectrum of the transitions corresponding to Dy3+ ion proves that the crystal practically does not contain Dy3+ ions or they occur in such low concentration that low intensity Dy3+ f-f transitions are not detectable. In the absorption spectrum of the SrCl2 crystal doped with DyCl2, however, obtained in a quartz tube, which caused the oxidation of Dy3+ to Dy2+, only narrow lines characteristic for Dy3+ ions are observed. This proves that DyCl2, which is used as an admixture, is not the source of any impurities that could give color centers in SrCl2. Hence, all the bands observed in the SrCl2:Dy2+ spectrum are associated with Dy2+ ion transitions. Divalent dysprosium is isoelectronic with trivalent holmium and hence their lowest energy state is the 5I8 multiplet originating from the 4f10 configuration. In the absorption spectrum of SrCl2:Dy2+, we do not observe the f-f transitions from the ground 5I8 multiplet levels to the excited levels of the 4f10 configuration. Dy2+ ion in SrCl2 is located at sites with local symmetry Oh. Hence, the intensity of the forbidden f-f transitions is too low to be observed in absorption. Moreover, except for the transitions from the ground 5I8 multiplet to the first two excited 5I7 and 5I6 multiplets, all transitions would be obscured by the strong allowed interconfigurational f-d transitions 4f10 4f95d1. However, the f-f transitions can be observed in the emission spectra. Figure 2 shows that in the emission spectrum of Dy2+ ion recorded at 77 K using 405 nm laser diode excitation one band at 7000 cm-1 is observed. Based on "Dieke diagram"21 for the isoelectronic Ho3+ ion and considering the reduction of energy of individual multiplets of divalent ions as compared to those of trivalent ions,7,22 which results from a reduction of the strength of the interelectronic interaction and spin-orbit coupling, the observed emission should be attributed to the transition 5I6 5I8. The excitation spectrum recorded during emission being monitored at 6852 cm-1 (Figure 2) exhibits a good agreement with the absorption spectrum (Figure 1), which corroborates the finding that the observed emission originates from Dy2+ ions.

Figure 2. a) Emission spectrum obtained by excitation with a wavelength of 405 nm and b) excitation spectrum recorded during emission being monitored at 6852 cm-1 for SrCl2:Dy2+ at 77 K.

Figure 3 shows an enlarged portion of the spectrum in the range in which well separated bands are observed. Most of the lines observed in the spectrum in this range are vibronic sidebands that accompany several zero-phonon lines. In SrCl2 crystal Dy2+ ions substitute for Sr2+ and possess a cubic eightfold coordination. The 4f95d1 configuration splits under combined action of crystal-field interaction (stronger) ACS Paragon Plus Environment


and spin-orbit coupling (weaker) into Γ6g, Γ7g and Γ8g levels. Therefore, the most intense vibronic sidebands should arise from even parity vibrations.

Figure 3. High resolution absorption spectrum recorded at 4.2 K for SrCl2:Dy2+ (0.2 % w/w) showing transitions from the 4f10(5I8) ground multiplet to the lowest energy states of the 4f95d1 configuration. The zero-phonon (ZP) lines identified from analysis of ν1(a1g) vibronic progressions are marked with arrows.

The most prominent feature of the spectrum is the ~212 cm-1 vibronic progression that arises from the totally symmetric ν1(a1g) stretch of the DyCl86- moiety. From analysis of vibronic pattern eight zero-phonon (ZP) lines can be distinguished in the lowest energy part of the spectrum, marked in Figure 3 with arrows. In higher energy part of absorption spectrum of SrCl2:Dy2+ the bands appear as broad and unstructured and the vibronic pattern is not as clearly resolved as for the lowest energy bands. This most probably results from the high degree of overlapping between the numerous zero-phonon transitions and the phonon ones, which contributes to the observed spectral lines and cause their broadening. The ZP line at 17499 cm-1 is the only one that can be identified in this spectral region. Overall, the spectral analysis has enabled identification of eight ZP lines with energy (in cm-1): 9930, 10744, 11061, 12209, 12263, 13369, 13700, and 13899. In the low energy portion of the absorption spectrum five bands are observed, which are indicated in Figure 3 as A, B, B ', C and D, whose intensity is gradually increasing from that for A to D. For heavy lanthanides (4fN), i.e. with N > 7, the spin-forbidden (sf) transitions to the high-spin (HS) levels occur at lower energy than the spin-allowed (sa) transitions. Therefore one may expect that the bands observed on the low-energy side of the spectrum may originate from the spin-forbidden (sf) transitions. Dorenbos17 based on a compilation of data on the f-d transitions of divalent lanthanide ions in inorganic compounds has proposed empirical formulas that enable predicting the position of the first f-d band for any Ln2+ ion, provided that the f-d transition energy is known for any other Ln2+ ion in the same matrix. For the SrCl2 crystal doped with Sm2+ or Eu2+ the first f-d transitions are observed at 15280 cm-1 (Ref. 5) and 25044 cm-1 (Ref. 6), respectively. Based on the data from Table 4 and Figure 2 of Ref. 17, one may expect that for Dy2+ the first spin-allowed transition (to the low-spin levels, LS) would occur at about 11100 - 11200 cm-1. This energy would correspond to the low intensity band designated in Figure 3 as B, for which the first ZP line was identified at 10744 cm-1. Then the band A (see, Figure 3) would be associated with the spinforbidden transitions to the HS states. The Dorenbos' model17 predicts that the energy difference between the sf transitions and the sa ones for Dy2+ should be about 3760 cm-1. This suggests that the spin-forbidden (sf) transitions with the lowest energy may be expected at about 7350 cm-1. However, in this spectral region no transitions are observed in our experimental spectrum. This outcome may, however, be due to the too low intensity of such transitions. It should be kept in mind, however, that in the case of Dy2+ the number of experimental data that could be included in Dorenbos' compilation17 was very limited. There are no experimental data on the 4f95d1 levels for the free Dy2+ ion, whereas the data on the energies of the transitions to the first LS state of the f95d1 ACS Paragon Plus Environment

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configuration are only available for three fluoride matrices: CaF2, SrF2 and BaF2. Moreover, such transitions are observed in the absorption spectrum as wide vibronic bands without resolved fine structure, which makes it difficult to accurately determine the energy of the zero-phonon transition. Therefore, the estimated energy of the first sa transition as well as the energy difference between the HS and LS states may be subject to relatively high uncertainty. Therefore, in order to obtain additional and more accurate information, which would enable assignment of the bands observed in the absorption spectrum, we have carried out appropriate crystal-field calculations. Energy levels and the 4f10→4f95d1 transition intensities of Dy2+ in SrCl2 were calculated using a theoretical model for nfN energy levels developed by Reid et al.19 This model incorporated also the interactions arising from the existence of the (n+1)d-electron. The calculations were carried out using the fshell programs package provided by M. F. Reid.23 The model19 assumes that for the excited nfN-1(n+1)d1 configuration the nfN-1 core experiences the same interactions as the nfN configuration. These interactions (and the respective model parameters) are: Coulomb interaction between nf electrons (Fk(ff)), spin-orbit interaction (ζnf(ff)), two-electron correlation corrections to the Coulomb repulsions (α(ff), β(ff), and γ(ff)), three-electron correlations (Ti(ff)), electrostatically correlated spin-orbit interactions (Pk(ff)) as well as spinspin and spin-other orbit interactions (Mj(ff)). However, due to the presence of the d-electron, the atomic part of Hamiltonian must be supplemented by (i) the spin-orbit interactions for the (n+1)d-electron, parameterized by ζ(dd), and (ii) the Coulomb interactions between the (n+1)d electron and the nfN-1 electrons, parameterized by the direct Slater parameters Fk(fd) (k = 2,4) and the exchange Gj(fd) (j = 1,3,5) ones. The crystal-field interactions of the nfN-1 and (n+1)d electrons with the lattice are parameterized by Bqk ( ff ) (k = 2,4,6) and Bqk (dd ) (k = 2,4), respectively. The components q (-k ≤ q ≤ +k) are restricted by site symmetry.24 Since the details of the energy level calculations can be found in Ref. 5 and Ref. 19, for the sake of space, they are not presented here. The complete list of parameters utilized for calculations of the energy levels within the 4f95d1 configurations is given in Table 1. For Dy2+ we were able to determine the energies of eight ZP lines, but only for the low energy part of the spectrum. Since the number of experimentally determined energy levels is significantly smaller then that of theoretically predicted levels, and particularly no ZP line could have been identified in the spectral region above 17500 cm-1, the following fitting procedure has been adopted. Instead of minimizing the differences between the observed and calculated crystal-field energy levels as usually done for 4fN configuration of Ln3+, we have minimized the differences between the overall experimental and calculated spectral profiles. The initial values of the Hamiltonian parameters for the 4f9 core electrons were estimated on the basis of the mean free-ion parameter values for the 4f9 configuration of Dy3+.24 The initial value of the parameter B04 (dd ) for the crystal-field interactions of the 5d electron was adopted as -20500 cm-1, i.e. the value determined for SrCl2:Sm2+.5 Likewise, the values of crystal-field parameters for the f-electrons: B04 ( ff ) and B06 ( ff ) , were estimated based on the values obtained for Sm2+,5 i.e. -710 cm-1 and 215 cm-1, respectively.

The Sm2+ values were extrapolated to those for Dy2+ assuming that the parameters change according to the same trend as for the Ln3+ series in Cs2NaYCl6.25 This yields the Dy2+ values as: B04 ( ff ) = -600 cm-1 and

B06 ( ff ) = 140 cm-1. The respective values obtained for SrCl2:Sm2+ (Ref. 5) were adopted as the initial parameter values for the Coulomb interactions between the 4f9 electrons and the 5d1 electron as well as for the spin-orbit interaction of 5d1 electron. The calculations utilizing the above fitting procedure have enabled to optimize the values of the parameters: F2(ff), ζ(ff), F2(fd), G1(fd), ζ(dd) and B04 ( dd ) . The constant empirical ratios F4(ff)/F2(ff) = 0.707, F6(ff)/F2(ff) = 0.516, that are typical for 4f9 configuration of Dy3+,24 were retained. The relations between F2(fd) and F4(fd), G1(fd) and G3(fd) as well as between G1(fd) and G5(fd) parameters were constrained by the fixed ratios: F4(fd)/F2(fd) = 0.470, G3(fd)/G1(fd) = 0.807 and G5(fd)/G1(fd) = 0.613 calculated using the Cowan's code.26 All other parameters were not allowed to vary in the optimization procedure and were kept at fixed values as listed in Table 1. The adjustable Hamiltonian parameters were optimized by changing their values within certain physically justified ranges, calculating of the energy levels and transition intensities, and repeating this procedure until the best agreement was obtained between the calculated and experimental spectra. The final values of the so-fitted Hamiltonian parameters are listed in Table 1. TABLE 1. The fitted or adopted Hamiltonian parameters for the 4f95d1 configuration of Dy2+ ion in SrCl2. The values in square brackets were not allowed to vary in the optimization procedure.



Parameter Value (cm-1) Parameter Value (cm-1) 2 6 F (ff) 93472 T (ff) [-294] F4(ff)a 66085 T7(ff) [403] F6(ff)a 48232 T8(ff) [340] ζ4f(ff) 1949 B40(ff)c [-600] α(ff) [17.86] B60(ff)c [140] β(ff) [-628] F2(fd) 14183 γ(ff) [1770] F4(fd)d 6666 M0(ff)b [4.46] G1(fd) 5990 P2(ff)b [610] G3(fd)d 4834 T2(ff) [326] G5(fd)d 3672 T3(ff) [23] ζ(dd) 1067 T4(ff) [83] B40(dd)e -20151 a The parameters F4(ff) and F6(ff) were constrained by the empirical fixed ratios: F4(ff)/F2(ff) = 0.707, F6(ff)/F2(ff) = 0.516, taken from [24]. b The parameters M2(ff), M4(ff), P4(ff) and P6(ff) were constrained by the Hartree-Fock fixed ratios [24]: M2(ff) = 0.56 M0(ff), M4(ff) = 0.38 M0(ff), P4(ff) = 0.75 P2(ff), P6(ff) = 0.5 P2(ff). c 4 B 4(ff) = √(5/14)B40(ff) = -365 cm-1, B64(ff) = -√(7/2)B60(ff) = -262 cm-1 . d The parameters F4(fd), G3(fd) and G5(fd) were constrained by the fixed ratio: F4(fd)/F2(fd) = 0.470, G3(fd)/G1(fd) = 0.807 and G5(fd)/G1(fd) = 0.613, calculated by us using Cowan's code [26]. e 4 B 4(dd) = √(5/14)B40(dd) = -12042 cm-1.

The optimized values of F2(ff) and ζ4f(ff) parameters for the 4f95d1 configuration are close to the values for the 4f9 configuration of Dy3+.24 Analogous trend is observed for Sm2+ ions in SrCl2, where the determined values of Fk(ff) and ζ5f(ff) for the 4f55d1 configuration are similar to those typical for the 4f5 configuration of Sm3+.5 A similar trend is also revealed by calculations using Cowan's code,26 namely, the free-ion values of the parameters Fk(ff) for the 4f95d1 configuration are placed between the respective values for the 4f10 and 4f9 configurations, but are significantly closer to those for the 4f9 configuration. The values obtained for the Coulomb f-d interaction parameters Fk(fd) - direct and Gj(fd) - exchange are 58.6 % and 56.0 % of the free-ion values, respectively. This reduction is similar to that obtained for Eu2+ (Ref. 6) or Sm2+ (Ref. 5) ions in SrCl2. The value of the crystal-field parameter for the 5d electron B04 ( dd ) , adjusted in the fitting procedure, equals to -20151 cm-1 and is similar to the value of -21296 cm-1 and -20500 derived for Eu2+ (Ref. 6) and Sm2+ (Ref. 5) in SrCl2. In order to make a comparison with experiment, we have simulated the whole unpolarised absorption spectrum of SrCl2:Dy2+ making a rough assumption that the vibronic sidebands coupled to each of the zerophonon line may be approximated by a single broad Gaussian shape band. In simulation, this band is shifted from the ZP lines by 300 cm-1 with FWHM (full width at half-maximum) of 500 cm-1 and its oscillator strength is proportional to the line strength calculated for a given ZP transition multiplied by the transition energy. The simulated absorption spectrum of SrCl2:Dy2+ is shown in Figure 4b. The agreement between the simulated and experimental spectrum is very good with respect to both the bands positions and their relative intensities.


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The Journal of Physical Chemistry Figure 4. a) 4f10 -> 4f95d1 absorption spectrum at 4.2 K of SrCl2:Dy2+. b) The spectrum calculated by us using the Hamiltonian parameters listed in Table 1. The sticks at the bottom indicate the calculated positions of zero-phonon lines, with the heights proportional to the predicted intensities. HS indicates bands corresponding to the transitions to the high-spin (6H)7LJ multiplets.

The results of our calculations show that the first spin-allowed transition from level 4f10(5I8) to the 4f95d1 configuration states is the transition to the multiplet exhibiting the main character (6H)5I8. The energy of the first transition to the components of that multiplet was calculated at 13490 cm-1, while that of the most intense line at 13945 cm-1. This indicates that to the first spin-allowed transition in our experimental spectrum corresponds the ZP line located at 13369 cm-1. This line belongs to the band represented in Figure 3 as D. The bands A, B and C lying lower in energy are associated with the spin-forbidden transitions to the levels (6H)7LJ. The first spin-forbidden transition was calculated at energy 7600 cm-1 and is not observed in our experimental spectrum due to too low intensity. It is worthwhile to note the increasing intensity of the spin-forbidden bands in the experimental spectrum with increasing energy and their location moving closer to the sa transitions. A similar trend is observed in the simulated spectrum. The intensity of the sf bands, corresponding in the experimental spectrum to the bands B, B 'and C, is much higher than that of the band A, whereas the sf bands located at lower energy have such a low intensity that they are practically invisible in the simulated spectrum. This effect is due to the fact that with increasing energy of the sf bands, the admixture of the LS states in the HS states increases. For example, the lowest level HS (7LJ), with energy calculated at 7600 cm-1, contains 5.7% admixture of the LS (5LJ) states. For comparison, the HS state with the calculated energy of 10628 cm-1, which corresponds to the experimental band B, contains 18.1 % of admixture of the LS (5LJ) states, while the state calculated at 12120 cm-1, which corresponds to the experimental band C contains as much as 21.3 % of admixture of the LS (5LJ) states. These results corroborate the observed correlation between the energy of the sf bands and the degree of admixture of the LS states in the HS states. A comparison of, e.g., the bands C and D in Figure 3 also reveals that the intensity distribution between ZP lines and vibronic sidebands is different for the bands sf and the bands sa. In the case of the band sa (Cband), the intensity of the first vibronic line is smaller than the intensity of the ZP line, whereas the reverse is observed in the case of the band sf (band D). The Huang-Rhys parameter (S) may be evaluated from the  Sn  following equation: I n ∝ e − S   , where In is the intensity and n is the vibrational quantum number of the  n!  27 terminal state. By taking into account the integrated area of vibronic peaks coupled to the first ZP line of band C (at 12209 cm-1) we obtained the value of S ~ 0.94. Analysis of vibronic intensities for ZP lines at 13369 and 13899 cm-1, being the members of band D, yields S ~ 1.48. The S value determined for the sa transition is practically the same as that obtained for Sm2+ in the same host (1.44 - 1.51).5 The obtained values of the parameter S indicate a rather weak electron-lattice coupling and suggest that the nuclear displacement ∆Qa1g is different for the transitions sf and sa.

IV. Summary and conclusions Major outcome of this study is that we have shown that it is achievable to stabilize divalent dysprosium ions in a chloride host and thus successfully synthesize SrCl2 doped with Dy2+ (0.2 % w/w). We have recorded high resolution absorption spectrum of Dy2+ in a chloride host SrCl2 at liquid helium temperature (4.2 K) as well as identified and, based on crystal-field calculations, assigned the absorption bands occurring in the spectral range of 5000 - 44000 cm-1 (i.e. 2000 - 222 nm). Up-to-now the absorption spectra have been measured only for Dy2+ in CaF2, SrF2 and BaF2 samples, which, however, were obtained by γ irradiation of precursor samples containing Dy3+ ions. Such synthesis procedure could be a reason that only broad and unstructured bands were observed in the spectra obtained so far. The novel aspect in these comprehensive experimental and theoretical studies is that in the spectrum presented herein fine structure of bands and zerophonon (ZP) transitions are revealed. This enables high resolution identification and thus assignment of ZP lines arising from the divalent state of another (Dy2+) lanthanide ion. The first spin-allowed transition is observed at 13369 cm-1. The lowest energy spin-forbidden transition is observed in the experimental spectrum at 9930 cm-1. The crystal-field calculations suggest, however, that the first sf transition should be expected at energy as low as 7600 cm-1, but because of the low intensity, the first sf transitions are not observed in the experimental spectrum. ACS Paragon Plus Environment


The computed energy of the lowest sf transition agrees roughly with that obtained using Dorenbos' model.17 However, the position of the first sa transition derived from the interpretation of the experimental spectrum and supported by crystal-field calculations, turns out to be significantly higher than that predicted based on Dorenbos' model. The experimental value determined by us is 13369 cm-1, whereas Dorenbos' model yields ~ 11100-11200 cm-1. Similarly, we have calculated the energy difference between the first HS and LS levels as approximately 5700 cm-1, which is markedly greater than the value 3800 cm-1 given by Dorenbos.17 On the other hand, it should be kept in mind that the HS levels in the experimental spectrum that belong to the band group designated as B or C contain a significant admixture (about 20%) of the LS states. This causes that their intensity is only slightly smaller than that of the lowest energy spin-allowed transitions belonging to group D. Therefore doubts arise if the transitions to the HS states with such a high admixture of the LS states should still be regarded as the sf transitions. In short, the findings presented herein point out to considerable difficulties in obtaining good quality samples, measuring high resolution absorption spectrum, and interpreting the f-d spectra for Dy2+ ions as well as provide a realization of how relatively little is known about the electronic spectra of Dy2+ ions as yet. These difficulties have been successfully, so partially, overcome in the present study, thus filling the gap in our understanding of spectroscopic properties of divalent lanthanide elements. Our results also indicate the need for further experimental work that would provide additional results that could corroborate or otherwise the interpretation proposed by us.

Acknowledgments This work was partially supported by the National Science Centre, Poland under grant number DEC2012/07/B/ST4/00581.

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