ARTICLE pubs.acs.org/JPCA
Spectroscopic Properties and Hydration of the Cm(III) Aqua Ion from 10 to 85 °C Guoxin Tian, Norman M. Edelstein, and Linfeng Rao* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
bS Supporting Information ABSTRACT: The optical absorption, fluorescence excitation, and emission spectra of the Cm(III) aqua ion in 0.001 M perchloric acid were studied in pure H2O, pure D2O, and in mixtures of H2O-D2O at temperatures from 10 to 85 °C. The quantum yield of the fluorescence of the Cm(III) aqua ion in pure H2O and D2O was also measured in this temperature range and the radiative decay rate constant was obtained from these data. The results indicate that, from 10 to 85 °C, the effect of temperature on the absorption, excitation, and emission spectra is very small. By correcting the observed decay rate constant for the radiative rate constant, a set of correlations between the observed fluorescence decay rate constant and the hydration number of Cm3þ in H2O at temperatures from 10 to 85 °C was developed. A weak temperature dependence was observed for the nonradiative decay rate constant for the 6 0 D 7/2-8S0 7/2 transition and described by the Arrhenius equation. The activation energy of the nonradiative decay was measured to be 0.9 kJ mol-1, approximately matching the energy gap between the first and the second (A1 and A2) levels of the metastable 6D0 7/2 multiplet of the Cm(III) aqua ion. On the basis of these observations, it is postulated that the slight increase in the observed fluorescence decay rate constant as the temperature increases from 10 to 85 °C is due to the effect of thermal population of the A2 level.
1. INTRODUCTION The coordination chemistry of actinide ions (An) in aqueous solutions is of great importance in developing new separation processes for the advanced nuclear fuel cycle and predicting the migration of actinides in the environment. Because of its relatively long half-life and high fluorescence efficiency, 248 Cm(III) has been studied as an analog for other An(III) ions in a variety of systems by time-resolved laser-induced fluorescence spectroscopy (TRLFS).1 However, the scarcity of this isotope limits the number of studies, and some important fluorescence properties such as the quantum yield and the radiative decay rate constant (kr = 1/τr, where τr is the radiative lifetime) have not been evaluated directly. A value for τr = 1.4 ms was obtained from the Judd-Ofelt intensity analysis of the solution spectrum of Cm(III) in 1 M HClO4 at 22 °C. However, there is a rather large error associated with this number due to the uncertainty of ∼65% in the fitted parameter Ω2.1,2 The fluorescence lifetime of Cm(III) in solution was first observed by Beitz and Hessler in 1980.3 Since then several groups have reported the lifetime for the 593 nm emission of Cm(III) in H2O and D2O. As shown in Table 1, the lifetimes at room temperature in pure H2O measured by different groups agree well with each other (∼65 μs), but the values obtained in D2O r 2011 American Chemical Society
show large variations (940-1370 μs) even when the degree of deuterium substitution for H2O is taken into account. In the TRLFS study from 20 to 200 °C,4 the fluorescence lifetime of the Cm(III) aqua ion was found to be temperature-dependent, suggesting that the correlation between the lifetime and the hydration number of Cm(III) previously reported in the literature5 may be applicable only to room temperature solutions. Lindqvist-Reis, et al.,4 determined the hydration number of Cm(III) in aqueous solution and diluted in La and Y triflate crystals by fluorescence spectroscopy at various temperatures. In aqueous solution, it was found that the emission band originating from the 6D0 7/2-8S0 7/2 fluorescent transition became broad and shifted to lower energy as the temperature was increased from 20 to 200 °C. This temperature dependence was attributed to an equilibrium between two different hydrated Cm(III) species, namely Cm(H2O)93þ and Cm(H2O)83þ. By peak deconvolution, the fraction of Cm(H2O)93þ was calculated to be ∼90% at 20 °C and ∼60% at 200 °C, as the fraction of Cm(H2O)83þ increased from ∼10% to 40%. However, it was also found that Received: December 7, 2010 Revised: January 19, 2011 Published: February 22, 2011 1933
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kobs ¼ kr þ
Table 1. Measured Lifetimes of Cm(III) in Pure H2O and D2O Solutions T
medium
lifetime (μs)
295(1) K
0.1 M HClO4
65(2)
3
0.1 M DClO4(99%D) H2O
940(20) 68(3)
3 6
RT
H2O
65(2)
7
RT
H2O
64.6(7)
5
RT
D2O
1270(20)
5
RT
0.01 M HClO4
65(2)
8
RT
0.01 M HClO4(99.95(D))
1200(30)
8
293 K
H2O
64(3)
4
293 K 20 °C
D2O H2O
1370(30) 65(2)
4 9
25 °C
0.001 M HClO4
67.1(1.0)
this work
25 °C
0.001 M DClO4 (99.9(D))
1320(30)
this work
2. EXPERIMENTAL SECTION 2.1. Background. For an ion in solution the measured lifetime is τobs = 1/kobs, where kobs is the measured rate constant. The true radiative lifetime τr (or 1/kr) is defined as follows:
τobs ¼ τr Φ
ð1aÞ
Φkobs ¼ kr
ð1bÞ
or
where Φ is the quantum yield for direct excitation of the emitting level. Following Hass and Stein,11 the quantum yield is written as follows: kr þ
∑i ki
kobs ¼ kr þ knrad þ kH2 O
ð4aÞ
kobs ¼ kr þ knrad þ kD2 O
ð4bÞ
and for pure D2O,
the lifetime at 200 °C for the 593 nm emission band was shorter than that at room temperature despite the smaller average hydration number at 200 °C. This latter result was surprising as it is expected that the lower the number of H2O molecules in the first coordination sphere, the longer the relaxation time.10 In this work, we study the fluorescent properties of the aqua Cm(III) complex in H2O-D2O systems in the temperature range of 10 to 85 °C. The absorption and excitation spectra of aqua Cm(III) in 0.001 M HClO4 were measured at six different carefully controlled temperatures in this range. The fluorescence lifetimes and emission spectra were also measured in varying H2O-D2O mixtures in order to determine the correlation between the lifetime and the hydration number of Cm(III) at different temperatures. Also, for the first time, the fluorescence quantum yield Φ of the Cm(III) aqua ion was measured, and the radiative decay constants of the excited 6D0 7/2 multiplet of Cm(III) were obtained as a function of temperature. A very weak temperature dependence was observed for the nonradiative decay rates in the 10 to 85 °C temperature range from lifetime measurements of Cm(III) in the H2O-D2O system after corrections for the measured radiative decay rate. An Arrhenius type temperature-dependent quenching mechanism of the fluorescence of Cm(III) is proposed.
kr
ð3Þ
obs
For the case of interest here, Cm(III) in dilute perchloric acid solutions, the nonradiative processes are primarily associated with the O-H(D) vibrations of the associated water molecules in the first coordination sphere.10 In this case for pure H2O,
ref
295(1) K RT
Φ¼
∑i ki ¼ τ1
ð2Þ
where ki is the rate constant for the ith nonradiative process and,
where kH2O and kD2O are the rate constants of the nonradiative processes involving H2O and D2O respectively, and knrad is the rate constant for other nonradiative processes that do not involve water. If it is assumed that knrad is the same for the Cm(III) dilute solutions of either H2O or D2O, then for mixtures of H2O and D2O, kobs ¼ kr þ knrad þ χH2 O kH2 O þ χD2 O kD2 O
ð5Þ
where χH2O and χD2O are the mole fraction of H2O and D2O, respectively, in the mixture and χH2Oþ χD2O = 1. Then, kobs ¼ ðkr þ kD2 O Þ þ knrad þ χH2 O ðkH2 O - kD2 O Þ
ð6Þ
We assume knrad is small and negligible for this system (with only H2O or D2O molecules in the first coordination shell) so there should be a linear correlation between kobs and χH2O. Equation 6 implicitly assumes, as others have shown,5,10 that each H2O or D2O oscillator acts independently of the others in the first coordination sphere. Therefore we can define the rate constant per H2O molecule8 k0H2 O ¼
k H2 O nH2 O
ð7Þ
where nH2O is the number of water molecules in the first coordination sphere of Cm(III). In dilute perchloric acid solutions of Cm(III) without any ligands other than water coordinating to Cm(III), we assume nH2O = 9.12 Then eq 4a becomes, kobs ¼ kr þ knrad þ nH2 O k0H2 O
ð8Þ
Assuming knrad is negligible for the temperatures of interest in this work, we then obtain eq 9 for determining the number of H2O molecules in the first coordination sphere of Cm(III) in water, with a linear correlation of nH2O versus kobs and 1/k0 H2O as the slope and -kr/k0 H2O as the intercept: n H2 O ¼
kobs kr k0H2 O k0H2 O
ð9Þ
The parameter kr can be experimentally determined by measuring the quantum yield as described in this paper. Then the parameter k0 H2O can be obtained from (kobs - kr), see eq 8, assuming knrad is negligible and nH2O = 9 in pure H2O. 2.2. Preparation of Cm(III) Samples in H2O, D2O and Mixtures of H2O/D2O. The curium material used in this work contains 96% 248Cm and 4% 246Cm. The stock solution of Cm(III) in 0.001 M DClO4 (D2O) was prepared as follows. A 1 mL solution of 1 M HCl containing 1 mg Cm(III) was dried and the residue was dissolved with 2 mL of 0.1 M DClO4. The drying and dissolving process was repeated twice to convert CmCl3 to Cm(ClO4)3. Two additional drying and dissolving steps with 2 mL D2O were performed to reduce the excess 1934
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Figure 2. Measured rate constants of Cm(III) in mixtures of H2O and D2O in 0.001 M perchloric acid) as a function of temperature.
Figure 1. Excitation and emission spectra of Cm(III) in 0.001 M HClO4 in H2O at different temperatures.
amount of DClO4. Finally, the residue was dissolved with 2.5 mL 0.001 M DClO4 (D2O). The concentration of Cm(III) in the stock solution was determined to be 0.00161 M by absorption spectrometry using the molar absorptivity of Cm3þ in perchloric acid solutions (ε = 52.9 cm-1 M-1 at 396.4 nm,13). The concentration determined by absorption spectrometry is very close to that calculated from the weight of the Cm material (1 mg) used for preparing the stock solution. Aliquots (0.1 mL) of the above stock solution (0.00161 M Cm(III) in 0.001 M DClO4) were taken to prepare 2.5 mL of Cm(III) solution in mixtures of H2O/D2O by adding appropriate amounts of D2O and H2O. The volume fractions of H2O in the mixtures are 0.20, 0.40, 0.60, and 0.80, corresponding to 0.2004, 0.4006, 0.6006, and 0.8004 H2O by mole fractions. The concentration of Cm(III) in the mixtures is 0.0645 mM. For the Cm(III) sample in pure H2O, 0.1 mL of the above stock solution (0.00161 M Cm(III) in D2O) was repeatedly dried and the residue was dissolved with 2 mL HClO4, 2 mL H2O, and 2.5 mL 0.001 M HClO4 in sequence. The concentration of Cm(III) in the final H2O solution is 0.0645 mM. 2.3. Absorption Spectra. The absorption spectra of Cm(III) in dilute perchloric acid solutions of H2O and D2O at different temperatures were collected on a Cary6000i spectrometer in the wavelength region of 350 to 500 nm (0.05 nm interval and 0.2 nm bandwidth). Quartz cuvettes of 10 mm path were used. The temperature of the sample was controlled within (0.2 °C with a temperature controller. 2.4. Fluorescence Spectra and Lifetime Measurements. The fluorescence spectra and lifetime of Cm(III) in H2O and D2O solutions at different temperatures were acquired on a HORIBA Jobin Yvon IBH FluoroLog-3 fluorometer adapted for
Table 2. Measured Lifetimes of Cm(III) in Mixtures of H2O and D2O as a Function of Temperature τobsa (ms) χH2O
10 °C
25 °C
40 °C
55 °C
70 °C
85 °C
0.00
1.324
1.320
1.312
1.303
1.294
1.289
0.2004
0.296
0.291
0.289
0.286
0.281
0.2768
0.4006 0.6006
0.165 0.113
0.162 0.112
0.160 0.110
0.159 0.109
0.156 0.107
0.153 0.105
0.8004
0.0856
0.0846
0.0836
0.0824
0.0811
0.0797
1.00
0.0682
0.0672
0.0663
0.0656
0.0643
0.0634
a
The relative values are consistent with each other although the absolute values of the data are estimated to have an accuracy of (1.2%.
time-resolved measurements. Ten millimeter quartz cuvettes were used. The sample temperature was controlled by a constant temperature water bath. The excitation spectra were obtained in the wavelength region of 350-500 nm (0.05 nm per step, 0.2 nm bandwidth) by collecting the fluorescence at 592 nm (5 nm bandwidth and 0.5 s integration). The emission spectra were obtained in the wavelength region of 520-650 nm (0.125 nm per step, 0.5 nm bandwidth, 0.5 s integration) by excitation at 396 nm (5 nm bandwidth). A xenon lamp was the light source for collecting the spectra. For the lifetime measurements, a submicrosecond xenon flash lamp (Jobin Yvon, 5000XeF) coupled to a double grating excitation monochromator was used as the light source. Cm(III) was excited at 396 nm (5 nm bandwidth) and the fluorescence was collected at 592 nm (2 nm bandwidth). The input pulse energy (100 nF discharge capacitance) was about 50 mJ and the optical pulse duration was less than 300 ns at fwhm. A thermoelectrically cooled single photon detection module (HORIBA 1935
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Table 3. Measured Quantum Yields and Radiative Rate Constants of Cm(III) in H2O and D2O as a Function of Temperature ΦCm(III)a(H2O)
kr(H2O) (ms-1)
ΦCm(III)a(D2O)
kr(D2O) (ms-1)
10
0.0323
0.466
0.621
0.467
25
0.0296
0.434
0.620
0.470
40
0.0319
0.473
0.615
0.469
55
0.0288
0.436
0.605
0.464
70
0.0296
0.455
0.594
0.459
85
0.0289
0.452
0.590
0.457
T (°C)
Average (10-85 °C) a
0.453
0.464
The measured values are estimated to have an accuracy of (10% in H2O, and 5% in D2O.
Jobin Yvon IBH, TBX-04-D) that incorporates a fast rise time PMT, a wide bandwidth preamplifier, and a picosecond constant fraction discriminator was used as the detector. Signals were acquired using an IBH Data Station Hub and the data were analyzed using the commercially available DAS 6 decay analysis software package from HORIBA Jobin Yvon IBH. The goodness of fit was assessed by minimizing the reduced function, χ2, and visually inspecting the weighted residuals. 2.5. Measurements of Quantum Yields. The primary reference compound utilized for quantum yield measurements was quinine bisulfate in 0.1 M H2SO4 with a quantum yield of Φ = 0.546 at 25 °C.14 In order to use absorption coefficients and fluorescence energies similar to those of Cm(III) in H2O solutions, Rhodamine B was calibrated with quinine bisulfate solutions and used as a secondary standard for Cm(III) in H2O solutions.15 This was necessary because of the relatively weak fluorescence of these solutions. The measured quantum yield of Rhodamine B in H2O was 0.251 ( 0.007 at 25 °C, which is in good agreement with the value of 0.25 at room temperature16 and the value of 0.31 at 20 °C.15 All reference standards were kept at 25 °C while the Cm(III) samples were measured from 10 to 85 °C. To reduce the uncertainties of the measured quantum yield of Cm(III), two measures were taken: (1) the areas, instead of the peak intensities, under the absorption bands and fluorescence emission bands were used to draw the fluorescence intensityabsorption plots; (2) five samples of the reference standards (quinine bisulfate and Rhodamine B) and five samples of Cm(III) covering appropriate concentration ranges, instead of a single sample, were measured. The slopes of the integrated absorption vs integrated fluorescence for the standards at 25 °C and for the Cm(III) solutions at various temperatures were determined. From the slope ratios (Cm(III) sample vs the reference standard) and the known values of Φ for the standards, the values of Φ for the Cm(III) solutions were determined. The details of the procedures for the measuring the quantum yields are described in the Supporting Information.
3. RESULTS AND DISCUSSION 3.1. Absorption, Excitation and Fluorescence Spectra at Different Temperatures. The absorption spectrum of Cm(III)
in 0.001 M DClO4 in D2O (>99.9%) at 25 °C is identical to the spectra of Cm(III) in 0.1 and 1 M HClO4, indicating there is no H/D isotope effect on the absorption of Cm(III) aqua ion, and the perchlorate anion is not involved in the inner coordination sphere of Cm(III) in this concentration range. The positions of all absorption bands in the UV-vis region remain unchanged as the temperature varies, but the absorbance decreases by
Table 4. Values of the Rate Constant Per H2O Molecule, the Ratio of the Radiative Rate Constant to the Rate Constant Per H2O Molecule, and the Correlation between kobs and nH2O of Cm(III) at Different Temperatures correlation between T (°C)
(1/k0 H2O) (ms)
kr/k0 H2O
kobs and nH2O of Cm(III)
10
0.64
0.30
nH2O = 0.64 kobs - 0.30
25
0.63
0.29
nH2O = 0.63 kobs - 0.29
40
0.62
0.29
nH2O = 0.62 kobs - 0.29
55
0.61
0.28
nH2O = 0.61 kobs - 0.28
70
0.60
0.28
nH2O = 0.60 kobs - 0.28
85
0.59
0.27
nH2O = 0.59 kobs - 0.27
Figure 3. Thermal quenching of Cm(III) in pure H2O and D2O (with kobs and kr in units of s-1).
∼15% from 10 to 85 °C. As expected, the excitation spectra are very similar to the absorption spectra, and there is very little change in the excitation spectra as the temperature is increased from 10 to 85 °C as shown in Figure 1 (upper part). The fluorescence spectra change with temperature. The lower part of Figure 1 shows that as the temperature is raised, the bandwidth of the emission increases, and the peak position shows a slight red-shift. 3.2. Fluorescence Decay in H2O/D2O Mixtures: Linear Correlation between kobs and χH2O. Neglecting knrad in eq 6 gives an equation that describes a linear correlation between kobs and χH2O: kobs ¼ ðkr þ kD2 O Þ þ χH2 O ðkH2 O - kD2 O Þ 1936
ð10Þ
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Table 5. Energy Levels of the 6D0 7/2 Multiplet of Cm(III) in Aqueous Solution and a Hydrated Crystal at Room Temperature T (°C)
sample
A1 (cm-1)
A2 (cm-1)
A3 (cm-1)
A4 (cm-1)
refs
Cm3þ aqueous solution
20
16841
16929
17091
17301
9,
Cm3þ in [Y(H2O)9](CF3SO3)3
20
16895
16949
17079
17307
9,19
In this work, we measured the values of kobs for Cm(III) aqua ion in mixtures of H2O and D2O at six different temperatures (10, 25, 40, 55, 70, and 85 °C). The values of kobs and corresponding values of τobs are shown in Figure 2 and Table 2, respectively. Indeed, kobs increases linearly as χH2O is increased from 0 to 1 at each temperature. From the data at 25 °C, the following correlation was obtained in units of ms-1: kobs ¼ 14:1χH2 O þ 0:621 This correlation is slightly different from that of a previous measurement at room temperature5 in units of ms-1: kobs ¼ 14:7χH2 O þ 0:786
kobs ¼ kr þ Aexpð-ΔE=RTÞ
The reason for this discrepancy is discussed in a later section. 3.3. Quantum Yield, Radiative Rate Constant and the Correlation for Calculating the Hydration Number. The
hydration number nH2O defined in eq 8 can be determined if we know kr, the radiative rate constant. Experimentally, kr can be obtained from eq 1 by measuring the observed lifetime and the Cm(III) quantum yield ΦCm(III): kr ¼
ΦCmðIIIÞ 1 ¼ τr τobs
lifetime of Cm(III) on temperature has been observed in the temperature range from 20 to 200 °C in a previous study.4 However, we find the fluorescence lifetime of Cm(III) to be only weakly temperature dependent in the limited temperature range of our measurements (10 to 85 °C). In general, the radiative rate shows little temperature or coordination sphere dependence as shown above, while nonradiative de-excitation involving the population of higher electronic levels of metal ions is temperature dependent.17,18 The temperature dependence for the nonradiative rate constant is related to the energy gap (ΔE) between the emitting level and higher electronic levels. We assume for Cm(III) in H2O and D2O systems that knrad is negligible, so:
ð11Þ
In this work, ΦCm(III) was measured in pure H2O and D2O systems from 10 to 85 °C. The experimentally determined ΦCm(III) and corresponding kr (from eq 11) are given in Table 3. The data show that the ΦCm(III) differ by more than 1 order of magnitude between the pure H2O and D2O systems but that there is only a small temperature effect for ΦCm(III) in either the H2O or the D2O systems. However, there is no temperature or isotope effect for the radiative decay rate constant, kr. Therefore, we have used the average values of 0.459 ms-1 for kr, and 2.18 ms for τr for the temperature range of 10 to 85 °C in this work. With the above value of kr (0.459 ms-1) and the measured kobs at different temperatures, values of k0 H2O at different temperatures can be obtained using eq 8 (again neglecting knrad and assuming nH2O = 9). The values of 1/k0 H2O and kr/k0 H2O are listed in Table 4. Using these parameters and eq 8, the correlations between kobs and the hydration number (nH2O) of Cm(III) at different temperatures are established (Table 4). The correlations between kobs and nH2O of Cm(III) from this work (Table 4) are derived by correcting kobs with the radiative rate constant that was derived from Φ, the experimentally measured quantum yield. On the contrary, the empirical correlations previously reported are obtained by fitting kobs as a function of nH2O for a number of Cm(III)-spiked lanthanide solids with known nH2O, without the correction for the radiative rate constant.5 The empirical correlations at 25 °C in the literature (nH2O = 0.65 kobs - 0.88)5 is in fairly good agreement with the correlation derived from this work (nH2O = 0.63 kobs - 0.29), because kr (∼0.46 ms-1) is much smaller than kobs (∼15 ms-1) for Cm(III) in pure H2O solutions. 3.4. Mechanism of Temperature Dependency of NonRadiative Decay. A strong dependence of the fluorescence
ð12Þ
where A is the frequency factor. With the values of measured kobs and kr, the plots of ln(kobs - kr) against 1/T were drawn for pure H2O and D2O systems (Figure 3). Two linear lines with small and nearly identical slopes were obtained (A = e9.92, ΔE/R = 102 K in H2O, and A = e6.06, ΔE/R = 106 K in D2O), suggesting that the de-excitation mechanisms for the 6D0 7/2 emission of the Cm(III) aqua ion in H2O and D2O have the same weak temperature dependence. The ratio of kH2O/kD2O at 25 °C was found to be 47.9, (from eqs 4a and 4b, and assuming knrad is negligible) almost identical to the ratio of the frequency factors, A(H2O)/A(D2O) = 47.5, which validates the assumption that knrad is negligible for the Cm(III) aqua ion. The observed activation energy is almost the same in H2O and D2O, about 0.9 kJ mol-1, which is very close to the energy gap between the first and second (A1 and A2) levels of the metastable 6 0 D 7/2 multiplets (see Table 5) for Cm(III) in aqueous solution and in a hydrated triflate crystal, as reported by Lindqvist, et al.9 Hence, the weak temperature-dependence of the fluorescence quenching of Cm(III) aqua ion in H2O and D2O can be ascribed to the thermal population within the 6D0 7/2 multiplet. In this case, as the temperature is increased, the thermal population of the A1 level is reduced and the thermal population of the A2 level (and higher levels) is increased resulting in a shortened observed lifetime of the A1 emitting level.
4. CONCLUSIONS The fluorescence spectra and lifetime of the Cm(III) aqua ion were measured in H2O-D2O solutions from 10 to 85 °C. The weak temperature dependency of lifetime and intensity suggests that Cm(III) aqua ion mainly remains as Cm(H2O)93þ in the H2O-D2O system from10 to 85 °C. The variation of the lifetime and spectra may be attributed to the thermal population of the second (A2) and higher levels of the metastable 6D0 7/2 multiplet. The radiative decay rate constant and fluorescence quantum yield of the Cm(III) aqua ion were determined directly for the first time. The value, τr = 2.18 ms, is in good agreement with the earlier value obtained from the Judd-Ofelt intensity analysis of Cm(III) absorption spectrum when experimental uncertainties are considered. The radiative lifetime does not show a temperature dependence or an isotope effect. The correlation between the hydration number and fluorescence lifetime in the 1937
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temperature range from 10 to 85 °C has been established so that the hydration numbers for Cm(III) aqua species at different temperatures can be readily determined by fluorescence spectroscopy.
’ ASSOCIATED CONTENT
bS
Supporting Information. The experimental details of the quantum yield measurements are given in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences; and Office of Nuclear Energy, the Fuel Research and Development Program under Contract No. DEAC02-05CH11231 at Lawrence Berkeley National Laboratory. The authors are indebted for the use of the curium-248 material to the Office of Basic Energy Sciences through the transplutonium element production facilities at the Oak Ridge National Laboratory. ’ REFERENCES (1) Edelstein, N. M.; Klenze, R.; Fangh€anel, T.; Hubert, S. Coord. Chem. Rev. 2006, 250, 948. (2) Carnall, W. T.; Rajnak, K. J. Chem. Phys. 1975, 63, 3510. (3) Beitz, J. V.; Hessler, Jan P. Nucl. Technol. 1980, 51, 169. (4) Lindqvist-Reis, P.; Klenze, R.; Schubert, G.; Fanghanel, T. J. Phys. Chem. B 2005, 109, 3077. (5) Kimura, T.; Choppin, G. R. J. Alloys Compd. 1994, 213/214, 313. (6) Beitz, J. V.; Bowers, D. L.; Doxtader, M. M.; Maroni, V. A.; Reed, D. T. Radiochim. Acta 1988, 44-5, 87. (7) Wimmer, H.; Klenze, R.; Kim, J. I. Radiochim. Acta 1992, 56, 79. (8) Kimura, T.; Nagaishi, R.; Arisaka, M.; Ozaki, T.; Yoshida, Z. Radiochim. Acta 2002, 90, 715. (9) Lindqvist-Reis, P.; Apostolidis, C.; Rebizant, J.; Morgenstern, A.; Klenze, R.; Walter, O.; Fanghanel, T.; Haire, R. G. Angew. Chem., Int. Ed. 2007, 46, 919. (10) Horrocks, W. D.; Sudnick, D. R. J. Am. Chem. Soc. 1979, 101, 334. (11) Haas, Y.; Stein, G. J. Phys. Chem. 1971, 75, 3677. (12) Skanthakumar, S.; Antonio, M. R.; Wilson, R. E.; Soderholm, L. Inorg. Chem. 2007, 46, 3485. (13) Carnall, W. T.; Fields, P. R.; Stewart, D. C.; Keenan, T. K. J. Inorg. Nucl. Chem. 1958, 6, 213. (14) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (15) Lopez Arbeloa, T.; Lopez Arbeloa, F.; Hernandez Bartolome, P.; Lopez Arbeloa, I. Chem. Phys. 1992, 160, 123. (16) Pringsheim, P. Fluorescence and Phosphorescence; Interscience: New York, 1949. (17) Dawson, W. R.; Kropp, J. L.; Windsor, M. W. J. Chem. Phys. 1966, 45, 2410. (18) Haas, Y.; Stein, G. Chem. Phys. Lett. 1971, 8, 366. (19) Lindqvist-Reis, P.; Walther, C.; Klenze, R.; Edelstein, N. M. J. Phys. Chem. C 2009, 113, 449.
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