Vij D.R. Handbook of Applied Solid State Spectroscopy
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13. Laser-Induced Fluorescence Spectroscopy
Table 13.2 Fluorescence spectroscopic data of alkaline earth uranyl carbonates.
Name Formula
Emission
Maxima
Fluorescence
Decay Time
Remark
Refe-
rence
[ [[ [ [
Liebigite Ca
2
UO
2
(CO
3
)
3
10 H
2
O
466.9
483.1
502.7
524.1
545.9
571.9
313 ± 10 s Natural [23]
Andersonite Na
2
CaUO
2
(CO
3
)
3
6 H
2
O
468.4
485.2
504.8
526.2
549.6
575.4
33.1 ± 3.9 s Natural [24]
Bayleyite Mg
2
UO
2
(CO
3
)
3
18 H
2
O
471.3
487.7
506.9
527.6
551.4
578.3
17.9 ± 0.5 s Synthetic [23]
Liebigite Ca
2
UO
2
(CO
3
)
3
10 H
2
O
465.4
482.9
502.7
524.5
545.4
571.5
145 ± 5 s Synthetic [23]
Sr
2
UO
2
(CO
3
)
3
8 H
2
O
482.5
488.8
502.8
522.9
545.4
569.2
77 ± 1 s Synthetic [23]
Ba
2
UO
2
(CO
3
)
3
6 H
2
O
469.9
487.7
507.3
528.9
552.2
570.5
16.3 ± 0.4 s Synthetic [23]
Andersonite Na
2
CaUO
2
(CO
3
)
3
6 H
2
O
470.6.
486.1
505.4
526.7
549.6
573.9
65.2 ± 0.6 s Synthetic [23, 24]
Swartzite MgCaUO
2
(CO
3
)
3
12 H
2
O
472.3
488.9
509.0
531.1
554.7
578.9
59.4 ± 0.1 s Synthetic [25]
u
u
u
u
u
u
u
u
582
13.3 Fluorescence Spectroscopy of Minerals
Table 13.3 Fluorescence properties of some uranium minerals at room temperature.
Mineral Formula
Centers of
Fluorescence
Emission,
nm
Fluorescence
Temperature
Remark
(Number in
Minerals
Collection
in Freiberg)
[
Asselbornite (Pb,Ba)(UO
2
)
6
(BiO)
4
(AsO
4
)
2
(OH)
12
3 H
2
O
488.6
502.2
525.0
549.1
570.6
595.6
3.54 ± 0.15
s
(76217)
Billietite 6[UO
2
/(OH)
2
] Ba(OH)
2
4 H
2
O 478.8
495.7
514.3
535.0
559.0
574.3
163 ± 7 ns (67259)
Johannite Cu(UO
2
)
2
(SO
4
)
2
(OH)
2
8 H
2
O 503.0
524.5
547.2
5.2 ± 0.6 s (78557)
Kahlerite Fe[UO
2
/AsO
4
]
2
10 H
2
O 417.5
429.3
443.0
447.4
0.75 ± 0.02
s
(37925)
Metalodevite Zn(UO
2
)
2
(AsO
4
)
2
10 H
2
O 487.6
502.1
524.2
548.3
572.0
600.1
6.3 ± 0.5 s
0.7 ± 0.1 s
(69929)
Ranunculite HAl(UO
2
)(PO
4
)(OH)
3
4H
2
O 486.5
501.2
521.7
544.1
569.3
594.7
2.8 ± 1.1 s (78816)
Triangulite Al
3
(UO
2
)
4
(PO
4
)
4
(OH)
5
5 H
2
O 478.8
495.7
514.3
535.0
559.0
574.3
1.3 ± 0.3 s (75824)
The fluorescence lifetimes are given in Table 13.2 for some natural and
synthetic samples. It can be seen that these data differ from each other.
However, there is no trend between the natural and synthetic samples. It can
only be deduced that impurities and lattice effects may influence the
fluorescence lifetime.
Decay Time
at Room
u
uu
u
u
u
u
u
583
13. Laser-Induced Fluorescence Spectroscopy
Comparing the data from Table 13.3 two conclusions can be drawn:
1. The centers of the fluorescence emissions vary significantly. It can be
expected that the fluorescent minerals should be distinguished by their
fluorescence properties.
2. The fluorescence decay times at room temperature also vary in a wide
range from nanoseconds to microseconds. At the present time there is
no common relationship between the fluorescence decay constant
(reciprocal fluorescence decay time) and the environment of the uranyl
ion as known, for instance, for fluorescent trivalent actinides.
13.4 FLUORESCENCE SPECTROSCOPY OF SURFACE
SPECIES AND IN SOLID PHASES
Gabriel [27] studied the interaction of uranium (VI) on the surface of silica
particles. The studies were performed in the presence of atmospheric CO
2
.
The initial concentration of uranium was 1 u 10
–6
and 1 u 10
–7
M UO
2
2+
. By
TRLFS two surface complexes were identified. These species are
characterized by fluorescence decay times of 170 ± 25 s and 360 ± 50 s.
these species occurs a release of 2 and 3 protons, respectively. Nevertheless,
by comparison of the fluorescence spectra with sorption data, a third uranyl-
silicate-carbonate surface species could be postulated, existing in a lower
amount in the pH region 8.0 to 9.0. The three species were assigned to be
ŁSiO
2
UO
2
°, ŁSiO
2
UO
2
OH,
–
and ŁSiO
2
UO
2
OHCO
3
3–
.
Spectroscopic techniques were used by Kowal-Fouchard [28] to study the
the species ŁAl(OH)
2
UO
2
2+
is postulated. At silanol groups two species were
assigned to be ŁSiO
2
UO
2
° and ŁSiO
2
(UO
2
)
3
(OH)
5
–
. The formation constants
for the sorption equlibria for these species were derived to be log K°
Al
= 14.9
± 0.2, logK°
Si
1
= –3.8 ± 0.2 and logK°
Si
2
= –20.0 ± 0.1, respectively.
Unfortunately no information about the number of the released protons is
available. Second the samples were dried before the spectroscopic
measurements were performed. Therefore, it is possible that the species
composition was changed during this process.
The sorption of uranium (VI) onto gibbsite was studied by Baumann and
coworkers [29] by TRLFS. The sorption experiments were carried out under
ambient conditions in a pH range from 5.0 to 8.5 and a uranium concentration
of 1.0 u 10
–5
M. At the surface of gibbsite two uranyl species were detected.
These species are characterized by fluorescence decay times of 330 ± 115 ns
and 5600 ± 1640 ns, respectively. The first species dominates the lower pH
range, whereas the influence of the second species increases at higher pH
values. The maxima of the center of the fluorescence emission of both species
These complexes should be a mononuclear species. During the formation of
interaction of uranium with montmorillonite and to model this behavior.
The authors suggest at least three uranium-bearing species. At the aluminol site
584
13.4 Fluorescence Spectroscopy of Surface Species and in Solid Phases
are located at 479.5, 497.4, 518.7, 541.6, 563.9, and 585.8 nm. The authors
conclude that the species with the shorter fluorescence decay time is attributed
to a bidentate mononuclear inner sphere surface complex. The uranium (VI) is
bound in this complex to two reactive OH
–
groups at the broken edge linked
to on Al. The other species is assigned to be a small sorbed cluster of a
polynuclear surface species.
To study the surface of a depleted uranium (DU) disc immersed in a Ca-
phosphate solution for 182 days, time-resolved laser-induced fluorescence
spectroscopy was used [30]. The weathering solution contained among other
components 2.49 u 10
–3
M calcium and 1.05 u 10
–3
M phosphate, which re-
present pore water concentrations of agricultural soils. Six fluorescence
emission bands at 486, 501, 522, 546, 573, and 601 nm, and two fluorescence
decay times of 50 ± 5 ns and 700 ± 25 ns characterize the TRLFS spectrum.
By comparison of the obtained TRLFS spectra it could be shown clearly that
meta-autunite, a U (VI) phosphate, had formed during the low temperature
alteration on the DU disc. This secondary uranium (VI) mineral phase was
identified at least in a fingerprinting procedure by comparing it with TRLFS
spectra from an in-house U (VI) TRLFS database, including U (VI) oxides, U
(VI)
hydroxides, U(VI) sulphates, and also U(VI) phosphates. The observed
spectrum agreed completely in the centers of emission bands, and also the
determined fluorescence decay times agreed very well.
Theuvenot and coworkers [10] reported on the fluorescence of americium
in a ThO
2
matrix. They doped ThO
2
with 5 u 10
–2
% Am
3+
and excited this
sample with laser light of 337 nm. The resulting fluorescence spectrum
consisted in two main groups of lines. One group was observed in the
wavelength range 670–750 nm; the other one in the range 800–850 nm.
Additional experiments were carried out in order to get information on the
quantum yield. These more diluted samples (10
–6
–10
–3
atom % Am
3+
) were
excited with 355-nm laser pulses. The resulting fluorescence spectrum was
less resolved and did show only two broad emissions centered at 712 nm and
820 nm. The fluorescence decay was found to be monoexponential with a
decay time of 420 s at 300 K. With decreasing temperature the fluorescence
decay time increased, resulting in a value of 870 s at 10 K. These values
were much larger than the fluorescence decay times observed in aqueous
solution, where values of 24.6 ns were observed [9].
Stumpf [31] studied the sorption of curium onto smectite and kaolinite. An
outer sphere complex was detected at low pH by fluorescence spectroscopy.
Increasing pH leads to an inner-sphere adsorption that occurs via aluminol
edge sites. The species detected on the surface of clay minerals were assigned
to be ŁAl-O-Cm
2+
-(H
2
O)
5
and at higher pH ŁAl-O-Cm
+
(OH)-(H
2
O)
4
, or
probably a bidentate species Ł(Al-O)
2
-Cm
+
-(H
2
O)
5
. The sorption behavior of
Cm (III) onto Ȗ-alumina was studied also by [32]. Here the same types of
species were detected. However, the peak maxima for the first species
differed slightly (598.8 nm and 601.2 nm, respectively). In the case of the
interaction with calcite, it could be shown that curium was incorporated into
585
13. Laser-Induced Fluorescence Spectroscopy
the crystal lattice of calcite [33]. The authors came to this conclusion by
studying the spectral shift of the fluorescence signal and the increase of the
fluorescence decay time of the formed curium species. As clearly multi-
exponential fluorescence decays were observed, changes of the equilibria
between the several species in the excited state were not expected. Some
structural suggestions on the environment of the curium ion on the surface
and included in the calcite lattice were given, explained by the increase of the
fluorescence decay time to 314 s and 1302 s. These fluorescence lifetimes
correlated to Cm species having one and no water molecules in the solvation
shell, respectively.
In the series of the several solid sample forms, the frozen samples have drawn
interest, since it is known that quench effects decrease at lower temperatures.
It is known that the UO
2
(CO
3
)
2
2–
and the UO
2
(CO
3
)
3
4–
species do not show
any fluorescence properties at room temperature. This is caused by the quench
effect of the carbonate ions and the water molecules in the solvation shell.
With decreasing temperature this dynamic quench effect is decreased as the
energy transfer from the excited uranyl ion decreases.
The fluorescence of the UO
2
(CO
3
)
3
4–
species can be observed at
temperatures lower than about 235 K. The temperature dependence of the
fluorescence of this species is shown in Figure 13.4 for the temperature range
180 K to 273 K. All other experimental conditions for the measurements were
the same in order to compare the intensity of the observed spectra directly.
Plotting the integrated fluorescence intensity as function of the reciprocal
temperature a straight line can be observed. This is shown for the data
obtained from the above spectra in Figure 13.5 [34].
This behavior has already been exploited in order to study the uranium
speciation of samples especially from the Hanford Site in the state of
Washington, USA. Wang and coworkers [35] measured the fluorescence of
aqueous pore water samples from the vadose zone at the site. By cooling the
samples to the temperature of liquid helium it was possible to observe for the
first time the fluorescence of the species UO
2
(CO
3
)
2
2–
, UO
2
(CO
3
)
3
4–
and
(UO
2
)
2
(OH)
3
CO
3
–
. The spectral data for these species are summarized in
Table 13.4.
By comparison of the fluorescence properties of these synthetic samples
with the data obtained from the pore water samples it was shown that the
UO
2
(CO
3
)
3
4–
species was the dominating species in the studied samples.
In 2005 Wang [8] published studies on the interaction of uranium (VI)
with calcite. It was found that two uranium (VI) species were formed during
the interaction. One of these species was clearly detected to be a
OF FROZEN SAMPLES
13.5 FLUORESCENCE SPECTROSCOPY
586
13.5 Fluorescence Spectroscopy of Frozen Samples
Ca
2
UO
2
(CO
3
)
3
species. On the other component, a less resolved fluorescence
spectrum, no assumptions were made.
460 480 500 520 540 560 580
0
10000
20000
30000
40000
50000
60000
180 K
200 K
215 K
234 K
252 K
276 K
Figure 13.4 Fluorescence spectra of the UO
2
(CO
3
)
3
4–
species as a function of the temperature
(5.2 u10
–7
M UO
2
2+
Figure 13.5 Fluorescence intensity of the UO
2
(CO
3
)
3
4–
species as a function of the reciprocal
The fluorescence of a series of uranium minerals was studied by Wang and
coworkers [36] at temperatures of 6 K. Some of the results are summarized in
Table 13.5.
).
temperature.
Fluorescence intensity (A.U.)
Wavelength (nm)
0.0040 0.0045 0.0050 0.0055
0
10000
20000
30000
40000
50000
60000
(A.U.)ytisnetni ecnecseroulF
T
−1
(K
−1
)
587
13. Laser-Induced Fluorescence Spectroscopy
Table 13.4 Fluorescence spectroscopic data uranyl carbonates at temperature of liquid helium
(synthetic samples).
Formula
Emission
Maxima
Fluorescence
Decay Time Remark Reference
UO
2
(CO
3
)
2
2–
477.4
496.4
517.2
539.8
563.5
962 s [35]
UO
2
(CO
3
)
3
4–
479.6
499.2
519.9
542.4
565.6
883 s [35]
(UO
2
)
2
(OH)
3
CO
3
–
523.0
542.3
561.3
144 s Not all
emissions
determined
[35]
Table 13.5 Fluorescence properties of synthetic uranyl silicate minerals.
Mineral Formula
Centers of
Fluorescence
Emission
Fluorescence
Decay Time
at 4.2 K
Reference
HK(UO
2
)SiO
4
1.5 H
2
O
505.4
535.2
558.0
579.4
266 s [36]
Cuprosklodowskite
(H3O)
2
Cu(UO
2
)
2
(SiO
2
)
2
2 H
2
O
502.9
524.7
547.5
573.5
196 s [36]
Kasolite
Pb(UO
2
)(SiO
4
)
H
2
O
531.2
552.2
574.2
25 s [36]
Sklodowskite
(H
3
O)
2
Mg(UO
2
)
2
(SiO
4
)
2
4 H
2
O
496.6
516.7
538.7
561.5
207 s [36]
Soddyite
(UO
2
)
15
[(OH)
20
/Si
6
O
17
8 H2O
504.5
527.4
551.0
576.3
300 s [36]
Uranophane
Ca(UO
2
)
2
[SiO
3
(OH)]
2
5 H
2
O
503.1
525.5
547.0
570.5
192 s [36]
Haiweeite
Ca(UO
2
)
2
Si
6
O
15
5 H
2
O
494.7
501.0
519.2
540.1
246 s [36]
K
2
(UO
2
)
2
Si
6
O
15
4 H
2
O
513.8
535.0
558.6
582.6
134 s [36]
UO
2
OSi(OH)
3
+
- 19.0 ± 4.0 s [37]
u
u
u
u
u
u
u
u
588
Boltwoodite
Weeksite
It is not the main aim of this chapter to deal with fluorescence spectroscopy of
should be made. Besides the uranium minerals many other fluorescent
minerals exist. It is known that already some disorder in the crystal lattice can
cause intense fluorescence of the sample, and so many fluorites show intense
fluorescence.
As an example of fluorescence properties not caused by the central ion, the
spectrum of the mineral ferghanite LiH[(UO
2
)
4
/(OH)
4
/(VO
4
)
2
] × 2H
2
O
(minerals collection Freiberg No. 20959) is shown in Figure 13.6 [26]. The
sample was excited with laser pulses of 266 nm. The resulting fluorescence
spectrum is strongly shifted to the near UV region if compared with common
known uranyl fluorescence spectra. However, it is clear that this fluorescence
light cannot be emitted by de-excitation of the uranyl ion. Taking into account
the extremely short fluorescence decay time of 25 ± 2 ns, this fluorescence
emission should be caused by defects in the crystal lattice.
The luminescence of the rare earth elements was used by Gaft [38] to
determine several minerals containing these elements and to distinguish
between them. The method of time-resolved measurements was applied to
these minerals, which contain rare earth elements with emissions in the same
spectral range. The elements Tm
3+
, Pr
3+
, Er
3+
, and Ho
3+
were studied in the
minerals apatite, scheelite, zircon, calcite, and fluorite. Additionally, the
luminescence properties of the element Eu
3+
were studied in the minerals
apatite, zircon, calcite, fluorite, danburite, and datolite, respectively. Also
results on measurements of Dy
3+
, Sm
3+
, Ce
3+
, Yb
3+
, Gd
3+
, Mn
2+
, and Eu
2+
were
second range (Ce
3+
in apatite) to the millisecond range (Yb
3+
in
apatite and
scheelite). Also the gate (exposure time of the CCD camera
controlled by
the multichannel plate system after one laser shot) was varied
ments.
On the luminescence obtained from zircon specimens, Gaft and co-
workers reported in details [39]. Usually, lanthanides can easily be included in
the ZrSiO
4
lattice where the Zr
4+
is replaced by the lanthanides. Besides, the
fluorescence emitted from the included lanthanides, several other fluorescence
emissions were observed in the zircon minerals. The authors were able clarify
the nature of some of these additional luminescence bands in minerals of
different origin. The yellow emission at around 575 nm and a decay time of
applied dose rates for neutrons and Į-particles were not reported. The
emission at 505 nm was only observed in synthetic samples and was linked to
the presence of uranyl ions and phosphorous. At 750 nm an emission with
solid samples containing nonactinide elements. However, some short remarks
13.6 FLUORESCENCE SPECTROSCOPY
589
reported. The decay times vary depending on the lanthanide
from the nano-
for several mea-
sure
~32s should be caused by radiation effects with neutrons and Į-particles. The
OF NON-ACTINIDE SOLID MATRICES
13.6 Fluorescence Spectroscopy of Non-Actinide Solid Matrices
13. Laser-Induced Fluorescence Spectroscopy
decay time of more than 3 ms was observed. This emission should be
connected with Fe
3+
. Other emissions (480 nm / ~300 s; 515 nm / ~500 s;
605 nm / ~10 s) were not detected in synthetic samples and could not yet be
interpreted.
Of interest is also another contribution of Gaft in [40]. The authors
reported on the luminescence of Pb
2+
. As the spectrum of this luminescence is
very similar to that of Ce
3+
and Eu
2+
, it is not easy to assign the spectrum. The
use of time-resolved methods allows the differentiation between such
luminescence centers. An intense emission at 312 nm with a decay time of
120 ns was observed in calcite. The authors assigned this spectrum to the
presence of Pb
2+
impurities in the host lattice. The excitation maximum was
found at 240 nm.
350 400 450
0
10
20
30
40
50
60
Figure 13.6 Fluorescence spectrum of the mineral ferghanite.
Piriou [41] has investigated the sorption behavior of europium on calcite
by TRLFS. The experiments were carried out at an elevated temperature of
323 K at the site and specific fluorescence measurements were made at 15 K.
Three different sorption sites were assigned. One type was found in all
samples and was assigned to be a species containing H
2
O or OH
–
groups. The
spectrum of these species consisted in broad bands and a fluorescence decay
time between 0.55 and 0.85 ms was observed. The second type of species also
had a hydroxylated or hydrated environment. However, the fluorescence
lifetime is shorter and resulted in a value of 0.45 ms. The third species was
characterized by a fluorescence decay time of 9.0 ms and a narrow emission
line at 590.1 nm. Due to its long fluorescence lifetime this species should
have lost the water molecules from the salvation shell and should be included
Fluorescence intensity (A.U.)
Emission wavelength (nm)
590
References
in the calcite lattice. Such a type of species was also assigned for Cm
3+
by
Stumpf [33].
13.7 OUTLOOK
The improvement of the detection, especially the introduction of
measurements at low temperatures, shown by Wang [3] as an example,
increases the number of species detectable by fluorescence methods.
Complexes with mixed ligands, which were not detected before, like
(UO
2
)
2
(OH)
3
CO
3
–
, could be determined by their spectroscopic properties. The
increase in quality was demonstrated by the detection of uranium (VI)
carbonate species in pore waters. It is expected that in the future an
enhancement of such measurements will occur.
The description of new fluorescent species like uranium (IV) and
protactinium (IV) motivates a strong development in the study of the behavior
of tetravalent actinides towards solids. The introduction of laser systems
providing ultrashort pulses (fs-scale), in combination with appropriate
detection systems, extends the possibilities for studying the systems with very
short fluorescence lifetimes
Laser-induced time-resolved fluorescence spectroscopy has become a very
important tool in the study of solid samples of different origin during the last
years. From the spectra a lot of information about the sample regarding its
composition and structure can be gathered.
ACKNOWLEDGEMENTS
The author thanks Prof. G. Bernhard for much helpful discussion during the
preparation of the manuscript. The Technical University Mining Academy,
Freiberg, is acknowledged for providing the minerals used in fluorescence
measurements.
Thanks are also due to Prof. emeritus R. Vochten from University of
Leuven, Belgium, and Dr. A. Scheinost, Institute of Radiochemistry,
Forschungs-zentrum, Rossendorf, for providing with some synthetic uranium
phosphate samples.
REFERENCES
[1] Lakowics, J.R. (1999) Principles of Fluorescence Spectroscopy Kluwer Academic/
nd
[2] Marquardt, C.M., Panak, P.J., Apostolidis C., Morgenstern, A., Walther, C., Klenze, R.
[3]
[4] Billard, I., Ansoborlo, E., Apperson, K., Arpigny, S., Azenha, M.E., Birch, D., Bros, P.,
Burrows, H.D., Choppin, G., Coustin, L., Dubois, V., Fanghänel, Th., Geipel, G.,
Plenum Publishers New York: 2 Edition.
& Fanghänel, Th. (2004) Radiochim. Acta 92, 445-446.
Kirishima, A., Kimura, T., Tochiyama, O. & Yoshida, Z. (2003) Chem. Comm. 910-911.
591