# Electronic structure of americium sesquioxide probed by resonant inelastic
x-ray scattering

S. M. Butorin  Condensed Matter Physics of Energy Materials, X-ray Photon
Science, Department of Physics and Astronomy, Uppsala University, P.O. Box
516, SE-751 20 Uppsala, Sweden  D. K. Shuh  Chemical Sciences Division,
Lawrence Berkeley National Laboratory, MS 70A1150, One Cyclotron Road,
Berkeley, CA 94720, USA

###### Abstract

The Am $5d$-$5f$ resonant inelastic x-ray scattering (RIXS) data of americium
sesquioxide were measured at incident photon energies throughout the Am
$O_{4,5}$ edges. The experiment was supported by calculations using several
model approaches. While the experimental Am $O_{4,5}$ x-ray absorption
spectrum of Am2O3 is compared with the spectra calculated in the framework of
atomic multiplet and crystal-field multiplet theories and Anderson impurity
model (AIM) for the Am(III) system, the recorded Am $5d$-$5f$ RIXS data are
essentially reproduced by the crystal-field multiplet calculations. A
combination of the experimental scattering geometry and theoretical analysis
of the character of the electronic states probed during the RIXS process
confirms that the ground state of Am2O3 is singlet $\Gamma_{1}$. An appearance
of the low-intense charge-transfer satellite in the Am $5d$-$5f$ RIXS spectra
at an energy loss of $\sim$5.5 eV, suggests weak Am $5f$-O $2p$ hybridization
which is in agreement with AIM estimations of the $5f$ occupancy from
spectroscopic data in Am2O3 as being 6.05 electrons.

##  I Introduction

The importance of americium oxides in the nuclear cycle is well recognized.
Besides the intention to have them as a part of fuels for the fourth
generation (GEN IV) nuclear reactors, the use of these oxides for the power
sources in the deep space missions is also being exploited [1, 2, 3]. However,
the experimental studies of the electronic structure of americium oxides are
scarce due to their substantial radioactivity.

X-ray spectroscopy is an important tool for this type of studies and allows
for measurements on very small quantities of material in question. It can
provide information about the chemical state of actinides in various systems,
(non)stoichiometry, oxygen/metal (O/M) ratio, local symmetry of the
environment and charge distribution, as well as other parameters important for
nuclear fuel performance. One of the attractive techniques is resonant
inelastic x-ray scattering (RIXS) which is very sensitive to changes in
chemical state and in symmetry of the environment of the studied element, in
particular by probing the elementary low-energy excitations ($5f$-$5f$
excitations in case of actinide systems). The availability of high resolution
allows one to monitor the distribution of the electronic states in greater
detail. In contrast to commonly used x-ray absorption spectroscopy (XAS), the
RIXS resolution is not limited by the core-hole lifetime broadening and
depends solely on the instrumental resolution. However, a number of pioneering
valence-to-core RIXS studies at U $3d$ edges of uranium compounds [4, 5, 6, 7,
8, 9] suffered from a limited resolution of utilized crystal-analyzers
allowing one to resolve only the U $5f$-ligand $2p$ charge-transfer
excitations which are mainly at higher energy losses than the $5f$-$5f$
excitations (only very recently, the high-resolution RIXS measurements at the
U $3d$ edge became available [10]).

Soon after the experimental proof [11] that valence-to-core RIXS probes
elementary electronic excitations ($d$-$d$ excitations in case of MnO), going
to shallow edges was a natural way to improve the resolution of the RIXS
technique. The $d$-$d$ RIXS measurements at the Cu $3p$ edge of the Sr2CuO2Cl2
cuprate [12] followed by the measurements at the Ni $3p$ edge of NiO (Ref.
[13, 14]) and NdNiO3 (Ref. [13]) which indicated the ability of the technique
to provide greater details on the $d$-$d$ excitations. As to $f$-block
systems, first RIXS measurements of $f$-$f$ excitations at the $4d$ edges of
lanthanides were performed for the Gd oxide [15], shortly followed by RIXS
experiments for Ce-based heavy-fermion materials [16], while the first RIXS
measurements at the $5d$ edges of actinides were carried out for UF4
(Ref.[5]).

At actinide $5d$ thresholds, the main absorption edge is very broad due to so-
called autoionization processes as it can be seen from XAS measurements (see
e.g. Refs. [17, 18, 19, 20, 21, 22, 23, 24, 25]) and electron energy loss
spectroscopy (EELS) [26, 27, 28, 29, 30, 31, 1]. Such a broadening makes it
difficult to determine the chemical shifts and obtain information about the
ground state of the system from the XAS spectra. In this situation, the
employment of the RIXS technique with its potentially unlimited resolution at
the actinide $5d$ edges can be very helpful.

Although, initially pioneering $5d$-$5f$ RIXS studies at the actinide
$O_{4,5}$ edges were carried out for oxides of uranium [24, 32, 33, 23, 34,
35], neptunium [34, 24, 35] (see also the Supporting Information section of
Ref. [36]), plutonium [34, 24] and curium [37], the RIXS technique at the
actinide $5d$ edge was only rarely used because of safety restrictions for
research on radioactive samples in the ultra-high vacuum environment at
sources of synchrotron radiation.

Here we present first Am $5d$-$5f$ RIXS data of Am2O3 recorded at a number of
incident photon energies at the Am $O_{4,5}$ edges. The data are analyzed with
help of the model calculations. The many-particle theoretical approaches turns
out to be successful in reproducing the experimental spectra due to a
significant localization of the $5f$ states in Am2O3.

##  II Experimental

The Am oxide sample used for measurements was fabricated by technique used to
prepare radionuclide counting plates at the Lawrence Berkeley National
Laboratory (LBNL; see the ”Preparation of counting sources” subsection in Ref.
[38]). The counting plate was prepared from an aqueous solution of about 2.0
mM Am-243 (better than 99.6% Am-243 by mass) in 0.1 M HCl that was delivered
by micropipette techniques to an area of $\sim$4 mm2 area on a high purity Pt
substrate (25.4 mm diameter). The aqueous droplets were allowed dry leaving a
residue that was ring-shaped. This was followed by inductive heating to nearly
700 oC under atmosphere to oxidize the material and fixing the material to the
Pt substrate to preclude loss when placed in the UHV spectrometer chamber
during the measurement. This process is expected to yield the Am oxide
sesquioxide with an approximate composition of Am2O3 (Ref.[37]). The counting
plated was trimmed to 3 mm x 3 mm around the center and mounted on the sample
holder with conductive tape as described below. The Am sample taken to the
Advanced Light Source (ALS) was close to 1 $\mu$g of Am-243.

A specially designed sample holder, which is described in Refs. [24, 39], was
used for the Am2O3 sample during the measurements. It is essentially a
cylindrical can with slots for incoming and outgoing radiation. The sample is
attached to the slab inside the can just behind the slot. Due to such a
design, the sample holder served as a catch tray for material that might come
loose during handling and the measurements, thus ensuring that no
contamination will be left in the experimental chamber after the experiment.

Experiments in the energy range of the Am $O_{4,5}$ x-ray absorption edge
($5d\rightarrow 5f,7p$ transitions) of Am2O3 were performed at beamline 7.0.1
of the ALS, LBNL employing a spherical grating monochromator [40]. The Am 5$d$
XAS data were measured in the total electron yield (TEY) mode using drain
current on the sample. The incidence angle of the incoming photons was close
to 90∘ to the surface of the sample. The monochromator resolution was set to
$\sim$50 meV at 115 eV during measurements at the Am 5$d$ edge.

Am $5d$-$5f$ RIXS spectra of the sample at the Am $O_{4,5}$ x-ray absorption
edge were recorded using a grazing-incidence grating spectrometer [41] with a
two-dimensional detector. The 400 lines/mm grating of this spectrometer (based
on the Rowland circle criterion) was employed to collect the RIXS data with
the high energy resolution and the 300 lines/mm grating was used to measure
the RIXS spectra in a larger energy range but with lower resolution. The
incidence angle of the photon beam was 10∘ from the sample surface and the
spectrometer was placed in the horizontal plane at an angle of 90∘, with
respect to the incidence beam. The total energy resolution of the RIXS spectra
was estimated to be $\sim$70 meV for the 400 lines/mm grating and $\sim$160
meV for the 300 lines/mm grating, using the full-width at half maximum (FWHM)
of the elastic peak. Measured RIXS spectra were normalized to the current in
the ring.

##  III Computational details

The XAS spectra at the actinide $5d$ edges were calculated using the formalism
described in Ref. [42] in order to account for the ’giant resonance’, highly-
inhomogeneous broadening of the spectral transitions (due to differences in
core-hole lifetime of various core-excited states) and Fano effect. In our
calculations, in addition to the interactions for the free ion, as used in
Ref. [42], the crystal-field splittings in the $5f$ shell and the
hybridization effects between actinide and oxygen states were taken into
account using the Anderson impurity model (AIM) approach [43].

To simplify the calculations of the Am $5d$-$5f$ RIXS map around the Am(III)
$O_{4,5}$ thresholds, the crystal-field multiplet theory approach was used
because the Am $5f$-O $2p$ charge-transfer effects were not expected to be
significant in americium sesquioxide (see e.g. Ref. [25]). The RIXS map was
calculated using Kramers-Heisenberg equation

$\displaystyle I_{qq^{\prime}}(\omega,\omega^{\prime})$ $\displaystyle=$
$\displaystyle\sum_{f}\Bigl{|}\sum_{m}{\langle
f|D_{q^{\prime}}|m\rangle\langle m|D_{q}|g\rangle\over
E_{g}+{\omega^{\prime}}-E_{m}-i\Gamma_{m}/2}\Bigr{|}^{2}$ (1)
$\displaystyle\times\delta(E_{g}+{\omega^{\prime}}-E_{f}-{\omega}),$

where, $|g\rangle$, $|m\rangle$, and $|f\rangle$ are the ground,intermediate,
and final states with energies $E_{g}$, $E_{m}$, and $E_{f}$, respectively,
while $\omega^{\prime}$ and $\omega$ represent energies of incident and
scattered photons, respectively. $D_{q}$ is the dipole operator, $\Gamma$
stands for the intermediate state lifetime (Lorentzian FWHM) and $q$ and
$q^{\prime}$ are polarizations of the light with respect to the quantization
axis. The experimental geometry for RIXS measurements was taken into account
in the calculations as described in Ref. [44].

The required Slater integrals $F^{k}$, $G^{k}$ and $R^{k}$, spin-orbit
coupling constants $\zeta$ and matrix elements were obtained with the TT-
MULTIPLETS package which combines Cowan’s atomic multiplet program [45] (based
on the Hartree-Fock method with relativistic corrections) and Butler’s point-
group program,[46] which were modified by Thole [47], as well as the charge-
transfer program written by Thole and Ogasawara.

##  IV Results and discussion

The recorded Am $O_{4,5}$ XAS spectrum of Am2O3 is displayed in Fig. 1. For
Am2O3, the autoinization processes can be characterized by three decay
channels following the $5d^{10}5f^{6}\rightarrow 5d^{9}5f^{7}$ excitation. The
$5d\rightarrow 5f$ absorption and $5d\rightarrow\varepsilon{f}$ ionization are
competing routes coupled to each other by the $\langle
5d^{9}5f^{7}|1/r|5d^{9}5f^{6}\varepsilon{f}\rangle$ configuration interaction
(CI). The decay of the excited $5d^{9}5f^{7}$ states via $\langle
5d^{9}5f^{7}|1/r|5d^{9}5f^{6}\varepsilon{f}\rangle$ can be called the
$5f\rightarrow\varepsilon{f}$ tunneling channel [42, 48]. The $5d^{9}5f^{7}$
core excitations can also decay via $5d-5f5(s,p)$ Coster-Kronig and $5d-5f5f$
super-Coster-Kronig channels so that the excited states are coupled to the
$5(s,p)^{-1}$ and $5f^{-1}$ ionization continua by the $\langle
5d^{9}5f^{7}|1/r|5d^{10}5f^{6}5(s,p)^{-1}\varepsilon{\emph{l}}\rangle$ and
$\langle 5d^{9}5f^{7}|1/r|5d^{10}5f^{5}\varepsilon{\emph{l}}\rangle$ CI
processes, respectively. The broadening of the $5f$ edge which is highly
inhomogeneous can be evaluated by calculating these CI matrix elements.

Figure 1: Expeimental and calculated XAS spectra at Am $O_{4,5}$ edges of
Am2O3. The spectra are calculated using atomic and crystal-field multiplet
theory for the Am3+ ion and Anderson impurity model, respectively.

Fig. 1 shows the calculated Am $O_{4,5}$ XAS spectra of Am2O3 compared with
experimental one. First, the XAS spectrum was progressively calculated using
atomic and crystal-field multiplet theory for the ($5d^{10}5f^{6}\rightarrow
5d^{9}5f^{7}$) excitation of the Am(III) ion. Only the super-Coster-Kronig
decay channel ($5d^{10}5f^{6}\rightarrow 5d^{9}5f^{7}\rightarrow
5d^{9}5f^{5}\varepsilon{\emph{l}}$) was taken into account in the calculations
as a dominating process [42, 48].

It has been established (see e.g. [49, 50, 51]) that the Slater integrals
describing the $f$-$f$ interaction as well as the interaction of the $f$
electrons with core holes in shallow levels such as $4d$ for lanthanides and
$5d$ for actinides need to be scaled down from their _ab-initio_ Hartree-Fock
values for a description of the XAS spectra at those levels. In our
calculations Slater integrals $F^{k}(5f,5f)$, $F^{k}(5d,5f)$, $G^{k}(5f,5f)$
and $R^{k}(5d5f,5d\varepsilon{\emph{l}})$ were reduced to 75%, 75%, 65% and
80%, respectively (see Table 1). The actual reduction values were also a
result of the optimization of the agreement between calculated and measured Am
$5d$-$5f$ RIXS spectra of Am2O3 (see below). Furthermore, the value of the
spin-orbit coupling constant for the $5f$ shell was reduced in the
calculations to 93% from the Hartree-Fock value for the same reason.

Table 1: The ab-initio Hartree-Fock values of Slater integrals and spin-orbit coupling constants for Am(III) ion. In XAS and RIXS calculations these values were reduced as described in the text. $F$, $G$ and $\zeta$ are given in eV, $R$ in $\sqrt{eV}$. Am(III) | Value
---|---
$F^{2}(5f,5f)$ | 10.086
$F^{4}(5f,5f)$ | 6.577
$F^{6}(5f,5f)$ | 4.823
$\zeta(5f)$ | 0.345
$F^{2}(5f,5f)$ | 10.270
$F^{4}(5f,5f)$ | 6.707
$F^{6}(5f,5f)$ | 4.923
$\zeta(5f)$ | 0.359
$F^{2}(5d,5f)$ | 11.434
$F^{4}(5d,5f)$ | 7.377
$G^{1}(5d,5f)$ | 13.599
$G^{3}(5d,5f)$ | 8.412
$G^{5}(5d,5f)$ | 6.005
$\zeta(5d)$ | 4.002
$R^{1}(5d\varepsilon{g},5f^{2})$ | 1.208
$R^{3}(5d\varepsilon{g},5f^{2})$ | 0.761
$R^{5}(5d\varepsilon{g},5f^{2})$ | 0.540

In the crystal-field multiplet calculations, Wybourne’s crystal field
parameters were set to $B^{4}_{0}$=-0.835 eV and $B^{6}_{0}$=0.100 eV based on
the estimates for the case of Am(III) in ThO2 [52]. In addition, the direct
interatomic exchange and superexchange, treated as a magnetic field along the
z-axis and acting on the spin $S$, were set to 0.001 eV to lift the degeneracy
of the states.

Furthermore, the Am $O_{4,5}$ XAS spectrum of Am2O3 was calculated using the
AIM approach (which also included the full multiplet structure) as shown in
Fig. 1. The hybridization of Am $5f$ states with oxygen $2p$ states was taken
into account in the AIM calculations for the Am(III) system. The same AIM
parameter values were applied as in Ref. [25] where the Am $N_{4,5}$ XAS and
$4f$ x-ray photoelectron spectra of Am2O3 were calculated. The ground state
was described as a mixture of the $5f^{6}$ and
$5f^{7}\underline{\upsilon}^{1}$ configurations with Am $5f$-O $2p$ charge
transfer energy $\Delta$, $5f$-$5f$ Coulomb interaction $U_{ff}$, core-hole
potential $U_{fd}$ acting on the $5f$ electrons and Am $5f$-O $2p$
hybridization strength $V$ were set to 6.5, 5.7, 6.0, and 0.7 eV,
respectively.

An inspection of Fig. 1 reveals that the calculated XAS spectra are rather
similar for all the three cases, i.e. taking into account the crystal-field
interaction and Am $5f$-O $2p$ charge transfer does not lead to significant
differences between calculated XAS spectra. This is because the ground state
of Am2O3 is singlet $\Gamma_{1}$ (notations are for $C_{4h}$ symmetry since
the finite exchange field is applied along $z$-axis) and because the $O_{4,5}$
XAS spectrum is significantly broadened due to the autoionization processes.
However, such a large core-hole lifetime broadening at the $5d$ edge does not
affect the resolution of the $5d$-$5f$ RIXS spectra which mainly depends on
the instrumental resolution in this case.

Figure 2: Calculated Am $5d$-$5f$ RIXS map of Am2O3 using the crystal-field
multiplet approach.

To make a comparison with experimental RIXS data, we calculated the Am
$5d$-$5f$ RIXS map of Am2O3 for incident photon energies varying throughout
the Am $O_{4,5}$ edges as shown in Fig. 2. For simplicity, the crystal-field
multiplet approach was used since the contribution of the Am $5f$-O $2p$
charge transfer excitations to the RIXS spectra was expected to be small. The
calculated RIXS intensities are displayed using the energy-loss scale in Fig.
2 and represent $5f$-$5f$ excitations. One can see that an elastic peak (at
zero eV energy loss) is missing in the calculated spectra because the
$5d$-$5f$ RIXS map in Fig. 2 was obtained for the scattering geometry used in
our RIXS experiment. For the singlet $\Gamma_{1}$ ground state, the
$\Gamma_{1}\rightarrow 5d^{9}5f^{7}\rightarrow\Gamma_{1}$ transitions are
forbidden for the scattering geometry employed. In this geometry, mainly
$\Gamma_{3}$ and $\Gamma_{4}$ manifold states are probed as the final states
of the RIXS process. The first scattering structure which gains a non-zero
intensity appears at an energy loss of 360 meV and is the most intense on the
map.

Figure 3: Experimental and calculated Am $5d$-$5f$ RIXS spectra of Am2O3 for
several incident photon energies at Am $O_{4,5}$ edges.

The experimental Am $5d$-$5f$ RIXS spectra of Am2O3 recorded with the 400
lines/mm grating at several incident photon energies at the Am $O_{4,5}$ edges
are compared with the calculated results in Fig. 3. All the experimental
spectra have huge elastic peaks as a result of the reflectivity contribution
in this energy range, nevertheless the first RIXS structure at $\sim$360 meV
is still resolved in the recorded data. The calculated energies of the $f$-$f$
(crystal-field) excitations and relative intensities of the calculated RIXS
structures are in good correspondence with the measured ones (except for RIXS
structures on the slope of the elastic peak). We avoided a subtraction of the
elastic peak from experimental spectra because, for the correct procedure,
elastic scattering needs to be measured from the surface with exactly the same
(smooth) profile but without Am2O3 which is difficult to achieve.

An inspection of Fig. 3 shows the RIXS structures are not significantly
resonating with varying incident photon energies as a result of the very short
core-hole lifetime in the intermediate state of the RIXS process. Some
dependence on the excitation energy can be seen for the structure at an energy
loss of about 3.3 eV. As a whole, in contrast to the discussion on the dual
character of $5f$ states in some actinide systems, a good agreement between
calculated results and experimental data in our case indicates a significant
localization of the $5f$ states in Am2O3.

Figure 4: Measured and calculated Am $5d$-$5f$ RIXS spectra of Am2O3 at an
incident photon energy of 118.0 eV together with calculated energy diagram for
$\Gamma_{1}$, $\Gamma_{3}$, $\Gamma_{4}$ and $\Gamma_{2}$ states of the
$5f^{6}$ ground state configuration.

The first RIXS structure at the energy loss of $\sim$360 meV corresponds to
transitions to the $\Gamma_{4}$ and $\Gamma_{3}$ states which are split by
0.001 eV. This structure is characteristic for the Am(III) chemical state (see
Fig. 4) and an existence of this structure in the RIXS spectra can be used to
distinguish between Am(III) and Am(IV) compounds. Fig. 4 shows experimental
and calculated Am $5d$-$5f$ RIXS spectra of Am2O3 at the incident photon
energy of 118.0 eV together with the calculated energy diagram for the
$\Gamma_{1}$, $\Gamma_{3}$, $\Gamma_{4}$ and $\Gamma_{2}$ states of the
$5f^{6}$ ground state configuration. Such a comparison of spectra with the
energy diagram indicates the influence of the chosen scattering geometry on
which states of the ground state configuration can be probed as a result of
the RIXS process.

For RIXS spectra displayed in Fig. 3 which were recorded with the 400 lines/mm
grating, the intensity is significantly cut-off for energy losses of around 5
eV and higher due to the use of the round detector (for example, a full cut-
off is observed for a spectrum recorded at the incident energy of 112.0 eV).
Therefore, the 300 lines/mm grating was used which provided the opportunity to
measure the Am $5d$-$5f$ RIXS spectra for a more extended energy range but
with lower resolution. Fig. 5 displays such spectra of Am2O3 for a number of
the excitation energies on the incident photon energy scale. The excitation
energies used can be identified by present huge elastic peaks. The
$\sim$360-meV-energy-loss RIXS structure is not resolved but the resonating
behavior of the $\sim$3.3-eV-energy-loss RIXS structure appears to be more
distinct.

Figure 5: Am $5d$-$5f$ RIXS spectra of Am2O3 for several incident photon
energies at Am $O_{4,5}$ edges measured with lower resolution but for a more
extended energy range.

In RIXS spectra recorded at excitation energies between 113.5 and 117.0 eV, a
new structure at $\sim$5.5 eV below the elastic peak can be identified which
behavior upon changing excitation energy is indicated by a dashed line. This
structure follows the increasing excitation energy (follows the elastic peak)
and can be assigned to RIXS structures. Since the crystal-field multiplet
calculations do not suggest the existence of the significant RIXS structures
at the energy losses higher than 4.6 eV, the new structure can be attributed
to the result of the Am $5f$-O $2p$ charge transfer (charge transfer
satellite). For the description of the ground state as a mixture of the
$5f^{6}$ and $5f^{7}\underline{\upsilon}^{1}$ configurations, the RIXS
transitions to the anti-bonding combination of the $\Gamma_{1}$ character is
also forbidden in the scattering geometry employed, while some transitions to
the weakly bonded states of the $5f^{7}\underline{\upsilon}^{1}$ configuration
are allowed (see also discussion in Ref. [44] related to RIXS spectra of
CeO2). These latter transitions are a result of an involvement of the O $2p$
states which are rather close to the valence band maximum. The observed 5.5-eV
energy loss of the charge-transfer satellite is consistent with the choice of
the value of Am $5f$-O $2p$ charge-transfer energy $\Delta$=6.5 eV as a
parameter in our AIM calculations (see also Ref. [25]), because the $\Delta$
value is related to the center of the O $2p$ band.

##  V Conclusions

The obtained Am $5d$-$5f$ RIXS data helps characterize the electronic
structure of Am2O3. The observed agreement between recorded RIXS spectra and
calculated results using the crystal-field multiplet approach indicates the
significantly localized character of the Am $5f$ states in this oxide. The
analysis of the RIXS structures by comparing the experimental and calculated
data confirms that the ground state of Am2O3 is singlet $\Gamma_{1}$ of the
$5f^{6}$ configuration. The experimental scattering geometry used, which leads
to restrictions for specific RIXS transitions, made the analysis easier,
despite the presence of the intense elastic peak due to high reflectivity
contribution. The observation of the weak charge-transfer satellite in the Am
$5d$-$5f$ RIXS spectra at the energy loss of $\sim$5.5 eV is consistent with
Am $5f$-O $2p$ charge-transfer energy $\Delta$=6.5 eV used in the AIM
calculations of XAS and x-ray photoemission spectra of Am2O3 (Ref. [25]),
estimating the $5f$ occupancy to be 6.05 electrons in the ground state.

##  VI Acknowledgments

S.M.B acknowledges the support from the Swedish Research Council (research
grant 2017-06465). The computations and data handling were enabled by
resources provided by the Swedish National Infrastructure for Computing (SNIC)
at National Supercomputer Centre at Linköping University partially funded by
the Swedish Research Council through grant agreement no. 2018-05973.

This work was supported in part by the Director, Office of Science, Office of
Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and
Biosciences Heavy Elements Chemistry program (DKS) of the U.S. Department of
Energy at Lawrence Berkeley National Laboratory under Contract No. DE-
AC02-05CH11231. This research used resources of the Advanced Light Source, a
U.S. DOE Office of Science User Facility under contract No. DE-AC02-05CH11231.
The Am-243 used in this work was supplied by the U.S. DOE through the
transplutonium element production facilities at ORNL.

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