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Resonant Inelastic Soft X Ray Scattering From Dyf3

Resonant Inelastic Soft-X-ray-Scattering from DyF3
S. M. Butorin,1 J.-H. Guo,1 D. K. Shuh,2 and J. Nordgren1
1Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden
2Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
INTRODUCTION
For highly correlated materials, such as rare-earth compounds, studies of low-energy electronic
excitations (intra-ionic f-f excitations in this case) can provide a better understanding of ground
state properties. In the case of weak hybridization effects, the interlevel coupling and consequently
J-mixing in the ground state of the system are often disregarded in the interpretation of
experimental data by applying a pure atomic approximation (mainly for high-energy
spectroscopies) or by using a first order crystal-field theory where the crystal field interaction is
assumed to act only within the separate J manifolds. This is partly due to complications in
extracting information about the ground state J-mixing directly from the data. For example, the
estimation of the J-mixing degree in high-order crystal-field theory by adjusting the crystal-field
parameters from the fit of optical absorption or low-energy electron-energy-loss spectra [1,2] may
result in a large uncertainty originating from difficulties calculating the intensities of dipole-
forbidden transitions. In turn, the possible influence of weak metal-ligand hybridization is difficult
to analyze quantitatively in the absence of so-called charge-transfer satellites in high-energy
spectroscopic data.
In this situation, the use of alternate spectroscopic means to obtain J-mixing information is
essential. Recently, Finazzi et al. [4] have shown that the ground-state J-mixing can be studied by
taking advantage of dichroic properties of rare-earth 3d x-ray absorption. However, the method is
restricted to magnetically ordered systems. In this report we discuss the potential of resonant
valence-to-core x-ray fluorescence spectroscopy (RXFS) to detect the ground-state J-mixing when
applied to compounds without distinct long-range magnetic order and significant metal-ligand
hybridization.
Similar to optical absorption and electron-energy-loss spectroscopies with respect to probing the
low-energy excitations in electron-correlated materials, RXFS at the same time provides the higher
level of the transition selectivity due to the element specificity and dipole selection rules. In contrast
to systems with the strong metal-ligand hybridization where the charge-transfer process leads to an
appearance of additional intense lines in resonant x-ray fluorescence spectra [5] as a result of inter-
ionic excitations, J-mixing in systems with weak hybridization effects is expected to manifest itself
in an intensity gain of some intra-ionic (f-f) transitions which are disallowed for the pure Hund-
rule ground state. In other words, transitions with ∆J other than 0, ±1, and ±2 are probed in the
resonant excitation-deexcitation process (speaking correctly J is not a good quantum number in this
case).
EXPERIMENTAL DETAILS
DyF3 (99.99%) was congruently evaporated from a water-cooled effusion source in a simple
preparation chamber utilizing a graphite crucible evaporator. The DyF3 film (28 Ã…) was grown on
ambient high purity platinum foil substrate at 4 x 10-9 Torr. The evaporation rate was calibrated by
a quartz crystal monitor and the film thickness is estimated to be within about 10%. The film was
stored under a nitrogen atmosphere and then loaded into the experimental chamber equipped with a
l
m
DyF3
k
c
g
i
h
j
d e f
a
b
x0.8
x0.8
Normalized intensity
x0.7
x0.7
x0.6
145
150
155
160
165
Photon energy [eV]
Figure 1. Total electron yield spectrum at the Dy 4d edge and resonant Dy 4f → 4d x-ray fluorescence spectra of
DyF3 normalized to the incoming photon flux. The letters correspond to the excitation energies indicated in the
absorption spectrum.
fluorescence spectrometer. The sample was mounted with its surface normal located in the
horizontal scattering plane.
The experiment was carried out at beamline 7.0 of the Advanced Light Source, Lawrence Berkeley
National Laboratory with a spherical grating monochromator. The Dy 4f → 4d x-ray fluorescence
spectra of DyF3 were recorded using a grazing-incidence grating spectrometer [6] with a two-
dimensional detector. The spectrometer resolution was set to 95 meV at 152 eV. The incidence
angle of the photon beam was about 2Ëš from the sample surface and the spectrometer was placed in
the horizontal plane at an angle of 90Ëš with respect to the incidence beam. To determine the
excitation energies, the Dy 4d x-ray absorption spectrum of DyF3 was obtained by measuring total
electron yield at the 90Ëš incidence angle of the incoming radiation. During x-ray absorption and
fluorescence measurements, the resolution of the monochromator was set to 76 meV at a photon
energy of 152 eV. All of the spectra were recorded at room temperature.
RESULTS AND DISCUSSION
The resonant Dy 4f → 4d x-ray fluorescence spectra of DyF3 (Fig. 1) show a dispersion-like
behavior upon tuning the excitation energy across the Dy 4d absorption edge. The spectra recorded
at the excitation energies labeled by a, b, and c appear as a single peak with other low-energy
structures being very weak. Further increase in the excitation energy gives rise to an enhancement
of these weak structures so that the appreciable spectral weight is observed within the 8.5 eV range
below the elastic line, for example, in spectra i and j. All of the low-energy peaks follow varying
excitation energies and therefore can be associated with resonant inelastic x-ray scattering. It is
rather unlikely that the spectral weight on the low-energy flank of the elastic line originates from
phonon relaxation because this weight consists of distinct structures instead of a continuous
structureless band and the structures show different dependence on the excitation energy.
There are two distinct groups of pronounced inelastic-scattering peaks in Fig. 1. The first group is
distinguished by small energy losses on the tail of the elastic line, whereas the second is
characterized by energy losses more than 2.5 eV. When the excitation energy approaches the main
broad maximum of the Dy 4d absorption edge, the first group still possesses significant intensity
while the structures of the second group become relatively faint. Regarding the energy scale on
which the spectral variations occur, the observed fluorescent transitions can be attributed to intra-
ionic f-f excitations. The energy gap between two groups of inelastic x-ray scattering structures
reflects the separation between sextuplets and quadruplets of trivalent Dy [7,8] which can be
reached due to the excitation-deexcitation process.
The results of preliminary atomic-multiplet calculations for the Dy3+ ion show that the dominant
elastic peak in all of the Dy 4f → 4d spectra from DyF3 is to large extent a consequence of strong
interference effects in the intermediate state of the coherent second-order optical process. The states
constituting the main 4d absorption edge have a lifetime broadening of about 2 eV largely because
of the 4d-4f4f Coster-Kronig decay.
A close inspection of experimental 4f → 4d spectra shows that there are some spectral structures
which are not revealed in calculations within the pure atomic approximation. Thus, the feature with
the energy loss of about 1.15 eV is observed in spectra h, k, and l, presented in detail by Fig. 2.
While atomic multiplet theory predicts the non-zero intensities for resonant inelastic x-ray scattering
transitions to the 6H13/2, 6H11/2, and 6F11/2 sextuplets of the 4f9 configuration, the energy of the
extra-feature in experimental spectra h, k, and l (Fig. 2) is close to those of 6F9/2 and 6H7/2
manifolds of Dy3+ in LaF3 [9]. This is an indication of J-mixing and the presence of J = 13/2 and
l
k
j
i
Intensity [a. u.]
h
g
f
-8
-6
-4
-2
0
2
Energy [eV]

Figure 2. Enlarged inelastic x-ray scattering part of the resonant Dy 4f → 4d spectra from DyF3.
J =11/2 components in the ground state of DyF3. Indications of other extra-structures missing
from atomic calculations can be seen in the energy range between -2.0 and -1.2 eV, as in spectra l
and m. However, the present level of statistics does not permit identification.
REFERENCES
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A. Kotani and H. Ogasawara, J. Electron Spectrosc. 60, 257 (1992).
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M. Finazzi et al., Phys. Rev. Lett. 75, 4654 (1995).
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S. M. Butorin et al., Phys. Rev. Lett. 77, 574 (1996).
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J. Nordgren et al., Rev. Sci. Instrum. 60, 1690 (1989).
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D. van der Marel and G. Sawatzky, Phys. Rev. B 31, 1936 (1985).
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W. T. Carnall et al., J. Chem. Phys. 49, 4424 (1968).
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J. L. Fry et al., J. Chem. Phys. 48, 2342 (1968).
This work was supported by the Swedish Natural Science Research Council and by the Director, Office of Energy
Research, Office of Basic Energy Sciences, Materials Science and Chemical Sciences Division, of the U.S.
Department of Energy under Contract No. DE-AC03-76SF00098.
Principal investigator: E. Joseph Nordgren, Physics Department of Uppsala University, Sweden. E-mail:
joseph.@fysik.uu.se