Multi-Atom Resonant Photoemission
^Charles S. Fadley, fElke Arenholz, *1Alex W. Kay,
t#
Javier Garcia de Abajo, ^Bongjin S. Mun, ^ee-Hun Yang,
*Zahid Hussain, and ^ichel Van Hove
*Department of Physics, University of California, Davis, Davis, CA 95616 USA
t
Materials Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, CA 94720 USA
* Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA
^Permanent address: Departamento de CCIA, Centro Misto CSIC-UPV/EHU,
San Sebastian, Spain
Abstract We report here on the first measurements and theoretical considerations of an
interatomic multi-atom resonant photoemission (MARPE) effect that can enhance
photoelectron intensities by as much as 100% and appears to be generally observable in solid
materials. MARPE occurs when the photon energy is tuned to a core-level absorption edge of
an atom neighboring the atom from which the photoelectron is being emitted, with the emitting
level having a lower binding energy than the resonant level. Large peak intensity
enhancements of 30-100% and energy-integrated effects of 10-30% have been seen by our
group in various metal oxides and in a metallic system, as well as by other groups now in
metal halides and an adsorbate system. The effect has also been observed in solids via the
secondary decay processes of Auger emission and fluorescent x-ray emission. Weaker effects
also appear to be present in gas-phase electron emission experiments. The range of the effect
is so far estimated from both experiment and theory to be about 2-3 nm, with further work
needed on this aspect. MARPE should thus provide a new and broadly applicable
spectroscopic probe of matter in which the atomic identities and other properties (e.g. magnetic
order) of atoms neighboring a given atomic type should be directly derivable. Such
interatomic resonance effects also may influence normal x-ray absorption experiments, and in
some cases, they may require a consideration of the degree of x-ray beam coherence for their
quantitative analysis.
INTRODUCTION
In this paper, we discuss a fundamentally new type of resonant photoemission or
photoexcitation process that has been termed multi-atom resonant photoemission or
MARPE [1]. In this interatomic effect, a photoelectron is ejected from a given core
level of a first atom "A" and the photon energy is tuned through an absorption
threshold for a core level on a neighboring atom "B". Although at first sight such an
interatomic resonant photoexcitation process might be thought to be rather weak, and
to depend critically on the overlap of electronic orbitals on the two atoms involved,
neither of these things is true: the photoemission from A can be enhanced by as
much as 100% (i.e. by a factor of two) and a first theoretical examination of the
effect shows that orbital overlap between A and B is not required for a B atom to
CP506, X-Ray and Inner-Shell Processes, edited by R. W. Dunford, et al.
© 2000 American Institute of Physics 1-56396-713-8/00/$17.00
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contribute to excitation from A [1,2]. In fact, the effect extends well beyond nearestneighbor contributions, having a currently estimated range of 2-3 nanometers [2,3].
MARPE is thus quite different from the well-known intraatomic single-atom
resonant photoemission, which we will denote as SARPE for clarity, a phenomenon
which has been studied and used for many years to enhance photoemission
intensities by an order of magnitude or more, and to study interesting many-electron
aspects of the photoexcitation process [4,5]. A classic set of SARPE results due to
Krause and co-workers [5] is shown in Fig. 1. Here, as shown in the energy-level
diagram of Fig. l(a), the excitation of a photoelectron from Mn 3d in a gas-phase Mn
atom is followed as the photon energy is increased and reaches -50 eV, an energy
that is just sufficient to excite a deeper-lying Mn 3p electron in the same atom up to
the first dipole-allowed bound excited state, which will also be of Mn 3d character.
When this occurs, the very strong resonant Mn 3p-to-Mn 3d excitation can be
considered to decay immediately so as to produce a free electron at the same energy
as the photoelectron directly excited from Mn 3d at the same photon energy, as
shown by the dashed arrows in Fig. l(a). This resonant process, which is formally
related to Auger electron emission, but more properly termed autoionization [4], is
simultaneous with and coherent with the usual direct excitation of the photoelectron,
and it can lead to significant increases and decreases in intensity, depending on the
relative phases of the direct and resonant channels. The variation of the intensity of
the Mn 3d photoelectrons has been measured as photon energy passes over this
particular resonance region and the resulting data [5a] are shown in Figs. l(b)~a
typical spectrum, and l(c)~the energy dependence of the Mn 3d intensity. Note the
change in sign of the resonance effect as photon energy varies due to a change in
phase between the direct and resonant channels, with the resonance curve generally
following a Fano profile [4]. Simple one-electron Hartree-Fock (HF) theory is
unable to predict this phenomenon, whereas many-body perturbation theory (MBPT)
is [5b]. The peak intensity is increased by a maximum of a factor of about 7 for this
case. As another overall measure of the strength of the resonance, the positive and
negative effects of the resonance (darker grey-shaded areas) can be integrated over
the full energy range over which the effect causes significant differences in intensity
from the simple non-resonant Hartree-Fock (HF) theory [5b] and compared to the
estimated non-resonant intensity (lighter grey-shaded areas); this leads to an overall
effect of about 63%.
The core-level interatomic effect to be discussed in this paper is closely related to,
but qualitatively distinct from, an intermediate type of interatomic resonant
photoemission in which excitation of a valence electronic level of a system that is
primarily associated with one atom (e.g. a valence level on one side of an interface
[6,7] or a lone-pair molecular orbital primarily localized on one atom [8]) is
enhanced as the photon energy passes through a core absorption threshold of another
atom in the system (an atom on the other side of an interface or another atom in the
same molecule, respectively). In what follows, we will focus on measurements on
solid samples in which two well-localized core levels on A and B are involved,
although other measurements of the intermediate type are certainly of interest for the
future. Beyond observations of such core-level MARPE by our group and its
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400
(b) Atomic Mn
hv = 50.8 eV
ON resonance
300
«S 200
photoelectron
"
100
30
25
20
10
15
binding energy (eV)
8r
7
experiment
theory: MBPT
theory: HF
(c) Atomic Mn
6
Mn3p
5
Mn atom
4
3
non-resonant:
100%
2
1
10
20
30 40 50
60
70
80
90 100 110
photon energy (eV)
FIGURE 1 Single-atom resonant photoemission (SARPE) for the case of Mn 3d emission from
atomic Mn [5]: (a) The energy level diagram, with the resonance occurring via the Mn3p-to-Mn3d
excitation. The direct and resonant excitations are indicated by the solid arrows, and the dashed
arrows indicate the autoionization decay via the Coulomb interaction, (b) A photoelectron spectrum
covering the Mn 3d region, with the photon energy set on resonance [5a]. (c) The measured variation
of the Mn 3d intensity with photon energy, together with two types of theoretical calculation: HartreeFock = HF and many-body perturbation theory = MBPT [5b]. The region shaded dark grey represents
the effect of the resonance; the overlapping region shaded light grey underneath the smooth HF curve
represents the estimated non-resonant intensity, with relative areas as integrated over energy indicated
in percent.
collaborators that will be discussed below [1,3,9], the effect has also by now been
seen by others in other solid compounds [10], in an adsorbate sitting on a metallic
substrate [11], and with reduced amplitude and more indirectly in photoemission
from gas phase molecules [12].
Beyond representing an interesting new aspect of the x-ray absorption process,
MARPE appears to constitute a new probe of matter in which emission/excitation of
one atom A can be directly used to sense the presence of other atoms B near to it, at
least on the nanometer scale that is commonly discussed in many current materials
science developments. Adding magnetic sensitivity via magnetic circular or linear
dichroism (as will be discussed below) should also permit selectively monitoring the
magnetic order of those atoms B that are near to A. This projected capability can be
compared to several current characterization techniques based on x-rays. Several
methods presently permit determining the bulk atomic structures of solids, including
x-ray diffraction and extended x-ray absorption fine structure (EXAFS) [13].
EXAFS is also element-specific via core-level electronic excitations, allowing the
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local structure around each atomic type to be determined, at least as to the radial
positions of shells of neighboring atoms. If the atomic structure near solid surfaces
is to be probed, one can add to this list low energy electron diffraction (LEED) [14],
and photoelectron diffraction (PD) [15], with the latter also being element-specific
via core excitation. However, as powerful and widely used as the foregoing methods
are, none of them permits directly determining the type of atom that neighbors a
given atom. That is, some of these techniques (e.g. EXAFS and PD) may be
element-specific for the central atom in the structure, but there is no simple way to
determine the near-neighbor atomic identities (atomic numbers) from them. In some
cases, use can be made of the differences in electron-atom scattering strengths
between different atoms in these last two methods, and then comparing experiment
with model calculations, but this is only unambiguous when atomic numbers are
relatively far apart, and even then, this procedure often provides only a semiquantitative distinction of atomic identity. From this brief overview, it thus appears
that MARPE should add a significant new aspect to the array of x-ray based methods
for characterizing matter. More broadly, the fact that all atoms beyond He have a
core level that can be used as one side of a MARPE experiment indicates that it
could have broader applicability across the periodic table than two other powerful
spectroscopic probes, Mossbauer spectroscopy and nuclear magnetic resonance,
which require the use of certain nuclei or nuclear pairs.
We now turn to a more detailed consideration of the experimental aspects of
measuring such multi-atom resonant photoemission effects, provide some examples
of data obtained to date, and summarize the systems measured to date. An outline of
the theory being developed to describe the effect [2] will also be given, and its
relationship to other phenomena such as the Forster effect [16-19] and normal x-ray
absorption spectroscopy will also be discussed. Finally, we discuss some
implications, applications, and future perspectives.
EXPERIMENTAL METHODOLOGY AND RESULTS
Observation via Photoelectrons
The most direct method of detecting MARPE is simply by measuring the intensity
of photoelectrons emitted in a given peak, with our first example being O Is
emission from MnO, as the photon energy is scanned through the Mn 2p absorption
thresholds. The measurement thus requires scanning photon energy continuously
over such thresholds, and a careful determination of the elastic or no-loss peak
intensities above a suitably subtracted inelastic background. A variable-energy
synchrotron radiation source is thus required. For the inelastic background, we have
chosen to use the iteratively-derived Shirley background [20], but other forms could
be used as well. All intensities were also normalized to allow for the time-dependent
decay of the electron current in the Advanced Light Source storage ring at which all
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measurements were performed; the time between fills during these measurements
was 4-5 hours, with current dropping to about 40-50% of its initial value just before
a refill.
Our photoemission experiments (and Auger experiments to be discussed below)
have been carried out at bend-magnet beamline 9.3.2 of the Advanced Light Source
(ALS) in Berkeley [21], in particular using the Advanced Photoelectron
Spectrometer/Diffractometer situated there [15,22]. This beamline permits scanning
photon energy continuously over the energy range from 30 to 900 eV, thus spanning
the resonant energies of interest here. The experimental geometry is shown in Fig.
2(a); the light was linearly polarized, with polarization vector 8 lying in the plane of
the figure. The higher brightness of the ALS (a third-generation synchrotron
radiation source), coupled with the higher transmission and resolution of the
rotateable Scienta ES200 electron spectrometer used to measure photoelectron
spectra [15,22], permits determining photoelectron intensities rapidly enough to
study any resonant effects observed as a function of the key experimental parameters
of photon energy hv, photon incidence angle (6hv), and photoelectron emission
direction (0,(|)). The ability to rotate the electron spectrometer over approximately
60° in the plane of the storage ring, together with the polar (0) rotation of the sample
goniometer, also permits varying 6 and 0hv independently, a feature that is important
for distinguishing resonant effects from more simple effects due to the onset of
significant x-ray reflection and refraction effects at the surface (as discussed further
below).
Manganese oxide-MnO
As a first example of experimental photoemission results, we show in Figs. 2(b)(e) various aspects of the interatomic enhancement of O Is emission from MnO, as
photon energy crosses the Mn 2p thresholds. The O Is level is a deep-lying core
electron with approximately 530 eV binding energy, and the deeper-lying Mn 2p3/2
and 2p1/2 thresholds (i.e. peaks in the x-ray absorption curve) occur at approximately
640 and 652 eV, respectively [23,24]. The photoelectron kinetic energy is thus in the
range of 110-120 eV in crossing these thresholds. The autoionization decay that
would occur for the Mn 2p3/2 resonance is indicated by the dashed arrows in Fig.
2(b): it involves the simultaneous deexcitation of Mn 3d to Mn 2p3/2 and excitation
of O Is to a free-electron state at the photoelectron energy. Fig. 2(c) further
illustrates that the peak intensity has to be measured carefully via inelastic
background subtraction, since the background increases substantially as the photon
energy goes above the 2p thresholds, primarily due to inelastic scattering of Auger
electrons that are produced in the decay of the Mn 2p3/2^3d excitations via
secondary Auger processes that are different from the resonant autoionization
channel. We have in fact used the photon-energy dependence of this inelastic
background intensity to measure the x-ray absorption coefficient of MnO in our
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(b)
Electron spectrometer
(Scienta ES 200)
(a)
direct
channel
rotatable, 60°
[001]
_ _ _ _ photoelectron
resonant
channel
..... - o - - M n 3 d
~
Coulomb
interaction
„
hv
9
v
independently
Mn 2p3/,
Mn 2p1/;
variable
O atom
ON
1.6 (c) Solid MnO
I
1.4 O1 s spectra
^ 1.2 ehv
Mn atom
1.4
= 20°, e = 90'
ehv = 20°, e = 90°
-----e h v = 40°, e = 90°
I ON
= 0.8
I 0.6
MnO:
resonant O1s peak area
1.2
0.4
534
532
530
528
X 1-0
binding energy (eV)
H-
(f)
1.4
1.2
i
MnO: x ray absorption
coefficient
——— This work
1.0
(d) Solid MnO
-11% resonant
enhancement
ehv = 20°, e = 90°
.2
o
£
—---- Butorin et al.
0.5
Mn2p3/2
Mn2p1/2
0.0
640
645
650
635
photon energy (eV)
640
645
650
655
photon energy (eV)
FIGURE 2 Multi-atom resonant photoemission (MARPE) for the case of O Is emission from
MnO(OOl) [1]: (a) The experimental geometry, with various angles defined, (b) The energy level
diagram, as in Fig. l(a), but with the resonance occurring via the Mn2p-to-Mn3d excitation, (c) O Is
spectra on and off resonance, showing the energy-dependent inelastic background that can be used to
measure the x-ray absorption coefficient, (d) MARPE enhancement of the O Is intensity above
background, with 0hv = 20° and 6 = 90° (normal emission), and the same shadings and percentage
indications as in Fig. l(c). (e) MARPE enhancement of the O Is intensity with the non-resonant
intensity subtracted and set equal to unity, for two different x-ray incidence angles of 6hv = 20° and
40°. (f) The x-ray absorption coefficient of MnO over the Mn 2p region, as measured in this work
(solid curve) and in a previous study [24].
experiment, and this is shown in Fig. 2(f) in direct comparison to an x-ray absorption
spectrum measured in a prior study [24]; there is excellent agreement between the
two curves. The final O Is intensity data for a 20° x-ray incidence angle in Fig. 2(d)
show a clear enhancement of the O Is signal in crossing the Mn 2p3/2 and 2p1/2
256
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absorption peaks: the peak intensity increases by about 42% at the 2p3/2 peak, and the
effect as integrated relative to a smooth background over the 2p3/2 region is about
11%. Thus, although the overall enhancement in intensity is about an order of
magnitude less than seen in the intraatomic case of Fig. 1, it is still pronounced and
easily measurable. Also, the energy-integrated effect is only reduced by a factor of
1/5-1/6 compared to the more dramatic intraatomic case.
In Fig. 2(e), we have also overplotted background-normalized resonance effects
for two different x-ray incidence angles of 0hv = 20° (solid curve) and 40° (dashed
curve), but with the electron exit fixed along the normal at 0 = 90°; this was achieved
by rotating the sample and the electron spectrometer by the same amount with
respect to the light beam between the two measurements. These two curves are
essentially identical, and these results provide crucial evidence that these effects in
MnO are in fact due to multi-atom resonant photoemission, rather than any kind of
variation in the exciting near-surface x-ray flux. For example, an increase in
reflectivity and reduction in x-ray penetration depth due to the strong Mn 2p
absorption could yield higher electric field strengths near the surface [25], thus
resulting in enhanced O Is photoelectron intensities due to their short electron
inelastic attenuation lengths that can be estimated to be in the 5-7 A range [26]. But
such phenomena can be ruled out in the present case, because the intensity
enhancement does not change when 0^v changes and because the exponential x-ray
penetration depths are in any case still expected to be much larger than the electron
escape depths, at approximately 32 A and 61 A for 0hv = 20° and 40°, respectively.
These latter numbers come from a detailed consideration of the MnO absorption
coefficient, including Kramers-Kronig analysis of the experimental data in Fig. 2(f)
to derive the real and imaginary parts of the dielectric constant (8 and (3,
respectively) and a subsequent x-ray optical calculation of the reflection coefficients
and detailed profiles of the squared electric field below the surface, as shown in Fig.
3 [27,28]. The maximum reflectivities of the x-rays occur at the Mn 2p3/2 resonance
and are calculated to be -0.003 and -0.00001 at 0hv = 20° and 40°, respectively, thus
indicating that the electric field strength near the surface cannot change significantly
due to the onset of reflection. The detailed calculations shown in Fig. 3 confirm this,
although we do note that a standing wave of about ±8% in strength is expected at the
Mn 2p3/2 resonance and for 0hv = 20°, roughly what is expected from the square root
of reflectivity, which would give ±5.4%. As a final comment, the electron inelastic
attenuation length Ae cannot change significantly from off-resonance to onresonance, since the overall electronic structure and electronic states involved in
inelastic scattering are not significantly perturbed by the x-rays used in even thirdgeneration measurements [26b]. However, these combined experimental and
theoretical results make it clear that the effects of enhanced reflectivity, refraction,
and reduced x-ray penetration must be allowed for or ruled out by working at high
enough incidence angles if MARPE effects distinct from them are to be clearly
distinguished.
The forms of the MARPE enhancement is also found to be rather closely linked to
the normal x-ray absorption coefficient. In Figs. 2(e), we have normalized the O Is
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Vacuum
= 20°
hv=634 eV
hv=639 eV
hv=640 eV (2p_/2)
----- hv=641 eV
hv=647 eV
hv=651 eV (2p1/2)
hv=634
hv=639
hv=640
hv=641
hv=647
hv=651
hv=660
-400
eV
eV
eV (2p3/2)
eV
eV
eV (2p1/2)
eV
-200 0
200
400
600
z = Depth (Angstrom)
FIGURE 3 Macroscopic x-ray optical calculation of the photon-energy-dependent field intensity IE I2
as a function of depth for MnO [28]: (a) x-ray incidence angle of Ghv = 20°, and (b) x-ray incidence
angleof6 hv = 40°.
data by subtracting off the smooth non-resonant background and setting its value
equal to unity. This permits a more direct comparison with the x-ray absorption
coefficient of MnO in Fig. 2(f), as measured previously [24] and repeated in our
work, as described above. There is a strong degree of similarity between the
normalized MARPE enhancement and the x-ray absorption coefficient that has by
now been observed by our group in several metal oxides [1-3,9], metallic alloys [3],
and metallic bilayers [3].
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As a final comment on our study of MnO, we note that the resonant process can
also be reversed in direction, in particular by choosing to emit photoelectrons from
the multiplet-split Mn 3s levels (-85 eV binding energy), and looking for a
resonance with the O Is levels (with absorption peaks over -530-542 eV). Strong
MARPE effects are also seen here in both members of the spin-split Mn 3s doublet,
with peak intensity enhancements of about 40% and energy-integrated effects of
about 26% [3]. The MARPE curve here is similar to the O Is absorption curve, but
not nearly as close to it as for the case of O Is-Mn 2p case shown in Figs. 2(e) and
2(f).
Other Transition-Metal Oxides
We have also observed MARPE effects in several other inorganic transition-metal
compounds, with all being studied as single crystals or epitaxial films with certain
low-index surfaces exposed in ultrahigh vacuum: Fe2O3(001), NiO(OOl), and
LaQ7Sr03MnO3(001), with the last being a collosal magnetoresistive oxide containing
mixed Mn3+ and Mn^+ oxidation states. Oxygen is bound directly to Fe or Ni or
Mn in all three of these materials. A high degree of surface crystalline order in all
four samples was verified by doing photoelectron diffraction measurements based on
O Is intensities, although this kind of order is not essential for observing the effect.
In each case, we have studied the resonance of O Is with the 2p3/2 and 2p1/2 levels of
the transition-metal atom that is known to be bound directly to the oxygen, as shown
previously for the first case of MnO in Fig. 2(b). In the perovskite-derived lattice of
La07SrQ3MnO3, we have also studied resonances between O Is and La 3d5/2?3/2, and
between Mn 2p and La 3d5/2?3/2. For reference, the binding energies of the levels
involved (relative to the Fermi energy) are: O ls-532 eV, Mn 2p3/24/2--644 eV, 656
eV, Ni 2p3/2>1/2-869 eV, 882 eV , and La 3d5/2j3/2»837 eV, 854 eV. Any level can in
principle resonate with another level at higher binding energy, so some of the pairs
possible here are: O Is with Mn 2p (resonant photoelectron kinetic energy = k.e. ~
110 eV), O Is with Ni 2p (k.e. - 344 eV), O Is with La 3d (k.e. - 302 eV), and Mn
2p with La 3d (k.e.-184 eV).
Figs. 4 and 5 summarize data from two of these oxides, Fe2O3 and
La0?Sr03MnO35 respectively. Several resonances are observed, with the peak
enhancements and energy-integrated effects being indicated directly on each plot.
Several resonant enhancements are shown here: O Is with Fe 2p in Fe2O3, and O Is
with Mn 2p, O Is with La 3d, and Mn 2p with La 3d in LaQ7Sr03MnO3. Our
measured x-ray absorption coefficient for Fe2O3 is shown in Fig. 4(d), and it is in
excellent agreement with a prior measurement at higher resolution [29]. In Fig. 4(b),
the resonant O Is peak area as measured over the Fe 2p3/2?1/2 region and for
259
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500
400
300
200
700
binding energy (eV)
resonant O1 s peak area -
1.6 (b) Fe203
700
705
715
720
photon energy (eV)
725
710
715
720
725
730
710
715
720
725
730
photon energy (eV)
.(d)Fe203
——— ehv = 20°, e = 90C
710
705
730
705
photon energy (eV)
FIGURE 4 MARPE effects on O Is photoemission and O KL23L23 Auger emission from Fe2O3(001)
[9]. The resonance here is O Is with Fe 2p: (a) An overall electron spectrum, with the photon energy
well above the Fe 2p edges at hv = 800 eV. The inset shows the experimental geometry, (b)
Resonant enhancement for the case of Ols photoemission, and with two different experimental
geometries, (c) Resonant enhancement for the case of O KI^L^ emission in the geometry of (a),
with the Auger peak used to measure intensity shown in the inset, (d) The x-ray absorption
coefficient of Fe2O3 over the same energy region as in (c), measured by partial electron yield.
photoelectron emission normal to the Fe2O3(001) surface is shown as the solid curve,
with the smoothly varying non-resonant intensity underlying it again determined and
used for normalization. Again, we see a significant resonance effect that for this
case exhibits a maximum 62% peak intensity enhancement over the smoothly
varying intensity without the resonance. The resonant intensity as energy-integrated
over the 2p3/2 region of 705 to 720 eV for this case yields a 24% effect. The
resonance enhancement also again follows very closely the x-ray absorption curve in
Fig. 4(d), but is not identical to it. Also in Fig. 4(b) we show the normalized
resonant intensity for an electron takeoff angle of 45° as the dashed curve. This
curve is again similar to the x-ray absorption coefficient, and perhaps closer to it than
for normal emission; the energy-integrated effect is here 17%. There is also a
significant difference between the curves for the two emission directions, indicating
some angular dependence-albeit small—of MARPE effects.
For
LaQ7Sr03MnO3(001), we show an overall high-energy electron spectrum in Fig. 5(a),
with the various resonances observed indicated by dashed arrows. In Fig. 5(b) is our
measurement of the x-ray absorption coefficient over the Mn 2p3/2?1/2 region, a curve
which agrees very well with prior data [30a], and in Fig. 5(c) our measurement of the
x-ray absorption coefficient over the La 4d5/2,3/2 region, which agrees well with prior
x-ray absorption data for La metal [30b]. Figs. 5(d)-5(f) show the MARPE
enhancements observed. In 5(d), the normalized O Is intensity in resonance with
260
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(a) La0.7Sr0.3Mn03
hv = 1486eV
above all
resonances
900
«c
2.5
o
2.0
0)
800
700
600
500
400
300
binding energy (eV)
10
(b)
l|l.5
§
c
I"S..2.
-e1-0
5
0.5
«
0.0
w
640
645
650
655
01s With Mn2p
~17%enh.
^2 1.0
o
640
645
650
655
photon energy (eV)
830
84C
850
photon energy (eV)
830
840
850
photon energy (eV)
FIGURE 5 MARPE effects on O Is and Mn 2p emission from La07Sr03MnO3(001) [expanded from
ref. 1]. (a) An overall electron spectrum for a photon energy well above all resonances studied at hv =
1487 eV. (b),(c) X-ray absorption coefficients over the Mn 2p and La 3d regions, respectively, as
measured via the inelastic background in photoelectron spectra, (d) Enhancement of O Is in
resonance with Mn 2p. (e) Enhancement of O Is in resonance with La 3d. (f) Enhancement of Mn 2p
in resonance with La 3d. Percentage resonance effects as energy-integrated over the absorption
region are again shown.
Mn 2p is shown. There is a resonant peak increase at the 2p3/2 position of about
33%, and an energy-integrated effect over 2p3/2 of 17%. The resonance is again seen
to follow very closely the Mn 2p absorption curve in Fig. 5(b) for this material [30a].
The O Is intensity is also found to resonate with the La 3d5/2?3/2 levels, as shown in
Fig. 5(e), with very large peak enhancements of up to 92-105%, an energy-integrated
effect over La 3d5/2 of 29%, and a form that follows very closely the x-ray absorption
coefficient in Fig. 5(c). Finally, Fig. 5(f) shows an analogous normalized resonant
enhancement of the Mn 2p intensity with the La 3d5/23/2 levels; with a peak
enhancement of 60-70%, a 20% energy-integrated effect, and a form that again
follows closely what we measure for the La 3d xay absorption coefficient in Fig.
5(c). Although the surface of this last sample was in no way treated after insertion
into ultrahigh vacuum, we nonetheless are able to detect three strong distinct
resonance effects among its constituents: O with Mn and La, and Mn with La.
Other solids and systems
We here briefly note that similar MARPE effects have by now been observed by
our group for Cr 2p emission in resonance with Fe 2p from Cr/Fe alloys [3] and
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Ce/Fe bilayers [3]; by Kikas et al. for Cl 2p emission in resonance with transitionmetal 2p in MnCh, VC12, CrCl2 [10]; by Gamier et al. for N Is emission in
resonance with Ni 2p from N2 adsorbed on Ni(OOl) [11]; and by Hemmers et al.
more subtly through non-dipole effects on Auger emission in CO [12], In all solid
compounds and the adsorbate system, the effects are of similar magnitude to those
reported here (-10-40% in peak intensity), and the MARPE enhancements are also
found to rather closely follow the x-ray absorption profile of the resonating level.
There is thus little doubt that this is a generally observable phenomenon. Our Cr/Fe
bilayer work [3] has also permitted directly estimating the phenomenological falloff
with distance from the emitter in the MARPE intensity enhancement, with an
exponential decay constant of about 15-25 A. This Cr/Fe work has also permitted
using MARPE effects to study compositional clustering and phase separation in
epitaxial Cr/Fe alloy films [3], and demonstrated that magnetic circular dichroism
can be seen in the effect [3], thus providing a new probe of the local magnetic order
(e.g. of Fe atoms that are known to be within a few nm of Cr).
Observation via Secondary Decay Processes
Auger electron emission
It is also of interest to ask whether the enhanced probability of creating a given
core hole as photon energy is scanned across a MARPE region can be detected from
the secondary decay processes involved, specifically, Auger electron emission and
fluorescent x-ray emission. Observation of the effect via these secondary decay
channels not only further verifies its origin, but also provides other experimental
methods for detecting it [9].
As one example of detection via Auger processes, we show in Fig. 4(c) the O
KL23L23 Auger intensity from Fe2Os, measured using the same sort of inelastic
background subtraction as for photoelectron intensities.
There is a clear
enhancement of this intensity on passing through the Fe 2p absorption curve, and it
again follows rather closely the absorption coefficient of the material, as shown in
Fig. 4(d) [9].
Although this is the only case studied to date, it seems clear that MARPE also
influences Auger decay as a secondary process to the initial photoexcitation.
Flourescent x-ray emission
The influence of multi-atom resonance on x-ray fluorescence intensities has been
studied for the case of MnO [9]. These measurements were performed on undulator
beamline 8.0.1 of the Advanced Light Source, using a special high-resolution grating
monochromator situated there [31]. The O Ka emission is resolvable from any other
peak, as shown in the spectrum of Fig. 6(a). The area of this peak, with a
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530
540
550
560
fluorescence energy (eV)
640
645
650
655
660
665
photon energy (eV)
630
635
640
645
650
655
660
665
photon energy (eV)
FIGURE 6 MARPE effects on O Ka x-ray fluorescence from MnO [9]: (a) Fluorescence spectrum
of MnO recorded at hv = 640 eV. The inset shows the experimental geometry, with a fixed angle of
90° between x ray incidence and exit, (b) Energy dependence of the O Ka fluorescence intensity over
the Mn 2p3/2,i/2 edges for different angles of x-ray incidence 0in and x-ray exit 0out = 90° - 0in. Curves
are shown for both the experimental data (solid curves) and for calculations based on Eq. 1 (dashed
curves), (c) The normalized total O Is cross section in MnO, including interatomic resonance effects,
as derived from dividing the two sets of curves shown in (b). The inset shows the experimental (open
symbols) and calculated (filled symbols) ratio between the resonant enhancement at the Mn 2p3/2 edge
as detected in the O Ka fluorescence intensity (MXE), and in the O Is photoelectron intensity (MPE) for
the different experimental geometries.
small background due to scattered light subtracted, has been measured as a function
of photon energy over the Mn 2p region and for different experimental geometries,
as shown in the inset in Fig. 6(a). Both the exciting x-ray incidence angle, 0in and
the outgoing Ka x-ray exit angle were varied over four values, with a fixed angle of
90° between them. The experimental O Ka intensity is shown in Fig. 6(b), and it
exhibits strong and well-known self-absorption effects which actually decrease the
emission significantly as one passes through the Mn 2p absorption spectrum.
However, the energy-dependent x-ray absorption coefficient in MnO can be
accurately determined from the data in Fig. 2(f) and a careful matching of it to
tabulated off-resonance values at lower and higher energies [24]. With jiMno(hv)
thus determined, we can allow for self-absorption via the standard expression [32]:
OK
I0(hvin )a°ols (hvin )eols (hvin
"***> < OK «
smy
smw
-1
out
(1)
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where Io(hVin) is the incident x-ray flux at energy hvin, aOis is the cross section for
producing an O Is hole at this energy (which we take to exclude MARPE effects and
thus be described by a smooth curve which varies little over the Mn 2p region), and
£ois(hVin) is the fluorescence yield at this energy (which we assume to be constant
over the small Mn 2p energy range of our measurements). Using this equation now
yields the four dashed curves shown in Fig. 6(b), which are consistently below the
experimental data when all curves are normalized to unity well below the Mn 2p
region. We can now estimate the MARPE effects on this data simple by taking the
ratio of each pair of curves, and this give the results shown in Fig. 6(c) for the four
experimental geometries studied. That is, this ratio we expect to be a measure of
GQIS / a ois' w^ere ^Ols i§ ^e true cross section for O Is hole production, including
any MARPE effects. The four curves shown are quite self consistent in all showing
a peak enhancement of 100-140%, and very similar shapes over the entire energy
range. These data also add another new element to the phenomenon by suggesting
that there is enhancement well above the Mn 2pi/2 threshold, although this needs
further experimental and theoretical confirmation. Finally, the fact that the effects
here are over twice as large as the approximately 40% seen in the direct
photoemission process from MnO (cf. Fig. 2) can be understood by noting that a
typical x-ray fluorescence emitter is well below the surface, and so can experience
resonant effects from atoms in a spherical cluster around itself, whereas in the case
of surface-sensitive photoemission, this cluster is essentially a hemisphere extending
inward from the surface. Quantitative calculations of the ratio of MARPE peak
enhancement on and off the 2p3/2 resonance in x-ray emission (MXE) and in
photoemission (MPE) confirm this qualitative explanation, as shown by the
comparison for experiment and theory in the inset of Fig. 6(c) [9].
Thus, although allowance for self-absorption effects will be critical in measuring
MARPE via x-ray fluorescence, the procedure outlined here appears to provide a
reliable method for doing this. Fluorescence detection also has the very desirable
feature of making the measurement more bulk sensitive, and thus applicable to a
broader range of systems.
THEORETICAL MODELING OF PHOTOEMISSION
Basic Theory
In order to further confirm our assertion of the interatomic nature of these
resonant enhancements, as well as to understand their origins and approximate
spatial extents, let us discuss some of the ingredients needed in the theoretical
analysis for the O Is-Mn 2p3/2,i/2 absorption region in MnO(OOl) [2]. The relevant
interactions and quantum-mechanical matrix elements involved are indicated in Fig.
7. Specializing to the MARPE case, and writing all expressions specifically for O
Is-Mn 2p3/2 for clarity, we have:
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Photoelectron
Mn3d
Mn2p
o
FIGURE 7 Schematic representation of the energy levels involved in Ols MARPE in MnO [ref.
2]. (a) Ols photoemission takes place via two different interfering channels: direct emission (thin
solid arrow) and emission assisted by the excitation of Mn2p to Mn3d (thick solid arrow), with
subsequent interatomic autoionization (broken arrows) produced by electron-electron interaction
(dashed line). Symbols for the various interactions are also indicated here, (b) The resonance
involves coherent addition of effects over several Mn sites in the solid situated at Rj with respect
to the emitter O atom.
• The dipole excitation by light with polarization vector e of an O Is electron from a
given oxygen atom to a photoelectron with kinetic energy E, angular momentum I =
1 =p as dipole-allowed, and magnetic quantum number mf = 0, ±1:
(2)
V
r°ad=<EP m f
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• The dipole excitation of a Mn 2p3/2 electron with magnetic quantum number m on
the jth Mn atom located at position Rj in a cluster surrounding the emitter O atom to
the first unoccupied Mn 3d level with magnetic quantum number m1 on that atom:
8.rlMnj2p 3/2m > ,
(3)
where we have included the effects of retardation via the exponential pre-factor in
the photon wave vector k hv and Rj, since the typical photon wavelength at the
Mn2p3/2 threshold of 640 eV energy of about 19 A, coupled with the minimum
interatomic O-Mn distance of d = 2.23 A leads to a phase of 42° or 0.73 radians that
can quickly accumulate to a non-negligible value in a sum over a cluster of several
times this interatomic distance in radius; and
• The autoionization of the Mn 3d state via a Coulomb interaction coupling the Mn
and O electrons:
VJj =< Epmf ,Mn2p 3/2>m I VCoul 1 01s,Mn3dm, > ,
(4)
where the Coulomb interaction written here as VCoul is an abbreviation for the
retarded form including both charge-charge and current-current interactions and
often referred to as the M011er expression [33,34]:
e
2
j + r 1 -r 2
-*
i k 1 v l R i + r 2 ~ r1l l(l-k
/i i
- -\
h v r 1 .r 2 ),
/c\
(5)
with this form having been used previously in high-energy Auger theory. Eq. (5)
includes both dipolar charge-charge interactions (the " 1 " in parentheses) and dipolar
current-current interactions (the "-k hv f 1 .r2 " in parentheses), as well as appropriate
retardation.
The enhancement observed in MARPE originates in the excitation produced by
the external radiation in every Mn atom j via the interaction described by Eq. (3) and
its subsequent de-excitation via Eq. (5). The effective spatial range of this interaction
is then determined by the sum over j of the phase factors appearing in Eqs. (3) and
(5), together with the inherent distance dependence of the O-Mn interactions and
possible dielectric screening effects. But considering only the sum over phase
factors, far-distance contributions to this sum suffer phase cancellations, so that they
play a minor role. Actually, a good approximation is already obtained when only
Mn atoms up to 10 times the nearest-neighbor distance away from the O emitter are
considered. Therefore, the range of MARPE in MnO can be estimated to be
approximately 22 A.
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As a final comment on the theoretical picture of MARPE, we should point out its
relationship to the inverse form of x-ray fluorescence holography [32,35], also
referred to as multi-energy x-ray holography (MEXH). In this off-resonance
measurement usually done at much higher energies of 6-15 keV, an incoming x-ray
beam scatters from various neighbors around a given fluorescent emitter, with the
relative phases of the different scattered-wave components thus encoding the local
atomic geometry. The intensity variation as a function of x-ray incidence angle is
thus a hologram, which can be inverted mathematically so as so directly produce an
image of the local atomic structure [32,35]. MEXH involves the scattering of virtual
photons from the various atoms surrounding the fluorescent emitter so as to add an
induced component to the local electric field that produces fluorescence. However,
when this picture is incorporated at the resonant edges in the 0.5-1.0 keV energy
regime utilized in MARPE, particularly for the case of MnO, the resulting induced
electric field produces a decrease rather than an increase in the magnitude of the total
electric field, largely due to the inelastic part of the x-ray scattering factor [36].
Therefore, the resonant nature of MARPE involves other processes not simply
accounted for by the simple scattering via virtual photons. This aspect of MARPE is
currently under investigation.
Relationship to the Forster Effect
As a final comment on the theoretical interpretation of MARPE, we contrast it
with the well-known type of intermolecular or interatomic energy transfer involved
in the Forster effect [16,18,19], which is also directly related to what has been
described as sensitized luminescence [17]. In this effect, a low-energy excitation of
the order of 1 eV that is very long-lived with respect to the core decay involved in
MARPE is produced on one atom/molecule. This excitation then propagates from the
"donor" atom or molecule to an "acceptor" atom or molecule with a matching
energy-level system, with the propagation time being much shorter than the lifetime
of the initial excited state. The relationship of these times dictates that coherent
quantum interference effects can be neglected, and the process is well described by a
transfer rate equation in which energy diffuses from one site to another via real
photons in multiple-step cascades. The interaction is again fundamentally
Coulombic, and it reduces due to the long wavelength of the excitation to a nonretarded 1/r3 for typical donor/acceptor distances of the order of 10 nm. The final
transition rates scale as the square of the potential or 1/r6.
MARPE is thus fundamentally different in several respects. It involves much
higher excitation energies of 100-1000 eV, much shorter relaxation times leading to
coherent interference effects, and much shorter wavelengths that can require
considering retardation effects at the high end of the energy scale given above. A
rate equation like that in the Forster effect is thus not appropriate, and quantum
interference with the direct process is then critical. MARPE thus involves virtual
photons and a single-step process, compared to real photons and a multiple-step
process in the Forster effect. Both processes can however be considered to involve
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multiple scattering of photons, whether real or virtual, and inclusion of this in
MARPE theory is also currently underway.
IMPLICATIONS, POTENTIAL APPLICATIONS,
AND FUTURE DIRECTIONS
Implications for X-ray Absorption Spectroscopy
We conclude by first pointing out some potential implications of the multi-atom
resonant photoemission effect on other measurements. It seems likely that MARPE
will have an effect on simple x-ray absorption spectroscopy, even though the effect
will be masked in that type of measurement by simple contributing to an alreadylarge increase in absorption in crossing a given core threshold. For example, in MnO
we estimate from our experimental data that the net change in the strength of the O
Is excitation process due to Mn 2pa/2 resonance is about 1.5% of the strength of the
primary Mn 2ps/2 absorption peak. But even at this kind of level, one can imagine a
process involving the type of inter-atomic interaction that makes MARPE possible,
but occurring between the same type of atoms in a sample and that this interaction
could affect the main absorption resonance on something like that magnitude scale.
In this case, one might refer to multi-atom resonant photoexcitation instead of
photoemission. Thus, in view of the strength with which such resonant effects are
found in photoemission from compounds, it seems quite possible that they would
represent a measurable correction to an absorption coefficient. One might also
expect such effects to be important in resonant x-ray scattering experiments, whether
elastic or inelastic.
Requirements on x-ray beam coherence
A further consequence of our discussion of MARPE (cf. Fig. 7(b)) and this line of
reasoning is that the incoming x-ray beam must be coherent over the full volume
necessary to excite all atoms j that can contribute significantly to the multi-atom
resonant excitation. Thus, the coherence volume of the x-ray beam becomes
important in order to "saturate" these effects. With an x-ray wavelength of A,x, a
wavelength resolution of AXX, and a beam divergence of 6div ,we can for the example
of beamline 9.3.2 at the ALS estimate the transverse coherence length from the
A,
formula dtrans - ——-— to be about 1200 A and the longitudinal coherence length
from the formula dlong = ——-— to be about 9700 A. These values are sufficiently
2(AA X )
large with respect to our present estimated range for the MARPE interactions of a
few nanometers or 20-30 A, that we do not believe our results have been influenced
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by beam coherence effects. Similar statements are probably true for most thirdgeneration spectroscopy beamlines with state-of-the-art optics.
However,
differences in x-ray absorption strengths for different beamlines and experimental
conditions have recently observed, and these may be linked to such beam coherence
effects [37].
Possible applications and future directions
Beyond their possible influence on x-ray absorption, or perhaps also on resonant
x-ray scattering phenomena, multi-atom resonant photoemission or photoexcitation
(MARPE) effects as we have discussed them here via photoelectron, Auger electron,
or x-ray fluorescence detection appear to have broad potential applications in physics
and chemistry, as well as in the materials and environmental sciences, and perhaps
also the life sciences. From data obtained by our group and now also others, these
effects are expected to be generally observable in condensed-phase materials,
provided that suitable core level spacings can be found in the two atoms to be
studied. The best estimates of the range ot the effect to date are as noted a few
nanometers, with nearest-neighbor atoms that are in more intimate bonding
interaction with the emitter expected to exhibit especially strong contributions [3].
However, it is also important to note that there may be several hundred resonating
atoms within a few nanometers of a given emitter in a typical solid. For many types
of systems exhibiting special bonding sites and/or heterogeneity on a nanometer
scale, MARPE measurements should permit determining the local composition,
bonding, and magnetic environment (e.g. via magnetic circular dichroism), as well as
the existence or non-existence of compositional clustering or phase separation [3], in
a non-destructive and unique way.
Further studies exploring the precise sensitivity of MARPE to different
interatomic distances and/or different pairs of core levels with varying energy
spacings are certainly called for. Further theoretical analysis to explain the observed
magnitudes of the effect and the low degree of angular sensitivity seen in experiment
and to more quantitatively determine nearest-neighbor effects and the effective range
of the effect are also needed. But even without more theoretical development,
calibrating such interatomic resonant enhancements against standard compounds
with the same or varying types of chemical bonding and/or heterogeneity may permit
estimating the number of near neighbors of a given type for unknown cases. The
well-known chemical shifts in photoelectron kinetic energies [15,22] should also
permit measuring resonances separately around different chemically-distinct species,
something which has already been done for N2/Ni(001) [11]. Measuring
photoelectron diffraction patterns [15,22] as differences on and off resonance may in
addition permit focussing on scattering processes associated with a given type of
near-neighbor atom, thereby leading to a more element-specific structural probe.
Although we have used single crystals here to better define these first
measurements of the effects, there is no general requirement of a single-crystal
specimen; however, with truly random atomic positions in an amorphous or glassy
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material, the range of the effect~and thus its magnitude-might be expected to be
reduced to something like the wavelength of the radiation due to phase cancellation
effects for distances larger than this. Further studies of such interatomic effects in
free molecules are also called for, with first observations showing effects in the 1%
range or smaller [12]. With soft-x-ray detection, as illustrated in Fig. 6, the
experiment becomes more bulk sensitive, considerably widening its applicability to
include perhaps the environment around active sites in systems of biological interest,
although a careful allowance for the enhanced absorption of the exciting flux at
resonance would be needed to unambiguously detect the resonant enhancement.
With photoelectron, x-ray, or Auger detection, the identities of neighbors to atoms
adsorbed at surfaces, around impurity atoms in solids, or around atoms at buried
interfaces should be detectable. Finally, exciting with circularly-polarized radiation
on magnetic samples should lead to resonant photoelectron spin polarization and/or
magnetic dichroism effects (e.g. by subtracting intensities for right- and left- handed
excitation, as already observed in Cr/Fe alloys [3]) that should be particularly
sensitive to the short-range magnetic order of near neighbors. Future studies
exploring and exploiting these various possibilities, as well as more accurate
theoretical modeling of all of these effects, are thus clearly of interest.
ACKNOWLEDGEMENTS
We are indebted to M.P. Klein and J.B. Kortright for helpful discussions, to EJ.
Moler for assistance with beamline tuning and maintenance, to R. Denecke for help
with initial experiments, and to S.A. Chambers, K. Krishnan, and J. Stohr for
providing samples. The expert assistance of M.W. West and M.J. Press with various
experimental aspects is also much appreciated. We also gratefully acknowledge a
collaboration with M.M. Grush, T.A. Callcott, D.L. Ederer, and C. Heske on x-ray
fluorescence experiments. This work was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences
Division, under Contract No. DE-AC03-76SF00098. Additional support was
provided by the Miller Institute (E. A.), as well as the Basque Government and the
Fulbright Foundation (J. G. de A.).
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