Photon Factory Activity Report 2019 #37 (2020)

Multi-atom Resonance in Simple Binary Molecules-Comparison between Solid and Aqueous Solution-

BL-27A/2019G032

Yuji BABA and Iwao SHIMOYAMAAdvanced Science Research Center, Japan Atomic Energy Agency, Tokai-mura, Naka-gun,Ibaraki, 319-1195, Japan
1 Introduction
Multi-atom resonance (MAR) by X-ray irradiation
is a phenomenon where intensity of photoelectrons(or Auger electrons, fluorescence X-rays) from anatom is changed by the resonant core-level
excitation of the neighboring atom. The MAR
phenomenon was firstly reported in photoelectronspectra for MnO [1]. On the other hand, MAR was
not observed in C Kα X-ray emission for TiNbC at
the Ti L2,3 resonance [2]. Although someexperimental and theoretical studies have beenreported after that, the uncertainty still remains as towhether MAR really happens or not.
In the present study, we re-investigated theexistence of MAR by measuring the X-ray emission
for simple binary molecules of alkali metal halide
and alkali-earth metal halides. In order to make theexistence of MAR more clearly, we particularly
focused on comparison between solid and aqueous
solution for the same molecule.

the Ca Kα X-rays was plotted, so the plots represent
general XAFS spectra. The peak at 4045 eV
originates from the resonant excitation from the Ca
1s to the unoccupied Ca 4p* orbitals [3]. The
intensity of the Cl Kα X-rays was plotted in Fig.1(b).
For solid sample shown as filled circles, the intensity
was unexpectedly decreased around the Ca 1s→4p*resonance. Since the concentration of CaCl2 was solow (1%) that the effect of self-absorption of theincident and fluorescence X-rays can be neglected.
Therefore, the observed decrease in the Cl Kα
intensity is considered to be caused by the Ca 1s→
4p* resonance excitation in the neighboring Ca atom.
On the other hand, the decrease in the Cl Kα
intensity is not observed for aqueous solution. Thisis simply because the CaCl2 was dissociated to Ca2+and Cl- ions, so few Cl atoms existed around a Caatom.

2 Experiment
The experiments were performed at the BL-27A
soft X-ray station, Photon Factory. We used thespecially designed X-ray fluorescence system [3], inwhich fluorescence X-ray absorption fine structure(XAFS) in soft X-ray region can be measured forboth solid and liquid samples. The X-rays wereirradiated from the bottom of the sample at the angleof 0.8°. The fluorescence X-ray was detected with Si
drift detector that was located perpendicularly under
the sample. The photon flux was normalized by thedrain current of aluminium mesh located in front ofthe sample.
For solid sample, the reagent was mixed withcellulose powder and pulverized using a mortar. The
concentration of the sample was 1wt%. Then, it waspressed into a pellet at a pressure of 3 ton/cm2. For
aqueous solution sample, the reagent was simplydissolved in pure water at a concentration of 1 wt%.
The solution was put into a vessel, whose bottom
was made of 4 µm mylar film.

Fig. 1: Photon-energy dependencies of the intensityof Ca Kα (a) and Cl Kα (b) for 1% CaCl2. Filled andopen circles represent solid and aqueous solution,

3.2 MAR for KCl
Fig. 2 shows the photon-energy dependencies offluorescence X-rays around the K K-edge for KCl.
The peak at 3615 eV is attributed to the K 1s→ KFig. 2: Photon-energy dependencies of the intensityof K Kα (a) and Cl Kα (b) for 1% KCl. Filled and open
circles represent solid and aqueous solution,
respectively.

3 Results and Discussion
3.1 MAR for CaCl2
Fig. 1 shows the intensity of the fluorescence Xrays for CaCl2 as a function of the X-ray energy
around the Ca K-edge. In Fig.1(a), the intensity of

4p* resonance [4]. The results were similar to thosefor CaCl2. For the intensity plots of Cl Kα X-rays (Fig.
-1-Photon Factory Activity Report 2019 #37 (2020)2(b)), the intensity of the solid sample was reduced
around the K 1s→4p* resonance. But the intensity
was constant for aqueous solution.

resonance excitation in the neighboring atom.
Considering that the probability of X-ray emission
after core-level ionization is determined by thefluorescence yield depending only on the atomic
number, we consider that the intensity of the X-rayemission was changed not by the changes in the Xray emission probability, but by the changes in theinner-shell ionization cross section itself.
Generally, the photoionization cross section of an
atom “A” (σA) by X-ray irradiation is independently
defined for the atom even for a molecule “A-B”.
However, at the energy of the core-to-valence

3.3 MAR for SrCl2
The former two results were MAR in chlorine(anion) induced by the resonant excitation in theneighboring cation. In order to check whether theMAR in cation induced by the counter anion
happens or not, we next investigated the MAR inSrCl2 following the Cl K-edge resonant excitation.
Fig. 3 shows the intensity of the Cl Kα and Sr Lα
fluorescence X-rays as a function of the X-ray
energy around the Cl K-edge for SrCl2. Deduced
from the reported Cl K-edge XAFS spectra for alkalichlorides, the peak at 2825 eV is considered tooriginate from the Cl 1s→4p* resonance [5]. In Fig.
3(b), the intensity of the Sr Lα X-rays was reduced
around the Cl 1s→4p* resonance for solid. But theintensity was not changed for aqueous solution.

Fig. 4: Schematic diagrams of single-atom resonance(upper figure) and multi-atom resonance (lower figure)for X-ray emission.

Fig. 3: Photon-energy dependencies of the intensityof Cl Kα (a) and Sr Lα (b) for 1% SrCl2. Filled andopen circles represent solid and aqueous solution,

3.4 Possible mechanism of MAR
Resonant X-ray (or photoelectron, Auger electron)
emission from a single atom is well establishedphenomenon. A typical example of X-ray emission
from a single atom following resonant core-level
excitation is shown in the upper part of Fig. 4, whichis called “resonant x-ray emission” (or resonant xray Raman scattering). In this case, the intensitychanges of X-ray emission can be simply explainedby the changes in the inner-shell photoionization
cross section (σA) of the atom. On the other hand, Xray emission following the core-level resonant
excitation in the neighboring atom is schematicallyshown in the lower part of Fig. 4. In this case, theintensity of X-ray (or photoelectron, Auger electron)
emission form an atom “A” is perturbated by thechanges in the inner-shell photoionization crosssection of the neighboring atom “B”. As describedabove, MAR was firstly observed in photoemission
from MnO where O 1s photoelectron emission wasenhanced by the resonant excitation at the Mn L3edge [1]. They interpreted this phenomenon in termsof the extension of resonant photoemission theory ina single atom to the interatomic case. As to theintensity, the reverse phenomena were observed in
the present study, i.e., the intensity of X-ray
emission from an atom was reduced at theresonance of the atom “A”, σA drastically increases,
which coherently induces the reduction of sB. Thismeans that we have to newly consider thephotoionization cross section of a molecule, σA-B, atthe resonance energy of one of the atoms.
Further theoretical study is needed to interpret thepresent results for MAR. Considering that thepresent phenomenon gives information about theatomic species of neighboring atoms, thephenomena will be applied to a method for structural
analysis, which compensates the conventional
synchrotron-based methods such as EXAFS andXANES.
References[1] A.W Kay, C.S Fadley et al., J. Electron Spectrosc.
Relat. Phenom. 101-103, 647 (1999).
[2] A. Moewes and E.Z. Kurmaev, Nucl. Instrum.
Meth. Phys. Res. A, 467–468, 1529 (2001).
[3] A. Bianconi, S. Doniach and D. Lublin, Chem.
Phys. Lett. 59, 121 (1978).
[4] J. Padeznik Gomilsek, A. Kodre, I. Arcon and R.
Preseren, Phys. Rev. A 64, 022508 (2001).
[5] R.K. Szilagyi et al., Photon Factory Activity
Report 2013 #31 (2014) B.
* baba.yuji@jaea.go.jp-2-