Photon Factory Activity Report 2017 #35 (2018)

BL-27A/2016G198

Surface Characterization of Polycrystalline Hemin
Hiromi IKEURA-SEKIGUCHI 1,* and Tetsuhiro SEKIGUCHI 2
Research Institute for Measurement and Analytical Instrumentation (RIMA),
National Metrology Institute of Japan (NMIJ),
National Institute of Advanced Industrial Science and Technology (AIST),
1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan
2
Japan Atomic Energy Agency (JAEA), Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
1

2 Experiment
Experiments were performed at beamline BL-27A withan InSb(111) double-crystal monochromator with anenergy resolution of ca. 0.9 eV around Cl K-edge. The
XAS spectra of polycrystalline hemin were measured inthe TEY mode as monitoring sample drain current and theAEY mode as monitoring Cl KLL Auger electrons near
the Cl K-edge (2815–2870 eV) and normalized to theincident flux with thin aluminum films. Such energies
were chosen to excite the Cl 1s core electron to 3p and 4p
unoccupied orbitals.
3 Results and Discussion
Fig. 1 shows Cl K-edge XAS spectra of polycrystalline
hemin measured in the TEY and the AEY modes. XAS
has the highest surface sensitivity in the AEY mode withthe probing depth of ~7.5 nm at Auger electron energy of~2.3 keV, which is about three times larger than theInelastic Mean-Free Path (IMFP). Although the TEY isknown as surface sensitive analysis, the probing depth of
the TEY is estimated to more than ~70 nm [1] in theexcitation energy regions of deep core levels.
Based on the previous study of metal chloride [2], preedge peak A is assigned to Cl 1s → Fe 3d hybridized with
Cl 3p orbital. Peak B is assigned to Cl 1s → 3p orbital.
We found that surface states of polycrystalline hemin are
higher in energy (~0.8 eV) than bulk ones as shown inpeak A. Peak B also shifts to higher energy state. Similarenergy shifts have been observed in N K-edge XAS
among hemin monomer, dimer, and solid states [3].
Considering the previous study [3], the shifts suggest that“dimer-like” layers with less intermolecular hemin-hemininteractions could be produced near surface regions. In

contrast, stronger hemin-hemin interactions could becaused partly by hydrogen interactions of side groups.

heminIntensity (arb. units)

1 Introduction
Hemin, iron(III) protoporphyrin IX, is one of metal
porphyrin complexes which are naturally occurring
molecules and found in a variety of important biologicalprocesses such as oxygen transport and photosynthesis.
Metalloporphyrin ring is composed of four pyrrole units
conjugated through methine bridges, with this highly
conjugated structure (Fig. 1) giving electron or charge
delocalization over a whole molecule. In order tounderstand electron and energy transfer processes, partial
density of unoccupied states was measured using X-ray
absorption spectroscopy (XAS) monitored by total
electron yield (TEY) for bulk and Auger electron yield(AEY) for surface.

BEA

2820 2822 2825 2827 2830
Photon Energy (eV)Fig. 1: Comparison of bulk- and surface- sensitive Cl Kedge XAS spectra of polycrystalline hemin measured by
the TEY (black line) and the AEY (blue dot line).
Acknowledgements
We thank the staff of the Photon Factory and BL27A(JAEA) for supporting experiments
References[1] M. Kasrai, et. al., Appl. Surf. Sci., 99, 303 (1996).
[2] B. Hedman, K. O. Hodgson, and E. I. Solomon, J. Am.
Chem. Soc., 112, 1643 (1990).
[3] R. Golnak, et. al., Phys. Chem. Chem. Phys. 17,29000 (2015).
*ikeura-sekiguchi@aist.go.jp