Beamline Practice at BL27SU
Soft X-ray photoabsorption spectroscopy using fluorescence yield method
Ver. 2014/09/03
Y. Tamenori (JASRI)
tamenori@spring8.or.jp

1. Introduction
The beamline BL27SU is used for X-ray absorption applications in the soft X-ray region(0.17-2.8 keV).

In this practical training course, the participants will learn how to measure soft X-ray

absorption spectrum.

The participants will gain experience in sample preparation, sample handling,

and data acquisition.

Soft X-ray absorption spectra will be measured by means of the total electron

yield method, the total fluorescence yield method using a PIN photodiode detector and the partial
fluorescence yield method using a silicon-drift-detector.

2. Practice plan
9:30～

Introduction

10:00～ Practical training-1 (Measurement of total electron and total fluorescence yield spectra)
12:00～ ---------- Lunch ---------13:30～ Practical training-2 (Measurement of partial fluorescence yield spectra)
17:30--------- Close -----------

3. What is a soft X-ray photoabsorption spectroscopy?
X-ray photoabsorption spectroscopy (XAS) is a widely-used technique for determining the
local geometric and/or electronic structure of matter [1,2].

Since the core electrons are localizedaround the atoms to which they originally belonged, XAS is an element-specific spectroscopic
technique.
Because the XAS spectra are characterized by many fine features near the absorption edge,
XAS is generally called X-ray absorption fine structure (XAFS) spectroscopy. The XAFS spectra areusually divided into two energy regions: the X-ray Absorption Near Edge Structure (XANES) and theextended X-ray absorption fine structure (EXAFS).

XANES typically represents the energy region

from the absorption edge to about 50 eV above the absorption edge.

The absorption structures near

the absorption edge were first interpreted by Kossel as due to electronic transitions to unoccupied
molecular orbitals.

The oscillatory structure extending for hundreds of electron volts past the edges

is called EXAFS, and it was theoretically interpreted by Kronig to the single scattering of the excited
photoelectron by neighboring atoms.

The energy region of XANES (extending over a range of about

100 eV) between the edge region and the EXAFS region has been assigned to multiple scattering1resonances.
XAS in the soft X-ray region is a powerful tool for chemical state analysis of the low-Z
elements, and it is often called NEXAFS (Near Edge X-ray Absorption Fine Structure) spectroscopy.
NEXAFS is synonymous with XANES, but XANES is more commonly used for solids and inorganic
compounds, while NEXAFS is used for organic compounds in the surface and molecular science.

In

the practical training course, the participants will experience how to measure XAS in the soft X-ray
region.

4. How to obtain soft X-ray absorption spectrum?
XAS is the measurement of the x-ray absorption coefficient [] of a matter as a functionof excitation energy.

The most faithful method to obtain the absorption coefficient is a transmission

method based on the Beer-Lambert law.

When X-rays of a narrow band width are irradiated on thesample, the absorption coefficient () can be determined from the following equation.
e(1)Here, (It) is number of x-ray photons that are transmitted through a sample, (I0) is the number of x-ray
photons irradiated on the sample, and (x) is the thickness of the sample.

However, the X-ray

transmission method, which is ordinarily used in hard X-ray measurements, is not appropriate for thesoft X-ray measurements, because the soft X-ray absorption coefficients are too large for practicalsample thickness to use the transmission method.

Therefore, to obtain the XAS data in the soft X-ray

region, we have to detect another signal which is proportional to the soft X-ray absorption coefficient.
The soft X-ray absorption coefficient can be observed by measuring either the electron yieldor the fluorescent yield. The fundamental phenomenon in soft X-ray absorption is an excitation or an
ionization of a core electron.

The resulting core hole is filled by an electron from another shell andfollowed by either an Auger electron or X-ray fluorescence emission.

Both channels can be used as aprobe of the probability of the creation of a core hole via soft X-ray absorption.

Here, it should be

noted that the spectra measured by electron yield and fluorescent yield methods are not identical tothat obtained by the transmission method.

In general, the electron yield is surface sensitive and the

X-ray fluorescence yield is bulk sensitive.

In other word, the surface or bulk sensitivity can becontrolled by different yield methods, and we can obtain deeper information on the chemical andelectronic states of matter from the XAS measurements.

In the practical training course, theparticipants will learn the techniques for measuring the chemical states of matter by using the electron
yield and the fluorescent method.

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5. Experimental
5-1. Light source and optics of beamline BL27SU.
Figure1 shows a layout of the BL27SU.

The light source is a Figure-8 undulator which can

produce linearly polarized radiation with low on-axis heat load [3].

Figure-8 undulator provides bothhorizontally and vertically polarized soft X-ray beams by choice of an appropriate undulator gap, thelinear electric vector lying horizontally (0°) from the integer-order-harmonic light and that vertically(90°) from the half-integer-order light.
The monochromator is a varied-line-spacing plane grating (VLS-PGM) fixed deviation
instrument of the Hettrick type [4].

The monochromator consists of the entrance slit (S1), sphericalmirrors (M21, M22), three gratings (G1, G2, G3), and exit slit (S2).

Two spherical mirrors and threegratings allow the scanning of an energy range from 170 to 2800 eV.

Energy sweep can be done onlyby the rotation of the grating.

Absorption spectra are measured by scanning the undulator gap as wellas the monochromator to keep the maximum intensity of the incident soft x-rays.

Three re-focusing

mirrors are designed for focusing the photon beam on three experimental stations arranged in tandem.

★
C3
Experime ntal
stations
M4
C1

S2

C2

M3
Top Vi ew

M21

178.8 deg
178.8 deg

Figure-8
Un dulator

Gratings

S1

M1

M0
42.7 m

M22

2m

12 m

Cylinder Bend
Mirror
CylinderMirrorToroidalMirror

12.5 m178 deg 175.4deg

Entrance Slit★

Exit Slit

CylinderMirror1m

Spherica lMirrors

7.5m

★

★

Side View

Gratings

Figure.1; Layout of the BL27SU<In the practice course, the alignment and the calibration of optics are omitted.
calibration procedures will be explained in the guidance. >3The outlines of5-2. Sample handling
The standard procedure in soft X-ray experiments is to install all equipment necessary forexperiments inside the high-vacuum chamber, since soft X-rays below 1.5 keV are fairly absorbed in
atmospheric air.

The high-vacuum chamber produces an ideally clean environment, therefore, it

gives good opportunities to investigate a clean surface and anaerobic materials.

On the other hand,the vacuum condition has drawbacks. Technologically significant phenomena take place underhigher-pressure regions (1~760 mbar).

One approach to overcoming the absorption in atmospheric

air is to use a Helium-path instead of an air-path.

Helium gas has a higher transmission than air inthe soft X-ray region due to its smaller atomic number.

In the BL27SU, a differential pumping

system was developed for use with windowless soft X-ray experiments under normal atmosphericpressure helium conditions [5].

This technique can eliminate the loss of incident photons due to theabsorption of vacuum windows.
Figure 3 shows the transmission curves ofsoft X-rays under atmospheric helium.

The
1.0

experimentally determined transmission curves
The

transmission at 550 eV (the oxygen K-edge
region) measured at 10 mm downstream of thefirst aperture is about 85%.

When the photon

0.8Transmission

are nearly identical to the simulated ones.

0.6

0.4

10 mm measurement
50 mm measurement
10 mm calculated
50 mm calculated

flight path in the atmospheric pressure region is
0.2

expanded up to 50 mm, more than 70% of the400

incident photons are transmitted to the detector.
Although the transmission is decreased in thelower photon energy region, about 60% of theincident photons are transmitted to the detector
at 350 eV.

500

600

700

800

900

1000

Photon energy (eV)Figure 3: Experimental and calculated transmission
curves of soft X-ray photons obtained under helium-path
conditions. The experimental transmission curves were
measured at the helium-path length of 10 mm (●) and
50 mm (○). The solid (10 mm) and dashed (50 mm)
lines are the simulated transmission curves of thefabricated differential pumping system.

In the practical training course, the participants will gain experience in sample preparation,
sample alignment inside a vacuum chamber.

Additionally, the soft X-ray experiments under theatmospheric pressure condition are also experienced.

46. Electron and Fluorescence yield methods
6-1, Electron yield detection
Electron elimination processes can be classified into three types.

First, the photoelectrons

can originate from both core and valence electron ionization, if the incident photon energy is higher
than the binding energies of these electronic states with respect to the vacuum level.

Secondly,following a core electron excitation or ionization, an electron in a higher energy level falls into thecore hole, and another electron is eliminated using a released excess energy. This second electron iscalled an Auger electron.

The kinetic energy of photoelectron depends on the incident photon energy.

On the other hand, in contrast to the photoelectron, the kinetic energy of Auger electrons ischaracteristic for specific Auger transition and is independent of the incident photon energy.

Thephotoelectron or Auger electron emitted in the solid may hit a neighboring molecule, so that it induces
an electron impact ionization of the neighboring molecule.

Third are secondary electrons which are

emitted by such an inelastic scattering processes with primary electrons.

In generally, the electron yield method is surface sensitive.

The sensitivity to surface andbulk compositions is different among the above three electron detection, since the mean free path ofelectron strongly depends on the kinetic energy, and it is well known as a universal curve [1].

Thesurface sensitivity of each electron yield method is estimated by the mean free path of electrons.
Further, while the secondary electron loses the memory of initial excitation site, the photoelectron and
the Auger electron are remembering an initial excitation site, because these electrons are eliminatedwith a specific kinetic energy.

Selective detection of such a specific energy electron improves theelement selectivity in the XAS measurements.

6-2. Total electron yield method
The total electron yield (TEY) method measures the yield of all photoelectrons, Auger
electrons, and secondary electrons.

Due to the large mean free paths of the low energy electrons, theelectron yield signal in TEY method is dominated by the low energy electrons with kinetic energies
below 10 eV.

One drawback of the electron yield detection is sensitive to sample charging.

Therefore, the application to the insulator samples is restricted.
Figure2 illustrates the setup of the total electron detection.

The detection of a sample

current is the simplest method for XAS in the soft X-ray region, and is equivalent to the total electron
yield (TEY) measurement.

In this method, the sample is connected to ground through an ammeterand the neutralization current is monitored.
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Alternatively, it is also possible to measure theelectron yield spectrum by detecting the eliminatedfree electrons. In this case, a micro-channel-plate(MCP) detector is commonly used.

The detection

efficiency of the MCP to the electron is about 30%, so
the detection efficiency of the electron lowers
compared with the sample current method.

On theother hand, the trajectory of the free electron can becontrolled by electric and magnetic fields.

By using

the energy filter to the incoming free electron, it ispossible to detect a specific energy electron.

Figure 2; An examples of TEY measurement
using sample current method and MCP detector.

6-3. Partial electron yield (not scheduled in this practical course)
The partial electron yield (PEY) method collects a fraction of the total electrons by using anelectron energy analyzer.

The simplest way to obtain the PEY signal is to placing a retarding electricfield in front of the electron detector, it is called a high-path filter method.
in the order of ~100 eV, which rejects low energy electrons.

The typical bias voltage is

The drawback of this method is that it

detects both photoelectrons and Auger electrons, as long as their kinetic energies exceed the bias
voltage.

The PEY method also detects electrons that undergo inelastic collisions with the substrateatoms, as long as their final kinetic energies are higher than the retarding bias voltage.
energy selectivity in retarding field method is restricted.

Therefore, theAnother PEY setup is to using a highresolution electron energy analyzer such as a cylindrical electron energy analyzer or a hemisphericalelectron energy analyzer.

For example, the kinetic energy of Auger electrons is characteristic for

specific Auger transition and is independent of the incident photon energy.

The Auger electron yield(AEY) can be obtained by setting the energy analyzer at specific transition energy.

6-4, Fluorescence yield detection
The core hole created by soft X-ray absorption is accompanied by either the fluorescencedecay or Auger decay.

Although the fluorescence yield detection is the most common method in thehard X-ray energy regime, it is relatively difficult in the soft X-ray region.

While the fluorescencedecay is a dominant channel for the relaxation of K-shell hole at Z > 30, its contribution is muchsmaller than the Auger decay at Z < 30.

For example, the relative fluorescent decay probability forthe oxygen K-edge is less than 1% [1].
6Although the yield is low, the fluorescence yield measurement has unique advantages overthe electron yield method.
(1) The detection of fluorescence yield is insensitive to sample charging.
measure the XAS of insulating materials.

It makes applicable toThis advantage is very important in the investigations oftransition metal compounds and inorganic materials such as the glasses and ceramics.
(2) It has a much deeper detection limit than the electron yield method.

Simultaneous collection ofboth fluorescence and electron yield provides an opportunity to distinguish the electronic properties
between surface and bulk of materials.
(3) Fluorescence measurement is possible under the non-UHV conditions, allowing studies of reaction
kinetics under in-situ conditions.

6-5. Total fluorescence yield
A fluorescence yield detector is mainly composed of PIN photodiode detector or
microchannel-plate-detector (MCP).

The PIN photodiode detector is possible to use under the

non-UHV condition and the detection sensitivity is higher than the MCP detector.
the PIN photodiode is highly sensitive to the visible light.

One drawback of

For example, the visible lights thateliminated by a filament of ion-gauge and enter from a viewing-port make higher background.
On the other hand, the pulse counting detection ispossible by using MCP detector.

By discriminatingeach pulse signal, the data of a high S/B ration can beobtained. Figure 3 shows the fluorescence yield
analyzer using MCP.

A negative high voltage isapplied on the entrance of the MCPs to repel electrons.
Further, a grid placed in front of the detector.

The grid

is used to prevent ions from coming into the MCPs.

Figure 3; An example of the total fluorescence
yield analyzer using MCP.

Then, the fluorescence can exclusively be detected by
the MCPs.

Practical training - 1
XAS of metallic sample will be measured by using the TEY and TFY method.

TEY spectrum isobtained by sample current method, and the TFY spectrum is measured using PIN photodiodedetector.

Participants will be experienced that the XAS measurement distinguishes the electronic

states of the oxidized surface and the bulk of metallic sample.
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6-6. Partial fluorescence yield
One major drawback of total fluorescence yield detection is its low S/B ratio to themeasurement of low concentration samples.
method is approximately 1%.

The detection sensitivity of the total fluorescence yield

An alternative approach is partial fluorescence detection which ispossible by using an energy-dispersive detector.
for each element as well as Auger electron.

The energy of fluorescence X-ray is characteristic

Selective X-ray fluorescence detection can improve thedetection sensitivity, and XANES spectra have been obtained for elements of concentrations lower
than 100 ppm.

For fluorescence measurements of concentrations above 0.1%, the total fluorescence

detection is preferable, however, for lower concentrations the partial fluorescence yield detection is abetter.
A silicon drift detector (SDD), which is a semi-conductor detector, is one of the widely
applied detectors to XAFS measurement [6].

The principle of the SDD was first proposed by Gatti

and Rehak [7], and is now commercially available.

The SDD is unique because it achieves a higher

energy resolution and higher count rate at shorter shaping times compared to conventionalsemi-conductor detectors, and it can be used near room temperature.

The high count rate of the SDD

can compensate for low fluorescence yield in the soft X-ray region, and it can distinguish thefluorescence line of interest from the peak caused by elastic scattering and from neighboring
fluorescence lines.
The SDD used in the present practice was purchased from OURSTEX Corporation Ltd. (XFlash Detector, 1201 series).

The active area of the photodetection element is 30 mm2.

The

entrance window mounted on the front of the detector is a MOXTEK AP3.3 window consisting of
ultra-thin polymer film (300 nm thick, density 1.4 g/cm3) and is supported by 38 µm thick rigid
silicon grid with 77% open area.

The SDD module equipped with the AP3.3 window provides an

X-ray transmission to the detector as low as B Kα X-ray energy (170 eV), i.e., our detector isapplicable to the XANES experiment at a boron K-edge.
not “helium-tight".

The AP3.3 window is vacuum-tight, but is

Therefore, it cannot be used in the helium path.

Practical training - 2
In the practical training-1, the participants will be experience that the detection limits of the TEY and
the TFY method is ~1%.
detector.

In the practical training-2, XAS is obtained by the PFY method with SDD

By selectively measuring the fluorescence signal from a specific element, it will be

experienced that the detection sensitivity is drastically improved.

8References[1] J. Stöhr, “NEXAFS Spectroscopy” (Springer, Berlin, 1992).
[2] J. G. Chen, Sur. Sci. Rep. 30, 1 (1997).
[3] T. Tanaka, et al., J. Synchrotron Rad. 5, 459 (1998).
[4] H. Ohashi, E. Ishiguro, Y. Tamenori, et al., Nucl. Instrum. Meth. Phys. Res. A 467, 533 (2001).
[5]

Y. Tamenori, J. Synchrotron Rad. 17, 243-249 (2010).

[6]

Y. Tamenori, M. Morita, and T. Nakamura, J. Synchrotron Rad. 18, 747-752 (2011).

[7] E. Gatt, and P. Rehak,. Nucl. Instu. and Meth. A 225, 608-614 (1984).

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