Beamline Practice at BL27SU
Soft X-ray photoabsorption spectroscopy using fluorescence yield method
Ver. 2010/09/27
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 alignment

inside a vacuum chamber, and data acquisition.

Soft X-ray absorption spectra will be measured bymeans of the total electron yield method, the total fluorescence yield method using a micro-channel
plate detector and the partial fluorescence yield method using a solid-state-detector.

2. Practice plan
9:15～

Introduction

9:30～

Guidance (beamline optics and control)

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)
17:00～ --------- 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 are usually divided into two energy regions: the X-ray Absorption Near
Edge Structure (XANES) and the extended 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 toelectronic 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.
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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 scattering 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.
‫ܫ‬௧ ൌ ‫ܫ‬଴e(ିఓ(ா)௫)(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. 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.
by the rotation of the grating.

Energy sweep can be done only

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
Experimental
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

Spherical
Mirrors

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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.
energy of photoelectron depends on the incident photon energy.

The kinetic

Secondly, following a core electron

excitation or ionization, an electron in a higher energy level falls into the core hole, and anotherelectron is eliminated using a released excess energy.
electron.

This second electron is called an Auger

In contrast to the photoelectron, the kinetic energy of Auger electrons is characteristic for

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

The photoelectron orAuger 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 bysuch 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.

The mean free path ofelectron strongly depends on the kinetic energy, and it is well known as a universal curve [1]. The
surface 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.

Alternatively, it is also possible to measure the TEY

spectrum by detecting the eliminated free electrons.
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In the later 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.

Onthe other hand, the trajectory of the free electron
can be controlled by electric and magnetic fields.
By using the energy filter to the incoming freeelectron, it is possible to detect a specific energy
Figure 2; Layout of the TEY measurement
using sample current method.

electron.

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].

5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 MCP. The
PIN photodiode detector is possible to use under the non-UHV condition and the detection sensitivity
is higher than the MCP detector.
visible light.

One drawback of the PIN photodiode is highly sensitive to the

For example, the visible lights that eliminated by a filament of ion-gauge and enterfrom a viewing-port make higher background.
possible by using MCP detector.

On the other hand, the pulse counting detection is

By discriminating each pulse signal, the data of a high S/B ration

can be obtained.

Figure 3 shows the TFY

analyzer using MCP.

A negative high voltage isapplied on the entrance of the MCPs to repel
electrons.

Further, a grid placed in front of thedetector.

The grid is used to prevent ions fromcoming into the MCPs. Then, the fluorescencecan exclusively be detected by the MCPs.
Figure 3; An example of the total fluorescence yield
analyzer using MCP.

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 MCP detector.
Participants will be experienced that the XAS measurement distinguishes the electronic states of theoxidized surface and the bulk of metallic sample.
66-6. Partial fluorescence yield
One major drawback of total fluorescence yield is its low S/B ratio to the measurement of
low concentration samples.

An alternative soft X-ray detector is an energy dispersive detector, suchas silicon-drift-detector (SDD) [5]. The energy of fluorescence X-ray is characteristic for eachelement as well as Auger electron.

Selective detection of X-ray fluorescence from element improves

the element selectivity of the XAS measurement.

For fluorescence measurements of concentrations

above 0.1%, the MCP or PIN photodiode is preferable, however, for lower concentrations the SDD
detector is a better.

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.

Reference[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] For example, http://www.amptek.com/

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