Atomic Excitation Exploited by Energetic-Beam
Characterization Methods
© C.Jeynes, G.W.Grime, 5th March 2012
University of Surrey Ion Beam Centre, Guildford GU2 7XH, England
An article for the COMMON CONCEPTS Chapter of the Wiley
Characterisation of Materials (2nd edition) on-line book
Abstract
Many disparate methods of compositional analysis of materials are underpinned
by the same fundamental atomic processes: the excitation of the electronic system of
the atoms followed by its subsequent relaxation. These methods include the electron
spectroscopies (XPS, AES) used for surface studies, the electron microscopies used for
elemental and structural characterisation (SEM using EDS and WDX; TEM using
EELS),
the X-ray methods (XRF, XAS) and ion beam analysis (PIXE) used for
elemental and chemical characterisation.
All rely on measuring the characteristic
energy absorbed or emitted by the unknown target atom when its electronic system is
excited by ionisation due to charged particles or electromagnetic radiation.
This
excitation is defined by the energy levels of the atomic electrons, determined primarily
by the atomic number of the atom. (Atoms can also be excited without ionisation, as in
optical and infra-red spectroscopy: this is outside the scope of this article.)
The theoretical description of the electronic structure of atoms is a major
intellectual triumph of the twentieth century and this body of knowledge is exploited in
the theoretical description of each of these methods, but the treatment of any particular
method is usually presented by specialists in that method in isolation from all others. In
this chapter we present a brief synthetic overview of materials analysis using atomic
excitation, highlighting those features and physical concepts which underpin all these
apparently disparate analysis methods. We hope to encourage modern analysts to
appreciate the truly complementary nature of the powerful methods at their disposal.
1
Contents
1
Introduction to the Techniques .................................................................. 4
2
Simple Introduction to the Atomic Processes ............................................ 8
2.1
3
Summary: extracting useful analytical information ........................ 11
Historical Introduction ............................................................................. 13
3.1
Early work ........................................................................................ 13
3.2
Detector Technology........................................................................ 14
4
Excitation Processes................................................................................. 16
4.1
Photon Excitation (XRF, XAS, XPS) .............................................. 16
4.2
Excitation by Electrons (EPMA, EELS, AES) ................................ 19
4.3
Excitation by Ions (PIXE) ................................................................ 20
5
Relaxation Processes................................................................................ 20
5.1
Fluorescence .................................................................................... 21
5.2
The Auger process ........................................................................... 21
5.3
Coster-Kronig and other effects ....................................................... 21
5.4
Summary .......................................................................................... 22
6
Energy loss mechanisms and Information Depth .................................... 23
6.1
Mass absorption coefficient: Photon Absorption (XRF, XAS) ....... 23
6.2
Particle Energy Loss (STIM, EELS, XPS, AES, EPMA, PIXE)..... 24
6.3
Information Depth ............................................................................ 25
7
Comparing the methods ............................................................................ 25
8
Depth profiling ......................................................................................... 29
9
Examples .................................................................................................. 30
10
Summary .............................................................................................. 31
11
Figures.................................................................................................. 35
References ........................................................................................................ 43
2
Tables and Figures
Table 1: Further Information on Techniques .................................................................... 5
Table 2a : Glossary of major techniques............................................................................ 6
Table 2b: Glossary of other terms. ..................................................................................... 7
Table 3 : Classifying Techniques by Probe and Resultant .............................................. 10
Table 4 : Classifying Techniques by Mechanism ............................................................ 33
Table 5: Information Depth ............................................................................................ 34
Figure 1: Atomic processes involved in AES, XPS, EELS, XAS, XRF, EPMA, PIXE. 35
Figure 2: Fluorescence yield for K- and L- shells as a function of atomic number. ...... 36
Figure 3: Henry Moseley's measurement of characteristic X-ray energies. ................... 37
Figure 4. STIM and PIXE analysis of Alzheimer's tissue ............................................... 38
Figure 6. XAS spectrum and explanation ....................................................................... 39
Figure 8. TEM-EELS chemical imaging ......................................................................... 40
Figure 7. M1 sub-shell ionisation cross-sections for protons to 5 MeV.......................... 41
Figure 5: Absorption cross-sections for 1 keV – 10 MeV photons in all elements. ....... 42
3
1
Introduction to the Techniques
Those materials scientists wishing to gain some overview of the bewildering
kaleidoscope of characterisation techniques might be forgiven for not noticing the
common principles underlying them. Practitioners of the individual techniques have
been – quite properly – concentrating on their own techniques,
and perhaps not
sufficiently describing the similarities and differences, overlaps and contrasts with
alternate and complementary techniques.
In this article we will directly address this with regard to the cluster of techniques
that make use of atomic excitation in one way or another. These techniques include the
apparently (and actually!) widely disparate : XRF, XPS, AES, EPMA (including EDS
and WDX on the SEM), PIXE, TEM-EELS, XAS, MeV-SIMS and STIM (see
Table 1 for details of where to find the main articles on these techniques in
Characterisation of Materials, and see Tables 2 for glossaries).
All of these methods: a) excite atoms with a probe beam, and b) follow one of
the resultant radiations for some characterisation purpose. The probe beam can be Xrays (photons), electrons or ions* and the resultant radiation can be X-rays, electrons, or
ions (see Table 3). Note that ions cannot be emitted from the sample as a result of
atomic excitation (except for the special case of MeV-SIMS), but must arise from
nuclear excitation (see PARTICLE SCATTERING,
and the ION BEAM ANALYSIS
chapter).
*
In this chapter we use the term ion to refer specifically to positively charged ions, atoms with fewer
electrons than their atomic number, though it should be noted that the actual effective charge of an ion
travelling in matter rapidly reaches an equilibrium value which may be different from that in the primary
beam at the surface. For the light ions extensively used in ion beam analysis, it can be assumed that the
ion is fully stripped of electrons when it interacts with an atom.
4
Table 4 summarises the variety of more or less complicated atomic excitation
processes which are utilised by the various techniques. We are concerned here only to
show the linkages (the similarities and differences) between the techniques; the reader
is directed to the articles indicated by the "Probe" column in this table (and see Table 1)
for much more detail about the techniques themselves.
There are many optical emission and absorption spectroscopies (atomic emission
spectroscopy and atomic absorption spectroscopy of their various kinds, see OPTICAL
IMAGING
AND
SPECTROSCOPY, INTRODUCTION) which are also atomic excitation
techniques and exploit the basic principles described here, but where the emitted or
absorbed energies are in the optical region (few eV) and only the outer electron shells
are involved. These methods will not be covered in this chapter.
Table 1: Further Information on Techniques
Technique
AES
EPMA
MeV-SIMS
PIXE
STIM
TEM-EELS
XAS
XPS
XRF
Cross-references to Characterisation of Materials articles
Chapter
Article
Electron Techniques
AUGER ELECTRON SPECTROSCOPY
Electron Techniques
ENERGY-DISPERSIVE SPECTROMETRY
Ion Beam Techniques
SECONDARY ION MASS SPECTROMETRY (AND TOTAL IBA)
Ion Beam Techniques
PARTICLE-INDUCED X-RAY EMISSION (AND TOTAL IBA)
Ion Beam Techniques
TOTAL IBA
Electron Techniques
SCANNING TRANSMISSION ELECTRON MICROSCOPY:
Z-CONTRAST IMAGING
X-Ray Techniques
XAFS SPECTROSCOPY
X-Ray Techniques
X-RAY PHOTOELECTRON SPECTROSCOPY
X-Ray Techniques
X-RAY MICROPROBE FOR FLUORESCENCE AND
DIFFRACTION ANALYSIS
5
Table 2a : Glossary of major techniques
Table 2b defines other acronyms.
These are categorised in Table 3 by incident and measured radiations and in Table 4 by mechanism.
Acronym
Name
AES
Auger electron
spectroscopy
EPMA
Electron probe
microanalysis
MeV-SIMS
Secondary ion mass
spectrometry with an
MeV primary ion beam
PIXE
Particle induced X-ray
emission
STIM
Scanning transmission
ion microscopy
TEM-EELS
EELS on the TEM
XAS
X-ray absorption
spectroscopy
XPS
X-ray photoelectron
specroscopy
XRF
X-ray fluorescence
Comment
Incident electron ionises atom which relaxes via the 3-electron
Auger process; results in emission of an electron with
characteristic energy. UHV technique sensitive to true surface
since EMFP is <~10nm. See also SAM, XPS.
Incident electron ionises atom which relaxes via the emission of
a characteristic X-ray. EPMA uses an SEM optimised for X-ray
analysis, often with multiple spectrometers, both energy
dispersive and wavelength dispersive. See PIXE and XRF for
excitation by (respectively) ions and photons. EPMA has a high
background on the X-ray lines due to primary electron
Bremsstrahlung.
High energy incident ion beams lose energy mostly through
interaction with the electronic lattice in the near surface region.
Gives rise to a gentle desorption process ("electronic sputtering"
involving a collective excitation) of surface molecules that
favours large molecular ions on insulating surfaces.
Incident ion ionises atom which relaxes via the emission of a
characteristic X-ray. Efficient ionisation is with ~3 MeV
protons or ~6 MeV alphas (comparable to ~1 keV electrons).
PIXE refers to ion beam excitation. See STIM. See also EPMA
and XRF for excitation by (respectively) electrons and photons.
PIXE and XRF have low background on the X-ray lines from
only secondary electron Bremsstrahlung.
As for EELS, the ions lose energy to the electronic lattice on
transmission through the sample.
Characteristic atomic
processes occur but cannot be resolved at these high primary
beam energies. Functionally analogous to X-ray radiography:
density maps are obtained using primary ion energy loss as a
contrast index. Ultra-low beam current (low damage) technique
frequently done sequentially with PIXE.
An auxiliary electron energy loss spectrometer can be used on
the transmission electron microscope to obtain inner shell
ionisation (and other) information from the sample.
Local structure determination in both disordered and ordered
materials. Needs an intense monochromatic X-ray source,
hence now done only on synchrotron light sources. This is a
cluster of techniques including XAFS (X-ray absorption fine
structure), EXAFS (extended XAFS), SEXAFS (surface
EXAFS), XANES (X-ray absorption near-edge structure) etc.
UHV technique sensitive to true surface since EMFP is <10nm.
1-electron process complementary to AES.
Incident photon ionises atom via the photoelectron. The atom
relaxes via the emission of a characteristic X-ray. See EPMA
and PIXE for excitation by (respectively) electrons and ions.
PIXE and XRF X-ray lines have low background only from
secondary electron Bremsstrahlung.
6
Table 2b: Glossary of other terms.
Acronym
Name
EDS or EDX
Energy dispersive X-ray
spectroscopy
EELS
Electron energy loss
spectroscopy
EMFP
Electron mean free path
ESCA
hr-PIXE
SAM
SEM
Electron spectroscopy for
chemical analysis
High (energy) resolution
PIXE
Scanning Auger microscopy
Scanning electron
microscopy
SEM-EDS
EDS on the SEM
SIMS
Secondary ion mass
spectrometry
static SIMS
sy-XRF
Synchrotron XRF
TEM
Transmission electron
microscopy
WDS or WDX
XAFS
XES
Wavelength dispersive
X-ray spectroscopy
X-ray absorption fine
structure
X-ray emission spectroscopy
Comment
Uses medium energy resolution (~140eV) semiconductor devices
as spectrometers with wide energy range. New superconducting
devices have a ultra-high energy resolution comparable to WDX
Usually now a TEM technique. Many energy loss mechanisms
include inner shell ionisation events. With ultra-high energy
resolution (>0.1eV) available the different forms of carbon are
easily distinguished. EELS is usually restricted to <~3keV.
In XPS and AES the information is carried by electrons with
characteristic energies, and the EMFP determines whether the
electrons can escape without scattering, hence with well-defined
energies.
Synonym for XPS.
Both WDX spectrometers and high resolution EDX detectors are
reported.
SEM with an electron spectrometer. See AES.
Versatile technique whose primary measurand is the secondary
electron yield which is informative about surface topography.
PIXE with electrons. Characteristic X-rays are observed (using an
auxiliary spectrometer) resulting from inner-shell ionisation events.
See EPMA in Table 2a. Typical electron energy up to ~30 keV
gives electron penetration depths ~5m or so with X-ray signals
available from this depth.
Uses a keV ion beam to sputter the surface of materials, with
sputtered ions (which can be positive or negative, and can be
molecular as well as atomic ions) detected and identified in a mass
spectrometer. For keV ions the sputtering process depends on
energy distributed to the surface through a nuclear collision
cascade.
SIMS at the so-called “static limit” where the primary ion fluence
is low enough for the sputtering not to change the sample
significantly. Static SIMS is a true surface technique where
“dynamic” SIMS is a depth profiling technique.
Synchrotron X-ray sources are accurately tunable as well as high
brightness, allowing chemically sensitive excitation of X-ray
fluorescence.
Versatile technique whose primary measurand is a projected phase
contrast image (in real or reciprocal space) of the sample in
transmission. ~200 keV beams require samples <~10m thick.
A crystal or grating spectrometer can be used to obtain very high
energy resolution (<1 eV) with limited energy range.
Important special case of XAS (q.v.)
General term for XRF, EPMA, PIXE etc.
7
2
Simple Introduction to the Atomic Processes
Figure 1 sketches the atomic processes involved in these techniques.
The starting point of all atomic excitation reactions is the stimulation of the
electronic shell structure by the electric field of a moving charged particle or photon
(Figure 1). This transfers energy from the stimulating radiation to the atom, leaving it in
an excited state. If the transferred energy is greater than the binding energy of an inner
shell electron then this can be ejected, leaving a vacancy in the shell – ionisation.
The
subsequent relaxation of the atom to the ground state may result in the emission of
photons or electrons. The magnitude of the energy transferred to the atom and the
energies of any emitted reaction products have a characteristic value which depends
primarily on the atomic number of the target atom. Measuring this provides the basic
analytical information in a wide range of analysis methods.
Perhaps the simplest methods in concept are those in which the characteristic
energy of the reaction product is measured, such as in XPS where an incident X-ray
beam directly produces a photoelectron whose energy is measured to determine the
identity of the emitting atom.
Analytical information can also be obtained by measuring the effect of the
interaction on the primary beam, though it can be more complex to extract it. For
example, in XAFS information is obtained about the structure of the neighbourhood of
the excited atom from the resonance effects (as a function of incident beam energy) of
the photoelectron on the absorption probability as it (the photoelectron) scatters from
nearby atoms. For EELS on the other hand, it is simply the characteristic energy loss of
the incident electron when it ionises the atom that is measured (although there are also
many other causes of electron energy loss).
8
The ionisation process is often shown as a collision that "knocks out" an inner
shell electron. This picture is of marginal utility, being actively wrong for positive
particles (ions) which tend to "suck out" electrons (opposite charges attract!). Fig.1
shows a more generalised view, valid for photons, electrons and ions. The incident
particle causes a transient electric dipole excitation of the electron wavefunctions of the
atom distorting the electron cloud relative to the nucleus.
For excitation by
electromagnetic radiation (photons) this is created by the electric vector of the photon.
The effects of the excitation depend in a complex way on the amplitude and duration of
the electric field, but large enough effects will result in the loss of one or more atomic
electrons – the ionisation event!
The probability of creating a vacancy in an electron shell (the ionisation crosssection) is related to the speed of the incident particle relative to the speed of the
electron in the shell. A proper treatment of this is quantum mechanical, but at an
intuitive level it can be imagined that the interaction time between the particle and the
electron is maximised if they have the same speed. Typical orbital velocities in inner
shells are around 10% of the speed of light. For electron beams this is achieved with an
energy of around 2.5 keV while proton beams need an energy around 4 MeV, which
explains why high energy ions are required for PIXE.
As an aside, this explanation also highlights why MeV ions are valuable for
focused beam applications. We are interested in an atomic excitations where energies of
several keV are transferred to the target atom; this is very large compared with the
energy of an electron beam, leading to a large deviation and energy loss of the electron,
but very small compared with the corresponding proton beam energy. In contrast,
protons are deviated by very small angles in each encounter, and lose little energy, so
9
can experience many collisions and penetrate long distances (tens of micrometres) in
effectively straight paths.
The ionised or excited atom is left in an unstable energy configuration and must
lose its excess energy and return to the ground state. There are only two branches for
this relaxation process.
The atomic shell structure rearranges,
losing the excess
(excitation) energy either radiatively with a resulting photon (XRF, EPMA, PIXE) or
non-radiatively with a resulting Auger electron (AES).
The probability of photon
emission is known as the "fluorescence yield". This is the same for all atoms regardless
of the process which created the vacancy, and is shown in Figure 2 as a function of
atomic number.
Table 3 : Classifying Techniques by Probe and Resultant
Techniques listed in bold face are those in which the effect of the
reaction on the primary beam is measured. In the other methods,
the energy of the resultant is measured.
Resultant
X-rays
XAS, XRF
X-rays
Electrons XPS
Ions
Probe
Electrons
EPMA
TEM-EELS, AES
Ions
PIXE
STIM, MeV-SIMS
The ionisation mechanisms are different for photons (XPS, XRF, XAS), for
electrons (AES, EPMA, EELS), and for ions (PIXE, STIM and MeV-SIMS). Photons
at energies and intensities normally used cannot give multiply ionised atoms, nor
(usually) can electrons. But ionisation with ions, especially heavy ions, can leave
atoms in very highly excited states; with significant complications in the emission
spectra readily observable using high resolution X-ray spectrometers. We should note
that the ionisation cross-sections are orders of magnitude smaller for photons than they
are for particles.
10
Ion resultants of ion excitation are included in Table 3 only for completeness.
The so-called electronic energy loss of ions in matter is one of the earliest observations
of atomic processes, heavily investigated during the development of the atomic theory
[1]. In principle STIM carries as much atomic information as EELS since the processes
are similar, but spectrometers with the eV energy resolution required to resolve this
information are not available for MeV particles. Through an electronic mechanism,
MeV-SIMS can result in the sputtering of a relatively high proportion of surface
molecular ions, where conventional (keV) SIMS has an atomic displacement (nuclear)
mechanism. But MeV-SIMS is the result of a collective process in which all atomic
information from the primary interaction between the ion and the target atoms is
obscured.
2.1 Summary: extracting useful analytical information
This has three aspects: identifying the atoms present in the sample, determining
their concentration, and determining their chemical state for the techniques capable of
high resolution (XPS, AES, sy-XRF, EPMA, hr-PIXE, EELS, XAFS).
Element identification is usually straightforward through the characteristic
energies absorbed or emitted in the process.
The following discussion relates
specifically to XES, but many of the considerations are applicable to all atomic
excitation techniques (see Figure 3). Characteristic X-ray emission energies can usually
be identified uniquely in a straightforward way. The difficulties are all special cases:
Each element emits several X-ray series (K, L, etc.) each containing several energies, so
X-ray line overlaps may give rise to well known analytical ambiguities (e.g., S K - Pb
M, Pb L - As K, Ba L - Ti K, Ti K - V K, V K - Cr K, Cr K - Mn K).
Semiconductor detectors generate spurious peaks (pileup and escape peaks) which can
11
create ambiguities such as the overlap of P K with the Ca K escape peak and Ni K
with Ca K pileup, both of which can affect, for instance, measurements in bone.
Secondary fluorescence, both from other regions of the sample and from other materials
in the system can be troublesome in some circumstances, especially where metallic
absorber foils are used to reduce the intensity of lines from major elements in the
sample.
In most cases the fact that each element emits several lines of similar energy
allows the ambiguities to be resolved, especially using modern spectrum processing
software.
Determining the concentration, or the number of atoms present, is less
straightforward and several physical effects must be quantified to use the atomic
excitation methods for accurate analysis: the ionisation cross-section of the atoms by
the primary beam, the fluorescence yield, and the absorption or energy loss of the
primary or emitted radiation within the material of the sample.
These depend on the
experimental parameters in a complex manner and need describing separately. These
effects are known collectively in EPMA as the “ZAF corrections” [2], that is: the effect
of Z (target atomic number) on the ionisation (and primary fluorescence) probability;
the effect of self-absorption on the final observed X-ray intensity; and the contribution
of (secondary) fluorescence to the observed X-ray intensity. Secondary fluorescence is
the XRF response of the sample to X-rays generated by the electron beam, and is
equally important in XRF and PIXE.
To these considerations need to be added the energy loss of the incident particles
in the sample, which have different treatments for photons, electrons and ions. The
ionisation physics is similar for the particle methods (PIXE, EPMA, AES, EELS), and
exactly the same for all the photon methods (XRF, XPS, XAS). Both particle and
12
photon methods share the fluorescence (or, equivalently, Auger) probabilities with the
other atomic excitation methods (EPMA, XRF, XPS, AES, PIXE). X-ray absorption
coefficients are also needed by the X-ray methods (XAS, XRF, XPS, EPMA, PIXE).
3
Historical Introduction
3.1 Early work
The relaxation mechanism of excited atoms is complicated. But the history of
atomic spectroscopy is very interesting and not well enough known. Christian Doppler
first proposed his eponymous effect as a means of detecting the motion of binary stars in
1842 [3], this was first observed (for sound, not light) by John Russell in 1844 [4].
Stellar spectroscopy was responsible for the discovery of helium in 1868 independently
by both Janssen and Lockyer [5]. Bohr's model of the atom [6] was a triumph in 1913
precisely because it solved the problem of the hydrogen Balmer lines (discovered by
Balmer in 1885 [7], generalised by Rydberg in 1888 [8] [9], and reviewed by Ritz in
1908 [10] including the newly discovered Lyman lines [11]).
Charles Barkla was responsible for the first recognition of characteristic X-ray
lines of elements, for which he received the 1917 Nobel Prize : it was in his 1911 paper
that he first named "X-ray fluorescence" (XRF), and introduced the "K" and "L"
notation : mid-alphabet letters being used since he expected both longer and shorter
wavelengths [12]!
In a landmark pair of papers rapidly following the publication of Bohr's model of
the atom,
Henry Moseley investigated the characteristic X-rays produced when
materials were bombarded with cathode rays (electrons). In his first paper [13] he
described the spectrometer (a crystal of potassium ferrocyanide), and pointed out that
his "elemental" targets were contaminated with impurities, saying, presciently: "The
13
prevalence of [X-ray] lines due to impurities suggests that this may prove a powerful
method of chemical analysis." In his second paper [14] he systematically measured Kand L-series wavelengths (see Fig.3),
the first use of a wavelength-dispersive
spectrometer (WDS). -PIXE was reported at the same time: by Chadwick in 1913
[15], and Thompson in 1914 [16].
The first XPS spectra were also recorded very early, by P.D.Innes in 1907 [17].
Of course, the photoelectric effect was discovered by Hertz in 1887 [18] and interpreted
by Einstein in 1905 [19] but Innes was the first to unequivocally energy analyse the
emitted electrons, interpreting them as due to atomic disintegration processes. It is
interesting that Innes confused atomic and nuclear disintegration processes, which is
perhaps not surprising since there was at that time no clear distinction between the atom
and the nucleus. High resolution XPS spectra were first published by Kai Siegbahn's
group in 1956 [20], and Siegbahn's work in establishing XPS as a useful analytical
technique was recognised with the 1981 Nobel Prize. The Auger relaxation process was
reported by Pierre Auger in 1925 [21], but as for the other techniques became usable as
an analytical technique only a generation later [22].
3.2 Detector Technology
The plethora of analytical techniques we have today were stimulated by the
development of the detector technology. We see repeatedly that new types of detectors
rapidly give birth to new methods of analysis.
Our overview here would not be
complete without some account of the technological history.
The
development
of
the
lithium-drifted,
silicon
detectors
(“Si(Li)”:
semiconductor solid state X-ray detectors using cooled FET pre-amplifiers) at the end of
the 1960s, gave a tremendous boost to X-ray elemental analysis, and also to the related
comprehension of details of the quantum electronic structure of atoms, frequently
14
needed for quantification purposes.
The first report of modern X-ray emission
spectroscopy (XES) using these detectors was by Bowman et al in 1966 using
radioactive sources. They reported Si(Li) detector energy resolution of 1.3 keV for Mn
K characteristic X-rays [23].
Energy dispersive spectrometry (EDS) analytical techniques rapidly emerged.
Electron probe microanalysis (EPMA) instruments acquired Si(Li) detectors with a
greatly improved energy resolution (<300 eV) [24]. Si(Li) detectors were also rapidly
fitted to scanning electron microscopes (SEMs), and applied to both X-ray fluorescence
(XRF) [25], and to PIXE by Johansson et al in 1970 [26] using proton beams from
small linear accelerators. The latter suggested that the trace-element detection limits of
PIXE could be as low as ng/g, and they analysed air pollution samples as an example.
This rapidly led to a report of the variation of trace metal concentrations along single
hairs [27].
Other highly cited examples using microbeam PIXE include measuring
concentration gradients of pollutants in aqueous systems [28] and measuring the absence
of Al in Alzheimer's disease samples [29] (see Figure 4). Today, the energy resolution
of Si(Li) detectors is close to its theoretical limit of about 120 eV (Mn K), and they are
widespread in many different fields of fundamental and applied sciences, especially in
SEM and XRF instruments.
Moseley used wavelength dispersive spectrometry (WDX) which is a high
resolution technique quite capable of picking up differences in the electronic structure of
the atoms due to different bonding states. This valence information is regularly used in
WDX-EPMA,
the electron spectroscopies (XPS, AES),
and the absorption
spectroscopies (EELS, XAS). It can also be used in PIXE if a high resolution detector is
used, which could be WDX [30] [31], or one of the new high resolution calorimetric
15
EDS detectors [32]. Of course, high resolution also allows disentangling of overlapping
peaks, which often occurs for the L lines [33].
It has become clear, using high resolution EDS detectors, that chemical state (or
the electronic environment of the atom) also significantly affects the relative intensities
of the families of transitions for each shell [34]. This effect is hard to demonstrate using
WDX detectors since the energy range of transitions for one shell usually far exceeds the
dispersion available (<400 eV) in any single measurement. But the superconducting
EDS detectors have relatively enormous dispersions (~15 keV). Thus, in the future the
valence state or atomic electronic environments might be probed not only using the
chemical shifts at the 1 eV level already well known from much basic work with ultrahigh-resolution electron spectrometers (XPS, AES, EELS, and with the equivalent
WDX photon spectrometry) but also from the relative intensities of lines which may be
separated by more than 1 keV using HR-EDS detectors.
4
Excitation Processes
Figure 1 shows the generalised atomic excitation process common to all
ionisation mechanisms. The details of the figure are for the case of PIXE, but photons
also ionise the atom in a similar way. Photon excitation is rather simpler than charged
particle excitation since the photon is either absorbed or not,
whereas in particle
excitation, some fraction of the particle’s kinetic energy is transferred to the atom
resulting in a slowing down and deflection of the particle.
4.1 Photon Excitation (XRF, XAS, XPS)
Photons ionise materials (excite atoms) when they are absorbed through the
photoelectric effect,
and the photo-ionisation probability (as represented by the
absorption) is visualised in Figure 5. The mass attenuation coefficient (
16
from the fraction of an X-ray beam
(initial intensity I0) that is absorbed during
transmission through an amount x of a material (measured in “thin film units” of mass
per unit area, or length*density). Note that the attenuation coefficient represents the
sum of the cross-sections for both the photoelectric effect which transfers energy from
the primary beam to electrons, and scattering of radiation out of the primary beam by
atomic electrons (Compton and Rayleigh scattering).
The energy absorbed by the
sample (that is, not including energy lost by scattering) is quantified by the mass
absorption coefficient, en.
The transmitted beam intensity I is given by:
ln (I / I0 ) = - x
(1)
Equation 1 determines the information depth of XRF (that is, the depth in the
sample from which information can be obtained, see §6.3) since the depth probed
depends on the energy of the probing beam, and the signal is always dominated by the
material closest to the surface. However, often quite penetrating beams are used so that
XRF can be used to analyse quite thick samples.
XAS works differently since it is the primary beam that is also detected. Thick
samples will attenuate the beam, but provided that there is enough signal the information
obtained is integrated over the whole sample thickness and chemical analysis can be
carried out by observing absorption edges. Again, due to the exponential absorption,
the entrance region where the probing beam has the highest intensity will have a greater
effect on the signal than the exit region.
 is a discontinuous function of X-ray energy. This is because absorption of the
X-ray suddenly becomes possible as soon as the photon energy exceeds the binding
energy of any particular atomic electron.
Therefore there are absorption edges
corresponding to the ionisation potentials of all the atomic sub-shells. Close to the
17
absorption edges the absorption shows a fine structure caused by quantum mechanical
interference effects, as shown in Figure 6. These depend on the structure of the valence
electrons of the atom, and so can be used to give information on the chemical
environment of the target atom and is exploited for this in XAFS using synchrotron
radiation.
In principle,
all atomic excitation techniques involve since every atomic
excitation has a probability of relaxing via a photon process,
and this X-ray
fluorescence will always ionise atoms within a certain distance (dependent on ) of the
first excitation creating additional resultant radiation.
Although this secondary
fluorescence can often be neglected it is always present and it is frequently important.
For the techniques involving photons as the measurand (XRF, EPMA, PIXE) the
absorption coefficient is also critical for the path of the X-ray towards the detector.
Therefore, in XRF is critical for both the primary and the detected radiation. (In XAS
of course it is the primary radiation that is detected.)
The overall accuracy of the absorption database is of continuing concern in
accurate work for all the techniques. How many photons survive at a given depth in the
material is clearly determined by (and very sensitive to) the value of .
All the
techniques require the photon intensity to be modelled as a function of path length
unless relative measurements are being made against sample-matched standards – a very
restrictive condition for the analyst.
The XCOM mass absorption coefficient
comprehensive values for 
database from NIST† includes
35] [36]. Work is continuing in
the community to make further critical measurements of this (and other) important
†
NIST: National Institute of Standards & Technology, Gaithersburg, the national metrology institute of
the USA
18
quantities. For example, the synchrotron group at the PTB, Berlin‡ recently used an
absolutely calibrated instrument to make a determination of mass attenuation
coefficients for Al [37] relative to previous values [38], finding internal inconsistencies
in them of up to 10%. EXSA§ has promoted the "International initiative on x-ray**
fundamental parameters" which is coordinating efforts by all of the PIXE, XRF and
EPMA communities to improve the various databases [39].
4.2 Excitation by Electrons (EPMA, EELS, AES)
EPMA spectra are usually fitted using a code based on the general semiempirical
determination in the 1980s of the depth distribution of the X-ray excitation function due
to the collision cascade of the electron beam. This utilises both Monte Carlo calculations
and an extensive series of measurements of tracer layers of one material in another
together with self-supporting thin layers [40] [41] [42].
This work built on and
systematised a considerable body of earlier work. The absolute accuracy with which
this excitation function is estimated depends on the uncertainty in the tracer layer
thicknesses (3%) and the demonstrable accuracy of the Monte Carlo (also about 3%).
The major difficulty with modelling electron excitation is in determining the size
and shape of excitation volume, which is usually described as a ‘tear-drop’ or ‘pearshaped’. The effective depth and diameter is determined by beam scattering within the
sample and depends primarily on the incident electron beam energy (and not the beam
diameter at the surface!). The average electron energy, and hence ionisation crosssection, depends on position in the excitation volume, thus an integration over the
whole excitation volume is needed to get the total excitation probability. The path
‡
§
**
PTB: Physikalisch-Technische Bundesanstalt, Germany’s national metrology institute
EXSA: European X-ray Spectrometry Association
Properly, "X-ray" is capitalised, since the "X" is an abbreviation for the proper name Röntgen.
19
length to the detector of a generated X-ray, and hence the probability of detecting the Xray, is also a function of the position in the excitation volume that the X-ray was created,
thus the absorption correction (which is more difficult to calculate than the mass or
fluorescence corrections) is obtained by integration of the excitation function. This was
critically compared to an extensive measurement database and found to have an
uncertainty of about 5% for ‘light’ elements (such as Ca and P). Thus, the total
systematic uncertainty for EPMA is estimated as 7% [43].
4.3 Excitation by Ions (PIXE)
For ions, semi-empirical models for the calculation of the probability of K-shell
ionisation have been established by Helmut Paul and co-workers [44]. L-shell ionisation
has been similarly determined by Miguel Reis and co-workers [45] [46] (see also [47]).
Reliable data for M-shell ionisation is not currently available in the same form, but
Figure 7 shows working values obtained by interpolation and extrapolation from
ECPSSR
††
calculations (Campbell et al [48]). Ionisation processes for ion beams can
become very complicated for heavy ions: these results are mostly for protons and He.
5
Relaxation Processes
The excited atom can relax by either radiating a photon (fluorescence) or by
emitting an Auger electron. The branching probability of fluorescence relative to Auger
relaxation is illustrated in Figure 2 and has been calculated by Chen & Crasemann using
ECPSSR theory for the K-shell [49], L-subshells [50] and M-subshells [51]. There are
also extensive experimental data for the L-shell transition probabilities which have been
critically reviewed by Campbell [52] [53].
††
ECPSSR: Energy-loss Coulomb-repulsion perturbed-stationary-state relativistic theory
20
5.1 Fluorescence
The simplest relaxation mechanism to describe is when an outer shell electron
occupies the vacant state resulting from the ionisation process. That is, the energy lost
by the outer shell electron is carried away by a photon with an energy equal to the
difference of the initial and final energy states – the characteristic X-ray. Quantum
selection rules apply, so that transitions between some levels are forbidden. After the
emission of the X-ray the atom remains ionised, since the outer shell now has the
vacancy and similar relaxation mechanisms can occur until the atom is fully relaxed.
Thus, relaxation involves a cascade of processes, starting with the most energetic. The
less energetic processes are not usually observable.
5.2 The Auger process
In the Auger process the energy lost by the electron transition from the outer
shell to the inner shell vacancy resulting from the ionisation process is given, not to a
photon but to another electron in one of the outer shells, which is emitted. Again,
quantum selection rules apply. As in the fluorescence process, the atom remains
ionised and will continue to relax with progressively less energetic processes.
Auger electron energies are equal to the corresponding characteristic X-ray
energy corrected for the binding energy of the two electrons involved. Because it is a
secondary process, Auger electron spectroscopy is rather more complicated to model
than photoelectron spectroscopy.
5.3 Coster-Kronig and other effects
The L-shell fluorescence yield is affected by the existence of the Coster-Kronig
(CK) transitions, which greatly complicate the modelling of this phenomenon, so that
there is still a heavy reliance on experimental measurements in this area [54]. CK
21
transitions are another class of nonradiative transition that transfers the vacancy from the
initial subshell to a higher subshell within the same shell; that is, a re-arrangement of
the electronic structure of the excited atom. The energy balance is preserved by the loss
of outer shell electrons with appropriate energies; of course, quantum mechanical
selection rules apply as usual in all these electronic structure re-arrangements.
There are very many CK transition probabilities to be determined, which can
have a large effect on the relative X-ray intensities in the L and higher series; these
intensities are therefore hard to determine accurately,
with large uncertainties
remaining. Campbell and co-workers have given semi-empirical fitted data for the K
[55] and L [56] series. Chen & Crasemann long ago calculated the relative line
intensities for the M series [57]; this remains the best dataset available, since good
experimental data for M-lines are hard to obtain. Therefore uncertainties for M-lines
remain high.
Recent work on L-lines using very high energy resolution EDS detectors has
underlined the complexity of this process, and also the – potentially large – gaps in our
understanding of it [58]. Not only can chemical effects strongly affect relative line
intensities, but second order effects can (as for the X-ray diffraction form factor) relax
the selection rules so that "forbidden" transitions (radiative Auger satellites in this case)
are in fact observed.
5.4 Summary
The relaxation mechanism of the excited atom is far from simple. Nevertheless,
the first event in this process, illustrated by Figure 2, is the initial simple (binary)
possibility of relaxation either by fluorescence (yielding characteristic X-rays) or by the
Auger effect (yielding energetic Auger electrons).
22
After the atom is ionised it can return to its ground state in a large number of
different ways,
often generating a whole cascade of X-rays, optical photons and
electrons at a variety of energies. The most important is the most energetic transition
which is well understood in so far as it is a purely atomic effect. But the atomic
environment (the chemistry, involving the outer electron shells) can have a significant
(sometimes large) effect on the details of the transitions observed. This means that
characterisation techniques based on atomic excitation have sensitivity not only to the
elemental composition of the sample but also to its chemistry.
The relaxation process is independent of the initial excitation mechanism, thus
developments in this area are applicable to many analysis techniques.
6
Energy loss mechanisms and Information Depth
The mechanisms discussed in this section determine the effect of the sample
matrix material on both the primary and resultant beams (attenuation of radiation or
energy loss and scattering of particles) as they travel to and from the target atom, which
is assumed to be located at some depth beneath the sample surface. Thus they determine
not only the quantitative yield of the process but the depth within the sample at which a
particular atom can be detected (information depth).
6.1 Photon Absorption (XRF, XAS)
Photon absorption behaviour was discussed above in the context of photon
excitation (§4.1, and see particularly Eq.1). Away from absorption edges absorption is
roughly proportional to the electron density in the photon path.
In XRF and XAS the probing X-ray beam is attenuated as it penetrates the
material. Of course the same is also true for XPS, but is not significant since the
photoelectrons can only escape without scattering from a very thin surface layer.
23
But for all the X-ray emission techniques (XRF, EPMA, PIXE) the detected Xray beam is also attenuated in exactly the same way on its path to the detector. Since the
penetration depth of the primary beam can be large for these techniques, even EPMA,
absorption can be a significant effect, especially for low energy X-rays. Clearly, any
quantitative work must take this into account.
6.2 Particle Energy Loss (STIM, EELS, XPS, AES, EPMA, PIXE)
A particle beam will lose energy inelastically by exciting atoms as it passes
through the sample. If the sample is thin enough to allow transmission of the beam and
a detector is placed behind it, then the average energy of the detected particles will be
determined by the average sample thickness and composition along the beam path. If an
electron beam is used (TEM-EELS) then the primary electron energy spectrum can be
analysed at high energy resolution to detect atomic excitations characteristic of elements
in the target. This is the inverse of AES or EPMA where the same electron excitation is
detected by the resulting Auger electron or X-ray emission. If an ion microbeam is used
(STIM), then an image of the sample density can be built up by using the mean energy
loss at each scan pixel as a contrast index. This is the ion analogue of X-ray radiography.
The energy loss process for electrons or ions is mostly due to scattering by outer
shell electrons (which are the most numerous). In principle these are also atomic
excitation and ionisation processes, but they are not treated as such since they do not
usually result in radiations of detectable energies escaping from the sample and the
energy is lost to heating of the lattice. However, since the inner-shell ionisation crosssections are strong functions of ionising particle energy it is important to know the rate
at which particles lose energy in the targets.
For ions there has been a huge
experimental and theoretical effort extending over the last century to determine the
energy loss of any ion beam in any target: started by William Bragg in 1905 [ref.1] and
24
now summarised in the SRIM‡‡ website [59] [60].
Electrons are treated rather
differently, but their energy loss in matter is equally well understood [61].
6.3 Information Depth
The expected depth from which information can be extracted by the various
techniques is summarised in Table 5. Depth profiling itself is discussed in §8.
In most of the techniques the information depth is controlled by the resultant
signal which is observed. XPS and AES are the prime examples of this: the excited
depth is microns (many microns in the case of XPS), but unscattered electrons can
escape only from the near surface (with an escape depth given by the EMFP). MeVSIMS is similar in that “sputtered” ions must arise from very close to the true surface.
For the transmission techniques (XAS, EELS, STIM) the primary beam (which
also carries the analytical information) has to penetrate the sample. XRF, EPMA, PIXE
are intermediate cases. For XRF a greater depth is always excited than signal can escape
from, since the fluorescent X-rays must be of lower energy than the primary beam. For
EPMA the excitation depth is given by the primary (electron) beam energy, and is in
general small compared with the X-ray absorption length. For PIXE, both primary ion
energy loss and X-ray absorption have a strong effect on the observed yield and it is not
possible to generalise about information depth.
7
Comparing the methods
It is important for analysts to appreciate that there are several analytical
techniques involving exactly the same atomic relaxation and X-ray absorption physics.
The excitation processes are similar (electric dipole excitation), but have to be treated
‡‡
SRIM: “The stopping and ranges of ions in matter”
25
separately to give the quantitative detail required for analytical purposes. Tables 3 & 4
compare the various techniques: Table 3 classifies the techniques by probe beam and
resultant signal, and Table 4 classifies them by mechanism. It is worth pointing out
again that atomic ionisation with ions has complexities not present for the simpler
ionisation with electrons or the photoionisation processes since multiple ionisation states
are much more probable.
Except that the excitation is via photo-ionisation instead of particle impact, the
physics of the X-ray fluorescence (XRF) technique and PIXE are similar, with similar
spectra, and detection limits only somewhat worse due to the presence of background
radiation originating directly from the exciting beam. Desktop XRF instruments using
X-ray tube sources are in wide use, but these do not give monochromatic beams and
calibrating the tube spectra is difficult. With synchrotron XRF a tunable monochromatic
source is available. This allows the ionisation of selected elements to be ‘turned on or
off’ by exciting above or below their ionisation potential, making this an exceptionally
powerful analytical tool.
However,
at present XRF mapping is still rather slow
(scanning the sample, not the X-ray beam), and XRF spectra give no direct access to
depth profiling, although other techniques such as XAFS and X-ray diffraction can be
carried out (sequentially) in the same installation. On the other hand, microbeam PIXE
is much easier to implement than microfocussed XRF, and mapping using a scanned ion
beam is very convenient.
Because the information depth for XRF is complementary to that of PIXE, there
is sometimes an advantage in an XRF/PIXE analysis [62] [63], as illustrated by the
results from the Alpha Particle X-Ray Spectrometer (APXS) of the NASA Mars
Exploration Rover missions. This generates mixed XRF/PIXE data, the analysis of
which is a tour de force that has established the presence of hydrated minerals on Mars,
26
an extraordinarily important result [64].
Note that PIXE will always excite secondary
fluorescence by XRF.
Electron-probe microbeam analysis (EPMA) is a scanning electron microscope
(SEM) method specialised for X-ray analysis. Typically, an EPMA instrument will have
both EDS and WDX detectors. Both EPMA and SEM-EDS (like XRF) have similar
physics and observed spectra to PIXE: the only difference is that in this case the
excitation is via electron impact rather than ion impact.
But SEM methods have
important analytical differences from PIXE. Electrons are 2000 times lighter than
protons, and the broad spectrum primary Bremsstrahlung X-ray background for protons
is negligible. Therefore detection limits for SEM methods are orders of magnitude
worse than for PIXE.
Also, because of the large lateral straggle for electron beams and for SEM
energies (usually <30 keV) and thick targets, the excitation volume for electrons is
determined by the electron energy and not by the probe diameter. But proton beams
penetrate to large depths from which few X-rays escape. So the excitation volume for
protons is determined essentially by the probe size. If ion microbeams can be built with
sub-micron spot sizes (see §6.4), then PIXE maps will have better spatial resolution
than SEM-EDS X-ray maps.
SEM-EDS and PIXE are similar in that for both techniques there is a
backscattered particle available. For PIXE it is the backscattered ions, which contain
composition and depth information in the energy distribution, and which can be
interpreted with very high (traceable) accuracy (Jeynes et al, 2012)[65]. Modern SEMs
usually have a BSE detector for ‘Z-contrast’, imaging sample regions with strongly
differing atomic number. The SEM-BSE signal cannot be treated quantitatively without
27
great difficulty (Monte-Carlo methods are needed), and therefore, as with XRF, any
depth profile is not accessible directly with SEM methods.
EDS detectors are also often installed on TEM instruments. In this case the
samples are always thin, but their absolute thickness is usually hard to obtain and so the
X-ray spectra are rarely treated quantitatively. However, modern TEM instruments
often also have an EELS attachment. This is the inverse process to AES: in EELS the
effect of the atomic structure (including chemical effects) on the transmitted electron
energy is observed.
The electron spectroscopies (XPS and AES) excite the atom with photons and
electrons respectively, energy-analysing the electrons resulting from atomic relaxation.
XPS is a one-electron process with the photoelectron observed directly. AES is a
process involving at least three electrons, which occurs when the atom relaxes nonradiatively. Of course, Auger electrons are also observed in XPS spectra. Because the
energy resolution available is high (<1 eV) chemical effects are readily observed.
Software for processing spectra from any of the X-ray emission spectroscopies
can be considered to have three components: modelling the excitation process (requiring
the ionisation cross section and the matrix effects on the primary beam), modelling the
fluorescence and detection processes (requiring the fluorescence yield, X-ray absorption
correction, secondary fluorescence correction and the detector response function) and
spectrum fitting (processing the spectra to extract peak areas in the presence of
background and spurious responses).
The second two are common to all methods, so
that in principle, X-ray spectrum processing software could be used interchangeably
provided that the initial ionisation processes can be modelled correctly. In practice,
software for EPMA has to take account of the pear shaped excitation volume rather than
the linear excitation path of XRF and PIXE, and so is less easy to generalise.
28
The X-ray absorption spectroscopies [66] should also be mentioned (see Fig.6).
These include XANES, EXAFS and NEXAFS as synchrotron techniques, and are
analogous to EELS in the same way that XRF is analogous to AES.
These techniques are frequently used in conjunction with others. For example, a
recent review of methods to visualise spatial distributions and assess the speciation of
metals and metalloids in plants addressed: histochemical analysis, autoradiography,
LA-ICP-MS§§, SIMS, SEM-EDX, PIXE,
XRF,
XAS,
and differential and
fluorescence tomography [67].
8
Depth profiling
None of these characterisation techniques gives direct information about the
depth within the sample of the target atom although of course the intensity of any signal
is strongly modified by the effects of the sample matrix on both the primary beam and
the resultant photon or particle. To access this information, the profiles must be inverted
from the spectra, and this problem is in general mathematically ill-posed.
If the sample structure is known a priori, then the structure parameters (major
element concentration and thickness for a sufficient number of layers to approximate the
sample) can be used as an input to the software to allow the matrix correction to be
calculated. This is routine for XRF and EPMA, for which software is commercially
available. The same thing is also frequently done in PIXE. But if the sample structure
is not known then single (XRF, EPMA, PIXE) spectra are almost always ambiguous
unless assumptions are made (e.g., the sample is a flat homogenous slab, all elements in
the sample are visible in the spectrum, with or without stoichiometrically bound oxygen,
etc). For PIXE of course, simultaneously collected particle scattering energy spectra
§§
Laser ablation inductively coupled plasma mass spectrometry
29
always carry direct depth profile information; this whole subject has recently been
thoroughly reviewed [68]. An equivalent facility is present in the SEM which routinely
use backscattered electron (BSE) detectors for Z contrast imaging: however, BSE
energy spectra are heavily complicated by the presence of multiple scattering and are
(almost) never used for determining depth profiles.
XPS has exactly the same problem of being insensitive to concentration profiles
in the sensitive depth, and the same systematic solution applies in principle to all the
techniques, although only XRF, PIXE and XPS routinely use them. That is to collect
spectra from the same sample under two or more different experimental conditions and
use the differences between the spectra to infer the depth profile. "Angle-resolved XPS"
has received careful attention [69], and "differential PIXE" can vary either the beam
energy or its angle of incidence [70] [71]. In principle AES is the same in this respect
as XPS, but the instruments are optimised in a different way and "angle-resolved AES"
is not used.
9
Examples
Two examples will serve to illustrate the range of ways atomic excitation
techniques can solve important materials problems. The first shows the use of X-ray
fluorescence methods, and the other shows electron spectrometry: both of them show
exemplary use of complementary techniques.
Fig.4 shows STIM/PIXE/EBS maps of brain tissue in an important study from
1992 which ruled out the presence of aluminium in brain tissue from Alzheimers
patients at levels greater than 15 mg/kg. The difficulty with previous studies is that the
plaques characteristic of the disease are almost impossible to see optically without
staining. But using STIM they can be easily visualised. Notice that in this case the
30
contrast with STIM is very much larger (with orders of magnitude smaller beam
fluence) than for PIXE.
There is currently great interest in possible routes to the silicon laser, and the
example shown in Figure 8 uses EELS to identify Si (Fig.8b), Er (Fig.8c) and O
(Figs.8d & 6e) in TEM images of nanostructures implanted in SiO2. The inset shows the
EELS peak for the O K edge which has a split edge structure for rare earth oxides, for
which a single Gaussian envelope will yield a lower peak energy and larger FWHM than
a Gaussian fit to the single O K peak of pure SiO2. Hence the TEM-EELS can image all
of Er, Si, and O in this sample. Intense photoluminescence is observed from this
sample and the analysis demonstrates that this is consistent with Er-O complexes
decorating the surfaces of crystalline Si nanoparticles.
10 Summary
We have compared and contrasted electron spectroscopies (XPS, AES) used for
surface science, electron microscopies (TEM-EELS, SEM-EDX, EPMA) used for a
wide variety of materials characterisation,
and X-ray (XAS, XRF) and ion beam
analysis (PIXE) methods used for characterising thin or thicker films (~100 m).
These all make use of the physics of atomic excitation, and they frequently
overlap – sometimes quite strongly – in their capabilities and applicability. The most
obvious example of this is the XRF/PIXE analysis of the Mars Rover data referred to
previously [72] [73].
We should also point out that most of these techniques are themselves regularly
used in conjunction with complementary methods:
TEM and SEM are imaging
techniques for which the X-ray and electron spectroscopies are only add-ons, the X-ray
techniques include a wide variety of diffraction and other methods; and ion beam
31
analysis includes nuclear reactions, elastic scattering, and SEM and SIMS methods as
well as PIXE.
Analysts,
methods,
and materials scientists making use of modern characterisation
should appreciate underlying similarities between techniques which may
appear to be unrelated, and they should also appreciate the complementary capabilities
of these techniques. We hope that this discussion of a set of techniques that are not
usually described together may help to nurture this appreciation.
Acknowledgements
We are grateful for help from Miguel Reis (Lisbon) and Elke Wendler (Jena).
Key References
This is a article intended as a new synthetic overview, and treating the subject in
a way which has not been published in this form before, drawing together information
from all the analytical techniques. For more detailed information please see the articles
cited in Table 1 and the key references therein.
32
Table 4 : Classifying Techniques by Mechanism
Mechanism
Technique
Probe
Measurand
X-ray
Indirect Photoelectron
XAFS
Synchrotron
X-ray
Indirect Ionisation
TEM-EELS
Electron
Ionisation, then relaxation
via Auger process
AES and SAM
Electron
Auger electron spectroscopy
XRF
X-ray
(Secondary) X-ray spectroscopy
PIXE
Ion
X-ray spectroscopy
EPMA
and
SEM-EDS
Electron
X-ray spectroscopy
Ionisation, then relaxation
via photon process
Comment
(values given are only indicative)
XPS (ESCA)
Direct Photoelectron
Purpose
Photoelectron spectroscopy
Primary X-ray absorption
spectra obtained by scanning
monochromatic primary beam
energy
Primary electron energy loss
spectra
Elemental
and
chemical
composition at surface (~10 nm)
Surface technique dependent on energy
analysing the photoelectron. Profiling
<1m with sputtering
Local
structure
in
the
neighbourhood of each element
averaged in thickness of sample
"Bulk" technique dependent on
interpreting the absorption spectra
Elemental
composition
averaged in thickness of sample
Elemental
(and
chemical)
composition at surface (~10 nm)
Elemental composition
surface ( < ~100 m)
Elemental composition
surface ( < ~20 m)
near
Elemental composition
surface ( < ~4 m)
near
near
"Thin film" (TEM sample) technique
recognising the absorption edges
Surface technique dependent on energy
analysing the Auger electron. High
lateral resolution by SAM. Profiling
<1m with sputtering
"Thin film" technique dependent on
recognising the characteristic X-rays.
Chemical (valence state) information
available, as for XPS and AES, from
chemical shifts in high resolution
spectra. Very high chemical
discrimination possible with (tunable)
synchrotron XRF.
33
Table 5: Information Depth
Technique
Information
Depth
Controlled
By
XPS
< 10 nm
Electron mean free
path
XAS
10 m – 1 mm
Energy and intensity
of the primary beam
XRF
10 – 200 m
Both primary and
detected X-ray
energies
AES
< 10 nm
Electron mean free
path
EPMA
< 1 – 5 m
Energy of the
primary beam
TEMEELS
< 1 m
Energy of the
primary beam
PIXE
~ 10 – 20 m
Both primary ion and
detected X-ray
energies
MeV-SIMS
~ 10 nm
Inelastic (electronic)
energy loss processes
STIM
~ 20-50 m
Energy of the
primary beam
Comment
Thicker samples regularly analysed using
sputter depth profiling
The effect of the whole thickness is
averaged. Thicker samples can be analysed
with more intense (synchrotron) beams
The whole excited volume (cylinder)
contributes to the line intensities. Depth
profiles cannot be unfolded from the spectra
without a model.
Thicker samples regularly analysed using
sputter depth profiling
The whole excited volume (pear drop)
contributes to the line intensities. Depth
profiles cannot be unfolded from the spectra
without a model.
The effect of the whole thickness is
averaged.
The whole excited volume (cylinder)
contributes to the line intensities. Depth
profiles can be unfolded from the spectra
without a model using particle scattering
signals.
The primary (fast heavy ion) beam is highly
damaging, so this must be treated as a static
SIMS technique
The effect of the whole thickness is
averaged.
34
11 Figures
+
+
+
-
free electron
+
+
E
+
e-m radiation
Figure 1: Atomic processes involved in AES, XPS, EELS, XAS, XRF, EPMA, PIXE.
Top: Ionisation can occur with an electron beam (AES, TEM-EELS, EPMA)
or a photon beam (XPS, XAS, XRF) or an ion beam (PIXE: this is shown). In all
cases the entire electronic structure is subjected by the incident radiation to a electric
dipole excitation which results in the loss of electron(s). Bottom Left: The excited
atom relaxes in a way consistent with the quantum selection rules; Bottom Right: the
2-electron Auger relaxation process is shown (AES is called a "three-electron
process" because of the primary electron), but the relaxation can also be radiative.
Bottom row modified from the Wikipedia article on Auger Electron Spectroscopy
[74]
35
Figure 2: Fluorescence yield for K- and L- shells as a function of atomic
number.
The Auger electron yield is the complement of the fluorescence yield. K-shell
values are modified from Bambynek et al [75]; the L-shell values are from Cohen
[76]. Reproduced from Johanssen & Campbell, Fig.1.1 [77]
36
Figure 3: Henry Moseley's measurement of characteristic X-ray energies.
Adapted by R.Nave [78] from Fig.3 of Moseley (1914) [ref.14]
37
Figure 4. STIM and PIXE analysis of Alzheimer's tissue
Scanning 3 MeV proton microbeam STIM, PIXE and elastic backscattering
(EBS) images (100 m x 100 m) from sections of unstained post-mortem tissue of a
patient suffering Alzheimer's disease. The spot size was 1 m x 1 m. Above: STIM
map of region containing neuritic plaque; Below: Maps of the same area for P & S
(PIXE), and C & N (EBS). Reproduced from Fig.2 of Landsberg et al, 1992 [29]
38
Figure 6. XAS spectrum and explanation
X-ray absorption spectrum of potassium tetracyanoplatinate K2[Pt(CN)4] near
the Pt LIII edge showing constructive and destructive interferences in the NEXAFS
signal (figure drawn by Farideh Jalilehvand, University of Calgary, and reproduced
from [79])
39
Figure 8. TEM-EELS chemical imaging
(a) High-angle annular dark field (HAADF) image of the mapped area of a
silica sample co-implanted with Si and Er and annealed to form Si nanocrystals
decorated with Er-O complexes, (b) Si L2,3 (98–101eV) and (c) Er N4,5 (167-177eV)
edge intensity maps. (d) FWHM and (e) peak energy of the Gaussian fits to the O K
edge (inset: blue=nanocluster positions, red=away from any nanoclusters). Dashed
circles and rectangles indicate correlations in the features of the images and the
contrast of the maps corresponds to a colour scale, which represents the linearly
normalized image intensity in arbitrary units. Reproduced from Fig.3 of Crowe et al,
J.Appl.Phys. (2010) [80]
40
Figure 7. M1 sub-shell ionisation cross-sections for protons to 5 MeV.
Reproduced from Fig.2 of Campbell et al, 2010 [81].
41
Figure 5: Absorption cross-sections for 1 keV – 10 MeV photons in all elements.
(Note: absorption is related to photo-ionisation). K, L, M, and N absorption
edges are also shown. From Wikimedia Commons: Photon_Cross_Sections.png and
the Wikipedia article Absorption cross section (downloaded 7th November 2011).
Figure prepared from code by Jaroslaw Tuszynski using his Matlab
module
PhotonAttenuation2 which accesses the NIST XCOM database.
42
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