2. WELCOME TO YOU ALL
PHYSICAL
CHEMISTRY
PRESENTATION
M.SC.-II –
(SEM. III )
2013–
2014
ELECTRON SPECTROSCOPY
Presented by :–
PAPER- II
Dharmendra R. Prajapati
RAMNIRANJAN JHUNJHUNWALA
COLLEGE
4. Electron Spectroscopy:The signal produced by excitation of the analyte consists of
a beam of electrons (rather than a beam of photons).
Excitation by X-ray --XPS (X-ray photoelectron Spectroscopy)
ESCA (Electron Spectroscopy for Chemical Analysis)
Excitation by UV radiation ---UPS (Ultraviolet photoelectron Spectroscopy)
Excitation by e beam ---AES (Auger electron spectroscopy)
SAM (Scanning Auger Microscopy)
Powerful tool for the identification of all of the elements
except H and He Provide information for surface layer (20-50
A) of solid.
6. X-ray photoelectron spectroscopy works by irradiating a
sample material with monoenergetic soft x-rays causing
electrons to be ejected.
Identification of the elements in the sample can be made
directly from the kinetic energies of these ejected
photoelectrons.
The relative concentrations of elements can be
determined from the photoelectron intensities.
7. Introduction (XPS) Analysis capabilities
Introduction (XPS) Analysis capabilities
Elements detected from Li to U.
None destructive (some damage to x-ray beam sensitive
materials)
Quantitative.
Surface sensitivity from 5 to 75 angstroms.
Conducting and insulating materials.
Detection limits that range form 0.01 to 0.5 atom
percent.
Spatial resolution for surface mapping from >10 mm
Depth profiling capabilities.
8. ESCA (also known as X-ray
photoelectron spectroscopy,
XPS) is based on the
photoelectron effect.
A high energy X-ray photon
can ionize an atom, producing an
ejected free electron with kinetic
energy KE:
KE = hυ − BE
hυ
Al Kα , hυ = 1486.6 eV )
BE=energy necessary to remove a
specific electron from an atom.
BE ≈ orbital energy
=photon energy (e.g., for
9. Instrumentation:
How are measurements made?
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Essential components:
Sample: usually 1 cm2
X-ray source: Al: 1486.6 eV;
Mg 1256.6 eV
Electron Energy Analyzer: 100
mm radius concentric
hemispherical analyzer; vary
voltages to vary pass energy.
Detector: electron multiplier
(channeltron)
Electronics, Computer
Note: All in ultrahigh vacuum
(<10-8 Torr) (<10-11 atm)
12. APPLICATION:APPLICATION:ESCA can be used to detect all elements except hydrogen and
helium, with a sensitivity variation across the periodic table.
It is most useful for solids, including powders and soft materials.
The qualitative and quantitative chemical state analysis
capabilities, combined with extreme surface sensitivity (usually a
few atomic layers) have made ESCA the most broadly applicable
surface analysis technique today.
Qualitative Analysis
low-resolution wide scan ESCA spectrum (survey
spectrum) elemental composition except H and He
Kinetic energy range 250 to 1500 eV
Binding energy range 0 to 1250 eV
Often peaks resulting from Auger e are found in ESCA
spectra, such peak are identified by comparing spectra
produced by two X-ray sources
13. AdvAntAges:-- surface sensitive (top few monolayers)
-- wide range of solids
-- relatively non-destructive
disAdvAntAges:-- expensive, slow, poor spatial resolution,
requires high vacuum
15. Auger Electron Spectroscopy
• Auger Electron Spectroscopy (AES), is a widely used
technique to investigate the composition of surfaces.
• First discovered in 1923 by Lise Meitner and later
independently discovered once again in 1925 by Pierre Auger.
Lise Meitner
Pierre Victor Auger
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16. Principles of AUGER:Auger electron spectroscopy (AES; pronounced [oʒe] in French) is a
common analytical technique used specifically in the study of surfaces
and, more generally, in the area of materials science.
Underlying the spectroscopic technique is the Auger effect, as it has
come to be called, which is based on the analysis of energetic electrons
emitted from an excited atom after a series of internal relaxation events.
PRINCIPLES OF OPERATION
(Auger Electron Spectroscopy)
• sample bombardment by electrons
•core electron removed
• electron from a higher energy level fall into the
vacancy
•release of energy.
•measured energy and defined sample
17. INSTRUMENTATION
The schematic of the experimental arrangement for basic AES is shown in
Fig. below.
The sample is irradiated with electrons from an electron gun.
The emitted secondary electrons are analyzed for energy by an electron
spectrometer.
The experiment is carried out in a UHV (Ultra high vacuum) environment
because the AES technique is surface sensitive due to the limited mean free
path of electrons in the kinetic energy range of 20 to 2500 eV.
The essential components of an AES spectrometer are
�UHV environment
�Electron gun
�Electron energy analyzer
�Electron detector
�Data recording, processing,
and output system
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18. Electron Energy Analyzer & Electron Detector
The function of an electron energy analyzer is to disperse the
secondary emitted electrons from the sample according to their
energies.
An analyzer may be either magnetic or electrostatic.
Because electrons are influenced by stray magnetic fields
(including the earth�s magnetic field), it is essential to cancel
these fields within the enclosed volume of the analyzer.
The stray magnetic field cancellation is accomplished by using
Mg metal shielding.
Electrostatic analyzers are used in all commercial
spectrometers today because of the relative ease of stray
magnetic field cancellation.
The dispersed secondary electrons are received in the electron
detector.
Detector communicates the energy with respect to time
data to the computer attached with it. The data is analyzed to
find out the Auger peak.
20. Auger Analysis Examples
A - Chemical composition, thickness and
spatial distribution of the elements on cerium
conversion layers deposited on galvanised
steel. Effect of the treatment time (30 minutes
and 24 hours)
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21. Application of Auger Electron Spectroscopy: Spatial resolution is high.
� Analysis is relatively rapid.
� Surface or subsurface analysis can be performed.
� It is sensitive to light elements (except H and He).
� It provides reliable semi quantitative analysis.
� Chemical information is available in some cases.
Limits of Technique and Disadvantages:•Surface Sensitivity: < 1 nm
•Lateral Resolution: < 50 nm
•Analytical Volume: 10-18 cm3
•Insulators are difficult to study due to surface
charging.
•Surface may be damaged by the incident electron
beam.
22. Comparison to XPS
Auger and X-ray photoelectron spectroscopy give similar
information, and the choice should be based on advantages and
disadvantages.
The Auger spot size is much smaller than the XPS and has the
capability of identifying fine features on the surface.
The XPS has the capability of determining surface
chemical structure and bonding through the use of chemical shifts.
Although Auger lines also exhibit chemical shifts, these are not
generally as large or as well-documented as those obtained by
XPS.
Also, X-radiation used in XPS imparts less damage to the
sample surface than does the electron beam used in SAM.
As mentioned above, the spatial analysis and imaging
capabilities of the scanning Auger microprobe make it a very
useful and complementary technique to XPS.
24. Principles: Ultraviolet photoelectron spectroscopy (UPS) refers to the measurement
of kinetic energy spectra of photoelectrons emitted by molecules which have
absorbed ultraviolet photons, in order to determine molecular energy levels
in the valence region.
The ultraviolet method (UPS) was developed by David W. Turner
There are two main areas UPS is used to study:1.Electronic structure of solids
2. Adsorbed molecules on metals
Specific examples of UPS studies include:1.The measurement of molecular orbital energies that can be compared to
theortical values calculated from quantum chemistry
2. Determination and assignment of bonding, nonbonding, and/or antibonding
molecular orbitals
3. The binding and orientation of adsorbed species on the surface of solids
4. Band structure mapping in k-space with angle-resolved techniques
25. Instrumentation:-
Figure : In
this instrument, there are no optics in use, nor is
there an electron multiplier.
This schematic shows separate chambers for the
sample and the analyzer, both of which are under UHV.
26. In early UPS, the sample was a gas or a vapor
that is irradiated with a narrow beam of
UV radiation.
More modern UPS instruments are now capable
of studying solids as well.
The photoelectrons produced are passed
through a slit into a vacuum region where they are
then deflected by magnetic or electrostatic fields to
give an energy spectrum.
UPS is sensitive to the very near surface region,
up to around 10 nm in depth.
27. Applications
The UPS measures experimental molecular orbital energies for comparison with
theoretical values from quantum chemistry, which was also extensively developed
in the 1960s. The photoelectron spectrum of a molecule contains a series of peaks
each corresponding to one valence-region molecular orbital energy level. Also, the
high resolution allowed the observation of fine structure due to vibrational levels of
the molecular ion, which facilitates the assignment of peaks to bonding, nonbonding
or antibonding molecular orbitals.
The method was later extended to the study of solid surfaces where it is usually
described as photoemission spectroscopy (PES). It is particularly sensitive to the
surface region (to 10 nm depth), due to the short range of the emitted
photoelectrons (compared to X-rays). It is therefore used to study adsorbed species
and their binding to the surface, as well as their orientation on the surface.
A useful result from characterization of solids by UPS is the determination of the
work function of the material. An example of this determination is given by Park et
al.Briefly, the full width of the photoelectron spectrum (from the highest kinetic
energy/lowest binding energy point to the low kinetic energy cutoff) is measured
and subtracted from the photon energy of the exciting radiation, and the difference
is the work function. Often, the sample is electrically biased negative to separate
the low energy cutoff from the spectrometer response.
29. Summary
ESCA,AUGER & UPS is
very important
analytical techniques used in
materials science to
investigate
molecular surface structures
and
their electronic properties
