"Fine Tuning Your Data"
Daresbury-Wednesday 9th July 1997
The meeting at Daresbury was opened by Dr. RK Wild from the
IAC-Bristol, Dr. MP Seah from NPL was the chairman for the morning session, Dr. S Evans
from the University of Wales chaired the afternoon session. The first paper was given by
Prof. Peter Weightman, from the IRC in Surface Science at Liverpool University, and was
titled:
The Daresbury Facility: What does it give you that
nothing else can?
Prof. Peter Weightman, IRC in Surface Science-Liverpool University
In answer to the question in the title, the Daresbury facilities offer Electron
Spectroscopy with;
- lots of photons
- tunable photons
- polarised photons,
plus complimentary information on;
- Electronic Structure, X-ray absorption, NEXAFS
- Physical Structure, X-ray Diffraction
Topography
EXAFS, SEXAFS
- Femtosecond monitoring of reactions
However, what doesn’t it give you that something else can? Photoelectron
Spectroscopy with;
- high sensitivity &
- high resolution &
- high energy photons
Where the "something else" is a laboratory XPS instrument. In
the view of the author, the most impressive experiments using synchrotton radiation are
structural characterisations of complex biological systems such as viruses. Also
impressive, but less complex are the structural studies on the crystalisation of polymers.
Moving to electron spectroscopy, the tunability of the low energy
photons provided by the synchrotron to band map the valence electronic structure of
solids. The tunable low energy photons can be used to exploit the Cooper minimum that
occurs in the photoelectron crossections of orbitals that have nodes such as the 4d levels
of Pd. By exploiting this characteristic it has been possible to separate out the the
contribution of the copper and palladium atoms to the conduction band density of states in
the CuPd alloys. EXAFS can also be used on the same systems, given detailed information on
there physical structure, quickly and relatively easily.
However, in the authors opinion, the real use of synchrotron radiation,
is to measure weak Auger transitions, with a good signal to noise!!! In such situations
the signal to noise is essentially determined by the background and in certain
circumstances the optimum experiments are performed using a tunable high energy photon
source. This is demonstrated by a study1 of a monolayer of Sb "delta
doped" 40Å below the surface of Si and by recent
studies of 1% of monolayer of Si 50Å below the surface of
GaAs. A provocative interpretation of the latter work is that it shows the potential of
synchrotron radiation for the study of processes occurring on a femtosecond timescale with
an accuracy in excess of that obtained with current laser technology.
References
- "Donor activation and electronic screening at an antimony d
layer in silicon" JMC Thornton, RJ Cole, DJ Gravesteijn and P Weightman, Phys. Rev. B
54 7972-8 (1996)
Prof. P Weightman
email peterw@is.ssci.liv.ac.uk
Surface Science on Beamline 4
V. R. Dhanak, IRC in Surface
Science-LiverpoolUniversity
The IRC beamline 4 consists of two branchlines, BL4.1 and
BL4.2. Beamline 4.1 provides photons in the VUV range 15 - 200 eV using a SGM. The
beamline and its capability is briefly described. The performance is illustrated using two
different surface science studies: SCLS measurements to probe rare earth overlayers on the
W(110) surface and photoemission studies of metals on TiO2 surfaces. Beamline 4.2 provides
photons in the soft X-ray range (1500 - 10000 eV) using a double crystal monochromator.
The performance of this beamline is described using two absorption experiments as
examples: adsorption of metals on quartz which is difficult to study using conventional
ESCA, and polymerisation of thiophene on Fe2O3.
Dr. V R Dhanak,
e-mail: vin@ssci.liv.ac.uk
Squeezing Extra Information out of Phosphorus L XANES
Dr. T Gaunlett, Shell, Thornton Research Centre,
Recently, claims have been made regarding the power of X-ray
Absorption Near Edge Structure (XANES)1 at the phosphorus L2,3 absorption edges
as a technique for the examination of polymeric phosphates2,3. XANES spectra at
the phosphorus L2,3 edges have much more fine structure, which is related to the symmetry
of the absorbing atom, than corresponding P 2p photoelectron spectra. The intensities of
the lowest spin orbit doublet bands in the XANES spectrum at the phosphorus L edges of
sodium phosphate glasses correlate well with the average chain lengths measured by end
group titration2. These bands are due to the 2p3/2*a1* and 2p1/2*a1*
transitions at phosphorus. They become more intense as the phosphate polymer chain
lengthens, tetrahedral symmetry is broken and the fraction of d orbital character in the
upper state increases2. This assumption can only hold if there is no further
distortion of the co-ordination around the phosphorus atoms due to interactions of the
phosphate oxygens with counter ions. There is, however, a further aspect of the XANES,
which can be used as a structural fingerprint4,5 and is applied to the near
edge in transmission Electron Energy Loss Spectroscopy6. The energy difference
between the shape resonance and the absorption edge step is related simply to the distance
of the first co-ordination shell from the absorbing atom4,5. By utilising both
the magnitude of this energy difference and the 2p3/2*a1* band intensity we are
better able to separate structural features of phosphate polymers. As P-O bond lengthening
due to interactions between phosphate oxygen and the associated cation can break the
symmetry at phosphorus, a study was made of calcium, iron and magnesium phosphates to
assess whether a cation effect could be detected.
References
- P J Durham in "X-Ray Absorption. Principles, Applications, Techniques of EXAFS,
SEXAFS and XANES," (eds. D C Koningsberger & R Prins), Wiley-Interscience, 1988.
- Z Yin, M Kasrai & G M Bancroft, "X-Ray Absorption spectroscopic studies of
sodium polyphosphate glasses,"Phys Rev B, 1995, 51, 742-750.
- M Kasrai, M Fuller, M Scaini, Z Yin, R W Brunner, G M Bancroft, M E Fleet, K Fyfe &
K H Tan, "Studies of tribochemical film formation using X-Ray absorption and
photoelectron spectroscopies," Proc 21st Leeds-Lyon Conference on Tribology, Elsevier
1995.
- A Bianconi, "EXAFS for Inorganic Systems, Daresbury Study Weekend 28 29 March 1981,
DL/SCI/R17, CLRC Daresbury Laboratory.
- A Bianconi, M Dell'Ariccia, A Gargano, & C R Natoli, Springer Ser. Chem. Phys. 1983,
27 (EXAFS Near Edge Struct.), 57-61.
- R Brydson, EMSA Bulletin 1991, 21, 57-67.
Dr. Trevor Gauntlett
j.t.gauntlett@msmail.trctho.simis.com
"Spin Polarised Photoelectron Spectroscopy: what is it,
and what information does it give us?"
Dr. E Seddon, Daresbury Laboratory, VUV Spectroscopy Group
Dr Seddon defined the basic construction and design of the
various types of spin polarized photoelectron spectrometers. The technique can be used
effectively on "real samples", several multi-element examples were shown,
including thin films of chromium on iron, and the oxidation of this system. The technique
measures asymmetry between two backscattered detectors, and a simple expression is applied
to the data to extract polarisation information. However, since only backscattered
electrons are used, the technique has very low sensitivity, for every 10,000 electrons
entering the analyser only 1 is detected. This minor disadvantage aside, the main
advantages and disadvantages of the technique are as follows:
+Ve
 |
Element specific |
|
Oxidation state specific |
|
Analysis surface or bulk |
|
Local magnetic order can be studied |
-Ve
|
No orbital information |
|
Indirect method of analysis |
|
Low sensitivity, really requires a very bright source |
However, the technique does have clear advantages over standard
photoelectron spectroscopy with regards to the analysis of local magnetic order in thin
films.
Dr. E Seddon
E.A.Seddon@dl.ac.uk
EXAFS: what can it tell us about real samples
L.M. Murphy X-ray Spectroscopy Group, Daresbury Laboratory
Extended x-ray absorption fine structure (EXAFS) is a technique which can
determine the detailed local environment surrounding an excited atom. The advantages of
the technique are as follows:
|
element spectrum |
|
geometrical content |
|
valence information |
|
in-situ experiment |
However, the disadvantages are the following:
|
multi site structure leads to analytical problems |
|
the analysis is NOT always routine |
There are now several different types of basic EXAFS experiments, such
as transmission, fluorescence, reflection and surface specific. It is possible to do
simultaneous EXAFS and diffraction experiments, combining both sets of data to solve the
problems associated with a given sample.
Further information on EXAFS can be found:
- http://srs.dl.ac.uk/XRS/index.html
- S.J. Gurman. J. Synchrotron Radiation 1995, Vol 2, pp 56-63.
Dr. LM Murphy
L.M.Murphy@dl.ac.uk
"The Origin and Practical Application of Auger Fine
Structure"
Prof. Peter Weightman, IRC in Surface Science-Liverpool University
Origin
Fine structure arises from states that can arise from different
ways of coupling the spin and angular momentum of electrons in unfilled atomic subshells.
These states are called TERMS and are cjaracterised by values of S,L and J quantum numbers
describing the angular momentum of each state. Two interactions are important;
- Coulomb interactions,
e2/r between states with described S and L quantum numbers.
- Spin-orbit interactions;
x nlL.S between states
with well defined J and MJ quantum numbers.
Irritation
Unfilled shells cannot be described by a basis set in which L,S,ML,MS,J
and Mj are simultaneously all good quantum numbers.
Choices
- LS Coupling: e2/r >> x nl
- jj Coupling: e2/r << x nl
- Intermediate Coupling (IC) e2/r » x nl Whatever the coupling we need good wavefunctions!
These can be obtained from the Hartree-Fock approach to atomic
structure and it is straight forward though complicated to evaluate the matrix elements
that generate the splittings and intensities of the LSJ fine structure components.
A simple atomic description of fine structure applies to Auger spectra:
- Of free atoms.
- Of transitions which create final states deep in the structure of atoms. i.e. far
away from the influence of molecular or solid state effects.
- Of transitions which create final states in otherwise filled, narrow d band systems
where they are a consequence of strong electron correlation. A situation described by
Cini-Sawatzky.
Practical Application of Auger Fine Structure
For transitions which create final states deep in the structure of
the atom, the Auger profile will be essentially independent of chemical environment.
However, the Auger energies will change with environment and these shifts can be measured
accurately by using sharp features in the fine structure. The combination of Auger shifts
and XPS shifts in the Auger parameter can be combined with a simple model of atomic
potentials to determine ;
- Differences in atomic charge in the initial states in different environments.
- Differences in electron screening of the final state in different environments.
For transitions which create final states in otherwise filled narrow d
bands the Auger fine structure depends on the ratio of the on-site electron screening U
(LSJ) to the single electron band width W. Information on electron correlation.
Prof. P Weightman
Email: peterw@is.ssci.liv.ac.uk
'The SRS: How the light source works'
Dr. H. Owen, Accelerator Physics Groups, Daresbury Laboratory
The Synchrotron Radiation Source produces X-rays using an
electron storage ring. This talk described the basic principles of synchrotron radiation
and particle accelerators, and then went on to describe the particular features and modes
of the SRS facility. Emphasis was made on how the photon output as seen on a beamline is
related to what is occurring within the storage ring.
Hywel Owen
h.owen@dl.ac.uk
http://accelerator.dl.ac.uk/staff/owen/
"The Daresbury Analytical Research and Technology
Service (DARTS)"
Dr. Steve Maginn, CCLRC Daresbury Lab, Warrington, UK
DARTS is a new initiative by CCLRC to grant easier, faster and more
cost-efficient access to the benefits offered by the use of synchrotron radiation (SR) to
industrial users. It has been designed to answer two perceived needs in the industrial
community - firstly, for companies inexperienced in SR techniques and uses, it offers a
level of consultancy and service that is tailored to meet the customer's requirements, so
that they never need to become familiar with the principles or details of the practice
involved; Secondly, it offers a simple raw data collection service to experienced users
who see SR techniques as a routine research tool and need faster response times than has
so far been available.
The presentation explained how the Service works, and how the above
need are met, along with information as to which SR techniques are included in the Service
and how to gain access to it.
The way in which the facility can cost the work has been changed, from
a minimum of 1 day to experiments that will take less than an hour. The time is
pre-allocated into the work programme, and effectively can be done by post. The turn
around time is expected to be 1 month, with typical costs being £500 for the raw data
from a diffraction experiment. At present not all the different Daresbury facilities are
available on the DARTS scheme, but more will be made available in the near future.
Dr. S Maginn,
sjm@dl.ac.uk |
