X RAY ABSORPTION SPECTRA

2. X-ray absorption
spectroscopy (XAS)c
I(x) = I0 e-µx
µ = linear attenuation
coefficient (depends on
material and photon
energy). The lost part is
due to absorption. In the
same material, the
absorption will have
very strong peaks when
the photon energy
exactly able to excite
certain energy level to
the unoccupied states.
This is called absorption
edges.
When x-ray (light) pass
a distance x in material,
the intensity drops
Dipole transition
from core-levels
to unoccupied
valence bands

3. Elemental
sensitivit
As in XPS,
XAS core-level
edges gives
same
elemental
sensitivity.

4. Chemical sensitivity
Spectra show one kind of Fe nano-particles
are Fe oxide
XAS is a
Dipole
transition
from core-
levels to
unoccupied
valence
bands, which
is sensitive to
chemical
surrounding
like XPS

5. Ways to measure XAS
And more ways…..?And more ways…..?
Different
surface
sensitivity
Saturation effects

6. More details
X-ray Absorption
Near Edge
Structure
and
Extended X-ray
Absorption Fine
Structure
Better for
very deep
core-levels

7. X-ray magnetic circular dichroism (XMCD)
Selection rules
For left-circular
∆ml = -1
∆ms = 0
For right-circular
∆ml = +1
∆ms = 0

9. How synchrotron works
e
hν
Sychrotron light source is essential part of XAS
(XMCD, EXAFS, etc), and very useful for XPS
etc. It generate in forward direction of relativistic
electrons with circular movement.

10. The beamline
SINS beamline
Bending magetnic HFM VFM
Gratings RFM
The beamline is the bridge form synchrotron to
workstation. It consists of many optics to let the light
source to be monochromatic(change photon energy) and
well focused on the sample.

11. The high intensity

12. Secondary Ion Mass Spectrometry
SIMS is based on
the mass/charge
ratio measurement
of both atomic and
molecular ions
ejected under
energetic particle
bombardment.
1-10 keV
Static with low energy for surface. Dynamic with high energy for
depth study

13. Three regimes
for sputtering

14. Energy Distribution of Sputtered Particles

15. Sputter Yields
Yields
depends on
atomic
number,
displacement
energy,
matrix of the
solids.

16. Quantitative description
I=- α T dN/dt= N A Y ν α T
The detected secondary ion intensity is described by:
α: Ionization probability to certain charge
state;
T: Instrumental transmission function;
N: density of surface atoms;
A: surface area with incident beam;
Y: sputter yield, number of secondary ions
per incident ion.
ν: the primary ion current
SIMS process can be seen as two stages: a. sputter
b. ionization.

17. A for different elements and their oxides(Matrix EffectMatrix Effect)

18. The choosing of ionsOxygen bombardment increases the yield of positive ions and
cesium bombardment increases the yield of negative ions. The
increases can range up to four orders of magnitude.

19. InstrumentationIon Sources: discharge type ion gun, ion source using
thermionic emitter, duoplasmatron type ion source
Mass Analyzers:Magnetic sector analyzer, Quadrupole
mass analyzer, Time-Of-Flight mass analyzer
Ion Detectors: Faraday cup, Daly detector, channeltron

20. Duoplasmatron
The duoplasmatron is the source of the ion
beam. It consists of a highly charged,
evacuated ``source-head'' which contains
several components. The ions are produced
within the source-head by bombarding atoms of
the chosen gas (carbon for here) with electrons.
The free electrons are produced by boiling them
off of a heated cathode which is charged, along
with an intermediate electrode (IE) to -50.15 kV.
Atoms of the gas are injected into a chamber
containing the cathode and a positively charged
(-50 kV) anode. There is a 150 V potential
difference between the cathode, the heated
filament, and the anode. As the electrons fly
toward the anode, they collide with the atoms of
the gas, producing ions. An electron can either be
absorbed by the atom thereby creating a negative
ion, or it can knock an electron off of the atom
producing a positively charged ion. The ions are
then focused electrostatically and magnetically
by the shape of the electric and magnetic fields
into a dense plasma in the region just before the
anode aperture. The plasma bulges slightly
through the anode aperture forming an
"expansion ball". The negative ions are then
selected out by an extractor which is at ground
potential. The ions form a beam flowing into the
beam-tube toward the accelerator.For C ions
Mixture of hydrogen and CO2 gas

22. Typical spectrum
Depth
Profiling
Typical spectrum