X-ray absorption spectroscopy (XAS) I(x) = I 0 e -µx µ = linear attenuation coefficient (depends...

X-ray absorption spectroscopy (XAS)

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

Elemental sensitivit

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

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

Ways to measure XAS

And more ways…..?And more ways…..?

Different surface sensitivity

Saturation effects

More details

X-ray Absorption Near Edge Structure

and

Extended X-ray Absorption Fine

StructureBetter for very deep core-levels

X-ray magnetic circular dichroism (XMCD)

Selection rules

For left-circular

ml = -1ms = 0

For right-circular

ml = +1ms = 0

How synchrotron works

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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.

The beamline

SINS beamline

Bending magetnic HFM VFMGratings 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.

The high intensity

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

Three regimes for sputtering

Energy Distribution of Sputtered Particles

Sputter Yields

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

Quantitative description

I=- T dN/dt= N A Y T

The detected secondary ion intensity is described by:

: 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.the primary ion current

SIMS process can be seen as two stages: a. sputterb. ionization.

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

The choosing of ions

Oxygen 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.

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

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

Typical spectrum

Depth Profiling

Typical spectrum