4. Intro to X-ray Spectroscopies

What are the basic X-ray matter interactions?

• Incident photons, I₀

• Photons transmitted, I

• Photons scattered, I’

• Photons emitted, I_F

What are the two fundamental photon group interactions?

• elastic and inelastic scattering - photon scattered - the photons that are still there after the incident photons are transmitted

• Photoelectric absorption - the photons that have disappeared

What are the elastic scattering techniques and uses?

• There is no exchange of energy

• can be diffraction of crystals: XRD or GIXRD

• or scattering: in amorphous and liquids - WAXS and SAXS

• Generally for the microscopic geometric structure

What are spectroscopy techniques and uses?

• spectroscopy = photoelectric absorption or Inelastic scattering

• Exchange of energy will take place

• Absorption: XAS< EXAFS, XANES

• Emission: XES HERFD

• Photoelectron: PES, XPEEM, ARPES

• Inelastic scattering: IXS, RIXS, X-Raman

• Gives the electronic structure, the local structure and the dynamics, interatomic and subatomic

What x-ray interaction is measure for photoelectron spectroscopy, x-ray emission spec and x-ray absorption spec respectively?

• photoelectron spec - electrons emitted from sample

• x-ray emission spec - the photons emitted

• x-ray absoprtion spec - photons transmitted through sample

What are the basic principles of x-ray absorption spec’s (XAS) absorption coefficient?

• key parameter measured is the absorptions coefficient, μ

• done with hard x-rays/high photon energies of 5-10keV, below 0.2 - 0.1nm wavelength

• I = I₀e^-μt

• μt = ln (I₀/I)

What is the setup of XAS?

• synchotron source generates high energy photons/x-ray

• polychromatic x-rays travel through a monochromator where x-rays of specifc energies are selected

• monochromatic x-rays travel to incident flux monitor and then hit the sample

• Transmitted flux monitor then sits after the sample

• I₀ and I are measured as a function of E_x - E_x will be the graph y-axis and I₀ will be the x-axis

What does the absorbance graph for XAS look like and the equation to determine μt?

• μ will depend on x-ray energy, E, atomic no, Z, density, ρ and atomic mass, A

• μ will have sudden jumps - absorption edges - that occur at energies characteristic of the element and its shell numbers

• heavier elements have more edges

• At very high Ex, the absorption tends to 0 as the material will become transparent to x-ray

How is μ generated in XAS?

• μ comes from the photoelectric absorptions

• x-rays with energy 1-200 keV are all absorbed by matter through photoelectric effect

• upon absorption of x-rays with certain energies, core-level electrons - K, L or M shells - from an atom are ejected

• Ejected electrons = photoelectrons

• The atom becomes excited and left with a core hole/absence of an electron

Which shell determines if the excited photoelectron is from K, L or M shells?

• shell n = 1 are K-edges

• 2s shells ar L1 edges

• 2P shells are L2 and L3 edges

What happens to the excited core hole and what happens during the auger effect?

• The excited core hole will return to ground state through having a higher level core electron fill the hole and emit its excess energy by fluorescent x-ray or auger electron

• Auger effect is very uncommon and is a non-radiative way of coming to the ground state

• energy of fluorescence or auger electron is the energy difference between the core levels

What data is measured/collected during XAS?

• measure energy dependence of x-ray absorption coefficient, μ(E) at and above absorpition edge of element

• It can be measured through transmission or fluorescence:

• Transmission: gives and EXACT measurement of μ, absorption measured directly by measuring what is transmitted through the sample, limited by sample quality, thickness, conc etc

• fluorescence: estimate of μ, radiative signals from re-filling of deep core hole is detected, μ(E) ~ I_f/I₀

What is XAFS?

• X-ray Absorption Fine Strucutre: oscillatory variation of x-ray absorption as a function of photon energy beyond absorption edge, AKA XAS

• has 2 measuring regimes

1. XANES: x-ray absorption near-edge spec - for low energy photons

2. EXAFS: Extended x-ray absorption fine-structure - for high energy photoelectrons, 50 - 100 eV from edge

What do XANES and EXAFS measure or obtain data form?

• XANES: covalency, local site symmetry, coordination chem, coord number, orbital hybridisation, orbital occupancy

• EXAFS: local structure (bond distance, no of or type of neighbours), e.g. types of ligands, distance of neighbouring ligands and bond angle info

What is XES vs XAS?

• XES is x-ray emission spec

• XES is highly sensitive to local magnetic moments, spin transitions

• XAS is x-ray absorption spec

• XAS is sensitive to element (characteristic binding energies), chemical (shape and position of edge), spin (absorp coefficient is different for circular polarised x-rays in magnetic materials)