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Probing Matter: Diffraction,Spectroscopy and Photoemission

Anders NilssonStanford Synchrotron Radiation Laboratory

What can wehope to learn?

Why X-rays?VUV?

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Photon is• Adsorbed• Elastic

Scattered• Inelastic

Scattered

Incident photon interacts with electronsCore and Valence

Electron is• Emitted• Excitated• Dexcitated

Cross Sections

Stöhr, NEXAFS spectroscopy

Photon Interaction

Below 100 keVPhotoelectric and elastic crosssection dominates

Spectroscopy-Scattering

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EMITTED PARTICLE• Elastic Scattering X-Diffraction, Speckle• Inelastic Scattering X-ray Emission Spectroscopy• Electron Emission Photoelectron Spectroscopy

NO EMITTED PARTICLE• Photon Adsorbed X-ray Absorption Spectroscopy

Detected Particles

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Diffraction

• X-ray diffraction

• Photoelectron diffraction (PhD)

• Extended X-ray Absorption FineStructure (EXAFS)

Long range X-ray diffractionInterference of many scattered photons

Short range PhD and EXAFSLocal scattering of electrons to nearestneighbor

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Spectroscopy

hν

Valence electrons Chemical Bonding

Core electrons Non interacting

Ionization Photoelectron Spectroscopy

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Core Levels -Atom Specific Information

X-rays probes core levels

Element Sensitive Chemical Shifts

Hufner, Photoelectron SpectroscopyStöhr et.al

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Core Level Spectroscopy

Unoccupied states

Occupied states

Fermi level

Core level

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Photoelectron Spectroscopy

kinbEhE != "

Hufner, Photoelectron Spectroscopy

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Core Level Electron Spectroscopy

hv

Electrons interact strongly

Surface Sensitivity

5-20 Å

Dependent on electron kinetic energy

Mårtensson et. al. Phys. Rev. Lett. 60, 1731 (1988)

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Photoelectron Diffraction

N

N

Forward scattering zeroorder diffraction

Molecular orientations

For a full structuredetermination

Energy dependentdiffraction together withmultiple scatteringcalculations

Nilsson et. al. Phys. Rev. Lett. 67, 1015 (1991)

Tonner et. al. ALS web page

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X-ray Absorption Spectroscopy

Molecular orbital or scattering picture

Stöhr, NEXAFS spectroscopy

1±=!lDipole selection rule

1s 2p

Ma et.al. Phys. Rev. A44, 1848 (1991)

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Chemical Sensitivity

Core level shifts andMolecular orbital shifts

Stöhr et.al

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EXAFSExtended X-ray Absorption Fine Structure

Interference ofoutgoingphotoelectron andscattered waves

[ ]! +"=i

l

iii

lkkrkAk )(2sin)()1()( #$

Nearestneighbordistance

Coordination shells

][

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Polarized X-raysOrientations and Directions

Probing Charge orientations and Spin directions

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Polarization Effectsin X-ray Absorption

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Core Hole Decay

fluoaug !+!=!

Core hole life time

Sum of all decaychannels

XES one electronpicture

AES two electroninteraction; complexCorrelation effects

Sandell et. al. Phys. Rev. B48, 11347 (1993)

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X-ray Emission and Photoemission

Hammer et. al Nature376, 238 (1995)

Nilsson et.al. J. El. Spec. 110-111, 15 (2000)

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Resonant Processes

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Methods

• X-ray Diffraction• Photoelectron Spectroscopy (PES)

Core level electron spectroscopyValence band photoemissionResonant photoemissionPhotoelectron Diffraction

• X-ray Absorption Spectroscopy (XAS)Near Edge X-ray Absorption Spectroscopy (NEXAFS)Extended X-ray Absorption Fine Structure (EXAFS)X-ray Magnetic Circular Dichroism (XMCD)

• X-ray Emission Spectroscopy (XES)Resonant Inelastic X-ray Scattering (RIXS)

• Soft X-ray ScatteringSpeckle

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Chemical Analysis

• Chemical Identifications• Speciation• Quantitative analysis

Cr L XAS C1s, O1s and Pt4f XPS

CO adsorption

Cr(VI) on Iron oxides

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Geometric Structure

• Lattice parameters

• Bond length

• Molecular orientation

Diffraction pattern of of Li metal

Representation of Li structure at44 Gpa pressure

XAS spectra of glycine adsorbed on Cu(110)

Local orientation ofglycine on Cu(110)

Two dimensionalstructure of glycineadsorbed on Cu(110)

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Electronic Structure

• Electronic Structure

• Band structure

• Electronic properties in complex materials

• Magnetism

Angular resolved PES

Photoemission spectra of W Measured band structure of quasicrystals

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Chemical Bonding

• Electronic structure

• Chemical Bonding

• Molecular orbitals

• Local ProbingX-ray emission process

XES spectra of N2 on Ni(100) Hydrogen bonding in water

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Magnetism

• X-ray magnetic circulardichroism (XCMD)

• Element specific• Spin and orbital moments• Magnetic Information

XMCD principle

Pt-Ni MultilayerNi L edge XAS spectrum and XMCD effectof Pt-Ni multilayer sample

Energy (eV)

Inte

nsity

(a.u

.)

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Microscopy

• Spectroscopy with spatialresolution

• Spatial chemical speciation• Magnetic domains

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Future

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Femtosecond Chemistry

Theoretical simulations, Mats Nyberg,Stockholm University

Probe pulse at differentdelay time ΔtBoth N atoms

New Ru Catalyst

Active site at steps

Hansen et.al. Science 294, 1508 (2001)

Haber-Bosch

N2 + 3H2 2NH3