XRay Absorption Spectroscopy Joachim Sthr Stanford Synchrotron Radiation

X-Ray Absorption Spectroscopy Joachim Stöhr Stanford Synchrotron Radiation Laboratory http: //www-ssrl. slac. stanford. edu/stohr J. Stöhr, NEXAFS SPECTROSCOPY, Springer Series in Surface Sciences 25, (Springer, Heidelberg, 1992). J. Stöhr and H. C. Siegmann MAGNETISM: FROM FUNDAMENTALS TO NANOSCALE DYNAMICS, Springer Series in Solid State Sciences 152, (Springer, Heidelberg, 2006)

Physical Processes

Quantum Theoretical X-Ray Interactions with Matter: The Basic Processes Fermi's Golden rule Kramers. Heisenberg relation

Tunable x- rays offer large interaction cross sections optical light electrons Photoemission neutrons

X-Ray Absorption and Scattering Cross Sections per Atom Thomson Cross section Fe atom Experiment Fe metal Kortright and Kim, Phys. Rev. B 62, 12216 (2000) f 1 and f 2 tabulated Henke-Gullikson factors http: //www-cxro. lbl. gov/index. php? content=/tools. html

X-ray Absorption Spectra in a Nutshell tabulated Henke-Gullikson

Absolute absorption coefficients from experimental spectra (from Henke-Gullikson compilation)

Names: XAFS – NEXAFS –XANES - EXAFS or XANES Interference of outgoing photoelectron and scattered waves Nearest neighbor distances Number of neighbors

Tunable x-rays offer elemental specificity

Experimental Soft X-ray. Techniques

Experimental X-Ray Absorption Techniques

X-Ray Absorption versus Photoemission

Electron Yield Sampling Depth

Surface sensitivity of total and Auger yield

total yield

Some Fundamental X-Ray Absorption Spectra -- soft x-rays --

NEXAFS spectra of polymers: building block picture

Chemical Sensitivity Core level shifts and Molecular orbital shifts

NEXAFS of Transition Metals Dipole selection rule 2 p 2 p “white lines” 3 d - strong 4 s - weak Ebert et. al. Phys. Rev. B 53, 16067 (1996). Total intensity reflect number of empty holes

Polarized X-Rays - Dichroism “dichroism” = pol. dep. absorption

Polarization definitions (high energy physics) Historical note: different “handedness” definitions in optics (space) versus high energy physics (time)

Polarized x-rays offer orientation sensitivity Orientational order Chirality Antiferromagnetic order Ferromagnetic order

X-Ray Natural Linear Dichroism

Linear Charge Dichroism in a d-electron system C. T. Chen et al. PRL 68, 2543 (1992)

J. Stöhr et al. , Science 292, 2299 (2001)

X-Ray Magnetic Linear Dichroism

Magnetic field splits p-orbitals sum

XMLD – spectra below and above TN Lüning et al. Phys. Rev. B 67, 214433 (2003)

XMLD spectra of two oxides

XMLD effects especially strong in multiplet peaks (Ni 2+, d 8) (Ni 1+, d 9)

X-ray Magnetic Circular Dichroism

Magnetic Circular Dichroism

Soft x-rays are best for magnetism

XMCD spectra of the pure ferromagnetic metals

The sum rules

X-ray Natural Circular Dichroism l s k=k 0 w/c Dipole =0 for x-rays Chirality

Pasteur’s and Faraday’s experiments