Joachim Stöhr Stanford Synchrotron Radiation Laboratory X-Ray Absorption Spectroscopy 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

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

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

Thomson Cross section Kortright and Kim, Phys. Rev. B 62, (2000) Fe atom X-Ray Absorption and Scattering Cross Sections per Atom Experiment Fe metal f 1 and f 2 tabulated Henke-Gullikson factors

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 Interference of outgoing photoelectron and scattered waves Nearest neighbor distances Number of neighbors or XANES

Tunable x-rays offer elemental specificity

Experimental Soft X-rayTechniques

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

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 Antiferromagnetic order Orientational order Chirality Ferromagnetic order

X-Ray Natural Linear Dichroism

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

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

X-Ray Magnetic Linear Dichroism

sum Magnetic field splits p-orbitals

XMLD – spectra below and above T N Lüning et al. Phys. Rev. B 67, (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

=0 for x-rays DipoleChirality  k=k 0  /c X-ray Natural Circular Dichroism

Pasteur’s and Faraday’s experiments