Probing Matter: Diffraction, Spectroscopy and Photoemission Anders Nilsson Stanford Synchrotron Radiation Laboratory Why X-rays? VUV? What can we hope to learn? 1
Photon Interaction Incident photon interacts with electrons Core and Valence Cross Sections Photon is Adsorbed Elastic Scattered Inelastic Scattered Electron is Emitted Excitated Dexcitated 2 Stöhr, NEXAFS spectroscopy Below 100 kev Photoelectric and elastic cross section dominates Spectroscopy-Scattering
Detected Particles 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 3
Diffraction X-ray diffraction Photoelectron diffraction (PhD) Extended X-ray Absorption Fine Structure (EXAFS) Long range X-ray diffraction Interference of many scattered photons Short range PhD and EXAFS Local scattering of electrons to nearest neighbor 4
Spectroscopy Valence electrons Core electrons Ionization Chemical Bonding Non interacting Photoelectron Spectroscopy hν 5
Core Levels - Atom Specific Information X-rays probes core levels Element Sensitive Chemical Shifts Stöhr et.al 6 Hufner, Photoelectron Spectroscopy
Core Level Spectroscopy Unoccupied states Fermi level Occupied states Core level 7
E = h"! Photoelectron Spectroscopy b E kin 8 Hufner, Photoelectron Spectroscopy
Core Level Electron Spectroscopy hv Electrons interact strongly Surface Sensitivity 5-20 Å Dependent on electron kinetic energy 9 Mårtensson et. al. Phys. Rev. Lett. 60, 1731 (1988)
Photoelectron Diffraction N N Forward scattering zero order diffraction Molecular orientations For a full structure determination Energy dependent diffraction together with multiple scattering calculations Nilsson et. al. Phys. Rev. Lett. 67, 1015 (1991) Tonner et. al. ALS web page 10
X-ray Absorption Spectroscopy Dipole selection rule!l = ±1 1s 2p Molecular orbital or scattering picture Ma et.al. Phys. Rev. A44, 1848 (1991) Stöhr, NEXAFS spectroscopy 11
Chemical Sensitivity Core level shifts and Molecular orbital shifts 12 Stöhr et.al
EXAFS Extended X-ray Absorption Fine Structure Interference of outgoing photoelectron and scattered waves [ l 2kr ( k ] l $( k ) = (" 1)! Ai ( k)sin i + # i ) i Nearest neighbor distance Coordination shells 13
Polarized X-rays Orientations and Directions Probing Charge orientations and Spin directions 14
Polarization Effects in X-ray Absorption 15
Core Hole Decay XES one electron picture AES two electron interaction; complex Correlation effects Sandell et. al. Phys. Rev. B48, 11347 (1993) Core hole life time Sum of all decay channels! =! aug +! fluo 16
X-ray Emission and Photoemission Hammer et. al Nature 376, 238 (1995) 17 Nilsson et.al. J. El. Spec. 110-111, 15 (2000)
Resonant Processes 18
Methods X-ray Diffraction Photoelectron Spectroscopy (PES) Core level electron spectroscopy Valence band photoemission Resonant photoemission Photoelectron 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 Scattering Speckle 19
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Chemical Analysis Chemical Identifications Speciation Quantitative analysis CO adsorption Cr(VI) on Iron oxides Cr L XAS C1s, O1s and Pt4f XPS 21
Geometric Structure Lattice parameters Bond length Molecular orientation Diffraction pattern of of Li metal Representation of Li structure at 44 Gpa pressure XAS spectra of glycine adsorbed on Cu(110) 22 Local orientation of glycine on Cu(110) Two dimensional structure of glycine adsorbed on Cu(110)
Electronic Structure Electronic Structure Band structure Electronic properties in complex materials Magnetism Angular resolved PES Photoemission spectra of W 23 Measured band structure of quasicrystals
Chemical Bonding X-ray emission process Electronic structure Chemical Bonding Molecular orbitals Local Probing XES spectra of N 2 on Ni(100) 24 Hydrogen bonding in water
Magnetism X-ray magnetic circular dichroism (XCMD) Element specific Spin and orbital moments Magnetic Information XMCD principle Intensity (a.u.) Pt-Ni Multilayer 25 Energy (ev) Ni L edge XAS spectrum and XMCD effect of Pt-Ni multilayer sample
Microscopy Spectroscopy with spatial resolution Spatial chemical speciation Magnetic domains 26
Future 27
Femtosecond Chemistry Haber-Bosch N 2 + 3H 2 2NH 3 Hansen et.al. Science 294, 1508 (2001) New Ru Catalyst Active site at steps Both N atoms Theoretical simulations, Mats Nyberg, Stockholm University 28 Probe pulse at different delay time Δt