Core Level Spectroscopies Spectroscopies involving core levels are element-sensitive, and that makes them very useful for understanding chemical bonding, as well as for the study of complex materials. The drawback is the interaction of the valence electrons/holes with the core hole. This leads to the formation of excitons and multiplets.
Spectroscopies involving Core Levels Core Level Photoemission Inverse Photoemission Tunneling Spectroscopy Core Level Emission 1 Hole 1 Electron 1 Electron/Hole 1 Hole 1 Core Level Absorption Electron Energy Loss RIXS Auger Resonant Inelastic X-ray Scattering Appearance Potential 1 Electron + 1 Hole 2 Holes 2 Electrons + 1 Hole 2 3
Core Level Photoemission (XPS, ESCA) E Fermi Core Hole Measures the binding energy of a core level by ionizing it Provides the energy reference for other core level spectroscopies Involves just a single quasiparticle, the core hole
X-ray Absorption Spectroscopy (XAS, NEXAFS, XANES) E Fermi Core Hole Probes an empty valence state by populating it from a core level Element sensitive, measures the local DOS at a specific atom Need to consider electron-hole interaction (2 quasiparticles)
Electron Energy Loss Spectroscopy (EELS) E Fermi Core Hole Same process as absorption spectroscopy, but with a virtual photon transferred from an external electron. Same dipole transitions in forward scattering, but additional multipole transitions at finite momentum transfer.
X-ray Emission Spectroscopy E Fermi Core Level Starts with a core hole (produced by absorption or EELS) The core hole gets filled by a valence electron The end result is a valencehole (no electron-hole interaction), i.e. one probes an occupied state
Resonant Inelastic X-ray Scattering (RIXS, X-ray Raman) E Fermi Core Level Combines core level absorption and emission coherently The end result is the creation of a valence electron-hole pair Becomes element-specific by involving a core level resonance
Auger Spectroscopy (AES) E Fermi Core Level Measures pairs of occupied states by creating two holes Becomes element-specific by starting with a core hole Convolution of the DOS of two valence holes blurs the results
Appearance Potential Spectroscopy (APS) E Fermi Core Hole Measures a pair of unoccupied states by adding two electrons Becomes element-specific via the remaining core hole Convolution of the DOS of two valence electrons blurs the results
Core Level Photoemission Element selective Synchrotron radiation X-ray tube (Al K α ) hν = 1400eV Intermediate oxidation states of Si at the Si/SiO 2 interface (key to Si technology!).
Varying the Probing Depth Fast electrons get farther (A = 0.1 nm) Not enough energy to excite plasmons ( 15eV) Si Ge GaAs
Chemical Information from X-Ray Absorption Spectroscopy Core to Valence Transitions : 1s 2p (π*, σ*), 2p 3d, Sharp levels (<1keV) for bond orbitals Deep levels (>1keV) for dilute species Magnetism Catalysts Bio Environment
Current Use of UV and X-ray Light Sources Valence Electrons Core Electrons Sharp Deep Photon Energy Wavelength 10eV 100eV 1keV 10keV 100nm 10nm 1nm 1Å Lithography, Nanostructures Proteomics Protein Crystallography Gratings Electronic Structure Crystals Atomic Structure
X-Ray Absorption Spectroscopy Photon energy hν related to: 1) Core level Element 2) Valence orbital Bonding
Decay Processes after X-Ray Absorption Photon in: Photon out: Electron out: Valence Orbitals: Empty Filled Core Level Absorption Emission Auger Electron Secondary Electrons Photoelectron
Information about Molecular Orientation Dipole selection rules: 90 0 20 0 l l ±1, here s p Electric field vector E parallel to the orientation of the molecular orbital C-H C-C Alkanethiol selfassembled monolayer (SAM ) 90 0 20 0
Chemistry of Bio-Interfaces Double-stranded DNA σ* π* π The N1s edge selects the π*-orbitals of the base pairs All π* orbitals are parallel to the axis of the double-helix
Breaking the Peptide Bond by Radiation: Photochemistry at the N Atom The peptide bond (= amide bond) form the backbone of proteins and polyamides (nylon, Kevlar, ) Observe the peptide bond directly at the N atom via N 1s spectroscopy. Both proteins and Kevlar are radiation sensitive. Find the microscopic mechanism. Which orbitals are eliminated, which are created?
The Peptide Bond N Two amino acids react. N forms the bridge. See the π* orbital of this double bond in X-ray absorption covalent + (zwitter)-ionic
Signature of the Peptide Bond at the N 1s Absorption Edge The π* of the peptide bond is the largest N 1s peak. Need a dimer to establish the π* peptide bond orbital. hν (ev) Xiaosong Liu et al., Langmuir 22, 7719 (2006) Gordon et al. 2003
Signature of the Broken Peptide Bond after Irradiation Peptide bond π* orbital weakens. New π* doublet grows. Universal: Small to large proteins, nylon,
Which Moiety Produces such a Characteristic Doublet? Model compounds for fingerprinting: Imine (top two panels) Nitroso (bottom two panels) Nitrile (not shown) An asymmetric imine has a doublet with the correct splitting. Symmetric imines have a single π*. The splitting is too large in nitroso and too small in nitrile groups.
Transition Metals: 2p 3d Absorption Edge Can detect the oxidation state, spin state, and the ligand field for one Fe atom inside a molecule. Fe 2+ Fe 3+
Time-Resolved X-Ray Absorption Spectroscopy Spin excitations in 100 picoseconds (Larmor frequency) Atomic motion in 100 femtoseconds (vibration period) Electronic motion in 1 femtosecond (Fermi velocity = nm/fs) Huse et al., J. Phys. Conf. Ser. 148, 012043 (2009) Pump-probe X-ray absorption spectra of a solvated Fe complex, from the low-spin ground state to a high-spin excited state.
Spatially Resolved X-Ray Absorption Spectroscopy for Catalysis Want this chemically resolved Chemically resolved, but still insufficient spatial resolution Fischer-Tropsch process for converting coal to liquid fuel. De Smit et al., Nature 456, 222 (2008)
PEEM and LEEM Photoemission Electron Microscope: Accelerate photoelectrons and run them through the magnifying optics of an electron microscope. Low Energy Electron Microscope: Use diffracted electrons instead.
Orientation of Nacre Platelets from PEEM with Polarized Light Oriented single crystals of Ca CO 3 act like bricks connected by a protein glue. Hard, but flexible to prevent cracking. Gilbert et al., JACS 130, 17519 (2008)