L14 - Biophysical Chemistry Chemistry 24a Winter Term...

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Biophysical Chemistry Chemistry 24a Winter Term 2009-10 Instructor: Sunney I. Chan Lecture 14 February 22, 2010 X-ray Absorption Spectroscopy X-ray Absorption Spectroscopy
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Electronic Spectroscopy of core electrons in atoms Take a transition metal element, e.g. Cu 0 Electronic Configuration : 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 1 According to Aufbau principle, atomic levels are given by Bound levels continuum ionization 1s 4p excitation of 1s core electron into 4p orbital by X-ray photon – K-edge absorption kinetic energy o photoelectron excitation of 1s core electron into continuum by X-ray photon: Cu + X-ray photon Cu + + e photoelectron L-edge absorption 4d 4p 4s 3d 3p 3s 2p 2s 1s Cu 0
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X-ray absorption edges of heavy elements found in biological systems Absorption edge (keV) λ of X-ray photon (Å) • S Z=16 2.4700 5.0185 • Mg 25 6.5376 1.8964 • Fe 26 7.1112 1.7435 • Co 27 7.7080 • Ni 28 8.3320 • Cu 29 8.9803 1.3806 • Zn 30 9.6607 1.2834 • Mo 42 20.0039 0.61978 X-ray absorption spectrum Absorption coefficient (not to scale) Cu Z = 29 K-edge L-edge M-edge E
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If Cu is d 9 , i.e., Cu (II), and ion is part of a tetragonal complex, then the edge is broadened, and often times fine structure weak (strictly forbidden) strong Typical x-ray absorption edge spectra for Cu(I) and Cu(II) compounds K-edge threshold Only the spectra up to 40 eV above the threshold is shown here Cu(I) Cu(II)
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X-ray absorption edge XANES threshold edge EXAFS oscillations Ionization threshold X-ray absorption edge The first ~25eV above the edge threshold is the near edge region, after called X-ray absorption near edge structure (XANES). XANES corresponds to the excitation of a (core electron) into a previously unoccupied state (such as a molecular orbital) or a resonance state (often called virtual molecular orbital). The region above the XANES region and just beyond the ionization threshold is referred to as the X-ray absorption edge.
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X-ray absorption edge The region beyond the ionization threshold energies is called the EXAFS region. In this region, the adsorption coefficient often exhibits oscillations or extended X-ray absorption fine structure, thus (EXAFS). EXAFS arises from the interference of the outgoing photoelectron wave with itself when it is back- scattered by neighboring atoms in a molecule or in condensed matter ( no EXAFS when there is no neighboring atoms) XANES for Cu(II) First intense transition occurs at 8986 eV A weak feature is observed at 8979 eV 8979 eV 8986 eV
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Assignments of XANES for Cu(II) a) 8979 eV 1s 3d
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L14 - Biophysical Chemistry Chemistry 24a Winter Term...

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