Photon Interaction. Spectroscopy

Transcription

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 Stöhr, NEXAPS spectroscopy Below 100 kev Photoelectric cross section dominates Spectroscopy Spectroscopy Valence electrons Core electrons Ionization Chemical Bonding Non interacting Photoelectron Spectroscopy hν 1

2 Core Levels-Atom Specific Information X-rays probes core levels Element Sensitive Chemical Shifts Stöhr et.al Hufner, Photoelectron Spectroscopy Polarized X-rays Orientations and Directions Probing Charge orientations and Spin directions 2

3 Unoccupied states Core Level Spectroscopy Fermi level Occupied states Core level Laser spectroscopy Excitations of valence electrons E Photoelectron Spectroscopy = hυ b E kin Hufner, Photelectron Spectroscopy 3

4 Core Level Electron Spectroscopy hv Electrons interact strongly Surface Sensitivity 5-20 Å Dependent on electron kinetic energy Mårtensson et. al. Phys. Rev. Lett. 60, 1731 (1988) Energy Scales Binding energies Solids relative to Fermi level Directly from spectrum Gases relative to Vacuum level Determined using calibration gases with binding energies obtained from optical measurements 4

5 Binding Energies: Two Pictures Ground state picture Koopmans teorem E Assuming the remaining electrons inert b = ε orbital eigenvalue b ε b Relaxation Valence electrons change due to electron removal E = -ε + E + E b b relax corr relaxation energy correlation energy Difference in total energy picture E b = E Ground TOT E Final TOT total energy of the whole system including all interacting atoms Chemical Shifts Chemical shifts of core levels of the same element due to different chemical surroundings 5

6 Spin Orbit Splitting 3p 6 + hυ 3p 5 (3p) Spin and angular momentum interaction -1 J = l ± s Intensities ratio of (2J3/2 + 1) /(2J1/2 + 1) = 4 / 2 = 2 For l = 1 and s = +1/2 J = 3/2 s = -1/2 J = 1/2 Two lines seen in spectrum3p3/2 and 3p 1/2 the two lines given by the population of the orbitals p shell p d shell d f shell f 3/2 5/2 7/2 and p and d and f 1/2 3/2 5/2 ratio 2 :1 ratio 3: 2 ratio 4 :3 Open shell interactions Coupling between open shells O 2 and NO paramagnetic molecules O 2 O1s 1π g 1s HOMO 2 Σ 4 Σ Open valence 4f shell for trivalent Yb 4f 13 Final state 4f 12 Multiplet splittings 6

7 Lifetime broadening The full width (Γ) of a spectral line is given by the lifetime (τ) of the final state Γ = / τ Valence hole states for free atoms no broadening Lifetime of core hole states is determined by sum of the rate for all decay channels Auger and fluorescence (X-ray emission) Γ = Γaug + Γ flu Γ = Γaug + Γ fluo Stöhr, NEXAFS spectroscopy Core Level Binding Energy Difference in total energy E b = E Ground TOT E Final TOT total energy of the whole system including all interacting atoms Relaxation Change of valence electrons 7

8 Relaxation Metallic screening + e - Electron transfer When N (electrons) are XPS binding energy is onset for XAS Chemisorbed C on Ni Image screening or polarization δ δ - No mixing of electrons between ionized atom and surroundings Electrostatic No relationship between XPS and XAS Physisorbed O 2 on graphite Z+1 Approximation + Z = Z+1 The valence electrons can not approximately distinguish an extra charge in the core region or in the nucleus Core ionized final states C*O = NO Ni* = Cu 8

9 N 2 and CO on Ni(100) E b = E Ground TOT E Final TOT Two different N atoms 1.5 ev binding energy shift The same ground state for both atoms N N O ΔE=1.5 ev N O N Difference in Adsorption energies O C O C Similar ground state energy O N O N ΔE=1eV Vibrations Core Levels Frank Condon Principle Core levels are non bonding orbitals No vibrational excitations expected? Relaxation in the ionized state Different potential energy curves Chemical shifted components Adsorbed CO on Ni C1s 9

10 Sudden Approximation Ionization much faster than motion of electrons Many different final states r 2 Ψ 2 ( r) He atom Monopole selection rule He 1s spectrum r Satellites Extreme shake-up intensities c(2x2) Ni (1001 Ne 1s Shake up and off spectra 2p np and 2s---ns excitations mp- Y-*@-+ I I I I,p%& BINDING ENERGY (et ) 10

11 Surface Core Level Shift (SLCS) E b = E Final TOT E Ground TOT ΔE = 0.3 ev ΔE S Surface segregation energy for Z+1 impurity in Z metal We have a lower binding energy for Au at the surface than in bulk for Pt (111) Variation across the 5d series The more open surfaces have a larger ΔE S Adsorbate Induced Shift N2 is physisorbed on Au CO is chemisorbed Pt is segregated to the surface in presence of CO CO forms stronger bonds to Pt compared to Au 11

12 Semiconductor Core Level Shifts Oxidation of Si Si/SiO 2 interface Si(100) Surface reconstruction Si2p chemical shift due to local charge on Si atom Electrostatic effects on Shifts Koopmans teorem Orbital eigenvalue Correlation between local charge on ionized atom and binding energy shifts Only special cases with ligands with large difference in electronegativity Do not work for metallic systems RELAXATION changes the picture Difference in total energy is the correct approach C1s shifts for different compounds 12

13 Photoelectron Diffraction N N Forward scattering zero order diffraction Molecular orientations For a full structure determination Energy dependent diffraction together with multiple scattering calculations Scattering angles at different energies Ambient pressure XPS (AP-XPS) at ALS BL Ambient Pressure XPS to pump to pump H2O on TiO 2 (110) p 0 p 1 << p 0 p 2 << p 1 H 2 O(v) TiO 2 X-rays from synchrotron Differentially-pumped electrostatic lens allows operation at p(h 2 O) < 5 Torr (equilibrium vapor pressure of water at +1 o C) H 2 O OH Binding Energy [ev] D.F. Ogletree et al., Rev. Sci. Instrum (2002). H. Bluhm et al., J. Electron Spectrosc. Relat. Phenom (2006). 13

14 Bridging the pressure gap: from UHV to near-ambient conditions Wettability tuned by the presence of OH: Cu(111) S. Yamamoto, J. Phys. Chem. C 111, 7848 (2007) Unoccupied states Core Level Spectroscopy Fermi level Occupied states Core level Laser spectroscopy Excitations of valence electrons 14

15 X-ray Absorption Spectroscopy Dipole selection rule Δl = ±1 1s 2p Molecular orbital or scattering picture NEXAFS or XANES Ma et.al. Phys. Rev. A44, 1848 (1991) Stöhr, NEXAFS spectroscopy Experimental details xxxxxxxxxxxxxxxxxxxxxxx 15

16 Comparison XPS and XAS qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq qqaaa XPS measures the photoemitted electron at fixed photon energy XAS measures the photo excitation and ionization cross section at different photon energies Valence shell properties Dipole selection rule Δl = ±1 molecules 16

17 EXAFS Extended X-ray Absorption Fine Structure Interference of outgoing photoelectron and scattered waves l [ 2kr ( k ] l χ( k ) = ( 1) Ai ( k)sin i + βi ) i Nearest neighbor distance Coordination shells Chemical Sensitivity Core level shifts and Molecular orbital shifts Stöhr et.al 17

18 Final state rule The energetic position of the spectral features are given by the properties of the final state XAS: core hole final state Z+1 approximation CO and N 2 π* chemical shift 18

19 Shape resonances Bond length with a ruler Intermolecular bond length aaaaaaa qqqqqq qqqqqq qqqqqq C. Puglia PhD thesis Dipole selection rule Transition Metals Δl = ±1 2p 2p 3d 4s Ebert et. al. Phys. Rev. B 53, (1996). Total intensity reflect number of empty holes 19

20 Properties of 3d metals Initial state rule The integrated spectral intensity reflects the number of holes in the intial state, ground state 20

21 Multiplet structure Polarized X-rays Orientations and Directions Probing Charge orientations and Spin directions 21

22 Linear Dichroism Molecular Orientations Surfaces, Polymers etc. Stöhr NEXAFS Spectroscopy Björneholm et.al. Phys. Rev. B47, 2308 (1993) Molecular orientations Glycine on Cu(110) loses acidic proton COOCH 2 NH 2 (110) surface two fold symmetry, spectra can be resolved in 3 directions π* π* σ* σ* Hasselström et al. Surf. Sci. 407 (1998)

23 Polarization Effects in X-ray Absorption X-ray Microscopy 23

24 Core Hole Decay XES one electron picture AES two electron interaction; complex Correlation effects Sandell et. al. Phys. Rev. B48, (1993) Core hole life time Sum of all decay channels Γ = Γaug + Γ fluo Resonant Processes 24