X-ray Spectroscopy. Interaction of X-rays with matter XANES and EXAFS XANES analysis Pre-edge analysis EXAFS analysis

Transcription

1 X-ray Spectroscopy Interaction of X-rays with matter XANES and EXAFS XANES analysis Pre-edge analysis EXAFS analysis

2 Element specific Sensitive to low concentrations ( %) Why XAS? Applicable under extreme conditions (high-pressure, high temperature, operando) Applicable to gasses, liquids and solids (+ surfaces, buried interfaces, impurities, etc.) Local geometric information Local electronic information

3 What do we learn from XAS? Metal valence during synthesis and reaction Metal coordination Binuclear centers, (very small) cluster sizes Metal site symmetry d-band occupation (3d, 4d or 5d; metal versus oxide, valence) Energy positions of empty bands of adsorbates (CO, H 2, on Pt, nature of adsorption site) 20 nm microscopy, 50 ps time-resolved

4 Interaction of x-rays x with matter Energy Spectroscopy Direction Structure Polarization Magnetism I (ω,k,q ) I(ω,k,q) I (E k,k,σ)

5 X-ray photoemission I(ω FIXED ) I (E k )

6 X-ray absorption I(ω,q) I (ω)

7 X-ray absorption edges Manganese Electron binding energies Label Orbital ev [literature reference] K 1s 6539 [1] L I 2s [3] L II 2p 1/ [3] L III 2p 3/ [3] M I 3s 82.3 [3] M II 3p 1/ [3] M III 3p 3/ [3]

8 X-ray absorption K L 2,3 M 2,3 2p 3d (3d) 3p 3d (3d) N 4,5 L 2,3 K 1s p (3d) M 2,3 N 4,5 4d 4f (4f)

9 X-rays from synchrotrons

10 X-ray absorption beamline (transmission) White X-ray I II III IV V VII VIII I Entrance slits II Monochromator III Exit slits IV Ionisation chamber V Sample VI Ionisation chamber VII Reference material VIII Ionisation chamber

11 X-ray absorption

12 X-ray absorption ln I 0 = ε c l Lambert-Beer I t µ = 1 ln x I 0 I t µ = absorption coefficient x = sample thickness Measure x-ray intensity before and after sample

13 X-ray absorption

14 Interaction of x-rays x with matter XAFS studies photoelectric absorption Thompson scattering (Diffraction) Compton scattering Photonuclear scattering (Mössbauer) Intensity (log) Photoelectric Thompson Compton Mn Photonuclear Electronpositron 100 1k 10k 100k 1M Energy (ev)

15 X-ray attenuation C Pb Fe Center For X-Ray Optics www-cxro.lbl.gov

16 X-ray attenuation 10cm air 1m air 1µm Fe 10µm

17 X-ray absorption of an atom

18 X-ray absorption of a molecule

19 X-ray absorption

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21 X-ray absorption Excitation of core electrons to empty states. Spectrum given by the Fermi Golden Rule (name Golden Rule given by Fermi; rule itself given by Dirac) I XAS ~ Σ f Φ f eˆ r Φ i 2 δ E f E ω i

22 X-ray absorption (single particle interpretation) Density of States (DOS) is the integral over k-space of the band structure. Core states have no dispersion. XAS preserves momentum (k)

23 X-ray absorption (single particle interpretation) Element specific DOS L specific DOS Dipole selection rule ( L= ±1) Edge at binding energy of 2p, 2s and 1s states I XAS ~ ε eˆ r c 2 ρ

24 X-ray Absorption Spectroscopy 2p 2s Phys. Rev. B. 40, 5715 (1989) / 48, 2074 (1993)

25 X-ray absorption

26 Electronic Structure

27 Electronic Structure

28 X-ray absorption: core hole effect TiSi 2 Final State Rule: Spectral shape of XAS looks like final state DOS Initial State Rule: Intensity of XAS is given by the initial state Phys. Rev. B. 41, (1991)

29 X-ray absorption: multiplet effects 3d Strong overlap of core and valence wave functions <2p3d 1/r 2p3d> Single Particle model breaks down 2p 3/2 2p 1/2 I XAS ~ 2 p 5 3d 6 eˆ r 3d 5 2 δ E f E ω i

30 X-ray absorption: multiplet effects Multiplet Effects in XAS

31 X-ray Absorption Spectroscopy Single Particle: 1s edges Hard x-rays Multiplets: 2p, 3s, 3p edges Soft x-rays

32 K edge XANES Element specific DOS L specific DOS Dipole selection rule ( L= ±1)

33 XANES: qualitative analysis Edge position gives valence

34 XANES: qualitative analysis Edge position gives valence

35 XANES: qualitative analysis Wong et al. Phys. Rev. B. 30, 5596 (1984)

36 XANES: qualitative analysis Edge position gives valence Pre-edge gives valence Different slopes

37 XANES: qualitative analysis Pre-edge intensity gives site symmetry

38 XANES: qualitative analysis Pre-edge intensity and center

39 XANES: qualitative analysis L edge of 4d-systems > number of empty 4d states

40 XANES: qualitative analysis L edge of 5d-systems > number of empty 5d states Difference between 5d 3/2 and 5d 5/2

41 XANES: qualitative analysis L edge of 5d-systems > number of empty 5d states Difference between metal and oxide

42 K edge XANES Element specific DOS L specific DOS Dipole selection rule ( L= ±1)

43 Core Hole Decay Fluorescence Auger 2p 3/2 2p 1/2 2s 2p 3/2 2p 1/2 2s 1s fluorescent radiation 1s

44 Fluorescence and Auger

45 γ ω µ ω µ θ ω µ ω µ ω µ ω )]sin ( ) ( [ )]sin ( ) ( [ ) ( ) ( F B F B f I B B f I µ ω µ ω µ µ ω µ ω µ ω 2 ) ( ) ( 2 2 ) ( ) ( ) ( γ θ X-ray Fluorescence ray Fluorescence

46 X-ray Fluorescence

47 X-ray Detection

48 What do we learn from XANES? Edge position gives valence K pre-edge center gives valence K pre-edge intensity gives site symmetry L edge intensity gives empty d-states HERFD-XANES gives details anti-bonding bands of adsorbates

49 XAS and EELS Identical spectral shape (only for soft x-ray edges!) [If EELS uses small q and high E] XAS at Synchrotron EELS with (S)TEM XAS: 0.1 ev with 20nm EELS: 0.3 ev with 0.5 nm XAS: extreme conditions EELS: vacuum

50 XAS and EELS Identical spectral shape (only for soft x-ray edges!) [If EELS uses small q and high E] XAS at Synchrotron EELS with (S)TEM XAS: 0.1 ev with 20nm EELS: 0.3 ev with 0.5 nm XAS: extreme conditions EELS: vacuum

51 TEM-EELS EELS of FeZSM fe4 0.8 Y Axis Title X Axis Title B Y Axis Title X Axis Title

52 In-situ X-ray X Absorption in Catalysis >3.5 kev: 1-30 bar (transmission) kev:~2 bar (FY, EY) <1 kev: 5 mbar (conversion EY) <1 kev: 1 bar (FY)

53 In-situ X-ray X Absorption in Catalysis >3.5 kev: 1-30 bar (transmission)

54 In-situ X-ray X Absorption in Catalysis heater outlet XAS at 5 mbar hυ 150 mm Sample heater X-ray-window 150 m m pump Synchrotron radiation sample gas inlet UHV Plattenventil valve pump Gasphase Signal I G Collector Plate Signal I C

55 Transmission X-ray Microscopy y (TXM) 20 nm soon 10 nm 200 nm ev Energy (ev)

56 Conditions XAS Hard x-ray XAS: extreme conditions > operando catalysis Soft x-ray XAS: 1 bar and ~250 C (TXM) > in-situ catalysis 10 mbar and ~500 C (electron yield) EELS: vacuum

57 Element specific Sensitive to low concentrations ( %) Why XAS? Applicable under extreme conditions (high-pressure, high temperature, operando) Applicable to gasses, liquids and solids (+ surfaces, buried interfaces, impurities, etc.) Local geometric information Local electronic information

58 X-ray absorption

59 EXAFS Intensity (a.u) 1,4 1,2 1 0,8 Decreasing background with increasing energy Oscillations present due to the presence of neighbours 0,6 0,4 0, Energy (ev)

60 EXAFS hν photoelectron Generation of photoelectrons (electron wave). Backscattering against neighbouring atoms. Interference with outgoing wave

61 EXAFS Constructive (in phase) Destructive (out of phase) Ψ = Ψout + Ψback

62 EXAFS a. 2.0 b. Absorption (a.u.) Absorption (a.u.) Energy (ev) Energy (ev) Absorption (a.u.) χ(k) 0.15 c. d Energy (ev) k (Å -1 )

63 EXAFS Amplitude 0,2 0,1 0-0,1-0,2 Remove step at edge Remove slope Change x-axis from energy to wave vector Energy (1/Å)

64 χ Shells ( k) = χ ( k) = A ( k) sinφ ( k) j j j j= 1 EXAFS formula Shells j= 1 Oscillations depend on Energy Distance and type of neighbours (j) Disorder (Debye Waller factor) The observed oscillations are the sum of different contributions from each shell of atoms around the absorber atom

65 EXAFS χ Shells [ ] ( ) 2 2σ k k = N F S k e kr + ( k) j ' ( ) sin ϕ e 2R sim j j 0 2 ' 2 j= 1 k jr j j / λ 2 2 j j Distance R Number of neigbours N Disorder: Debye-Waller factor, dependent on temperature.

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70 XANES versus EXAFS EXAFS: local geometry (N, R, σ) XANES: valence, site symmetry XANES much easier/faster to measure XANES easier to crudely interpret XANES more difficult to interpret ab-initio HERFD-XANES gives many new possibilities XANES easier at high-temperature