1. Part II
XAFS: Principles
XANES/NEXAFS
Applications

2. X-ray Absorption Spectroscopy (XAS)
X-ray Absorption spectroscopy is often referred to as
- NEXAFS for low Z elements (C, N, O, F, etc. K-edge, Si, P, S, L-edges) or
- XAFS (XANES and EXAFS)
for intermediate Z and high Z elements.

3. NEXAFS, XANES and EXAFS
NEXAFS (Near Edge X-ray Absorption Fine Structures) describes the absorption features in the vicinity of an absorption edge up to ~ 50 eV above the edge (for low Z elements for historical reasons).
It is exactly the same as XANES ( X-ray Absorption Near Edge Structures), which is often used together with EXAFS (Extended X-ray Absorption Fine Structures) to describe the modulation of the absorption coefficient of an element in a chemical environment from below the edge to ~ 50 eV above (XANES), then to as much as 1000 eV above the threshold (EXAFS)
NEXAFS and XANES are often used interchangeably
XAFS and XAS are also often used interchangeably

4. XAFS of free atom
In rare gases, the pre-edge region exhibits a series of sharp peaks arising from bound to bound transitions (dipole: 1s to np etc.) called Rydberg transitions

5. XAFS of small molecules
 Small molecules exhibit transitions to LUMO, LUMO + and virtual orbital, MO in the continuum trapped by a potential barrier (centrifugal potential barrier set up by high angular momentum states and the presence of neighboring atoms), or sometimes known as multiple scattering states

6. XAFS: the physical process
Dipole transition between quantum states
• core to bound states (Rydberg, MO below vacuum level,
-ve energy, the excited electron remains in the vicinity of the atom)- long life time-sharp peaks
• core to quasi-bound state (+ve energy, virtual MO, multiple scattering states, shape resonance, etc.); these are the states trapped in a potential barrier, and the electron will eventually tunnel out of the barrier into the continuum-short lifetime, broad peaks
• core to continuum (electron with sufficient kinetic energy to escape into the continuum) - photoelectric effect.
XPS &EXAFS

7. XAFS: the physical process cont’
Scattering of photoelectron by the molecular potential
– how the electron is scattered depends on its kinetic energy
• Low kinetic energy electrons - Multiple scattering (typically up to ~ 50 eV above the threshold, the region where bound to quasi-bound transitions take place); e is scattered primarily by valence and shallow inner shell electrons of the neighboring atoms - XANES region
•High kinetic energy electrons ( eV) are scattered primarily by the core electrons of the neighboring atoms, single scattering pathway dominates - EXAFS region

8. Electron scattering
Free electron (plane wave) scattered by an atom (spherical potential) travels away as a spherical wave
Electrons with kinetic energy > 0 in a molecular environment is scattered be the surrounding atoms
Low KE e- is scattered by valence electrons, undergoes multiple scattering in a molecular environment
High KE e- is scattered by core electrons, favors single scattering
Multiple Scattering
Single Back- scattering

11. NEXAFS of free atom & unsaturated diatomic molecules
States trapped in the potential barrier are virtual MO’s
or multiple scattering states (quasi bound states)
Free atom
free e with KE >0
bound
states
diatomic with
unsaturated
bonding
N2, NO,
CO etc. *
Asymptotic wing of the coulomb potential supports the Rydberg states

12. NEXAFS of Free Atom: Ar L3,2-edgehv
Ar (gas)
[Ne]3s23p6
2p6 2p54s1
2p14s1
Core hole
j = l ± s
Hund’s
rule
2p1/2
2p3/2
(lower binding)

13. NEXAFS of atom and small moleculesKr
HBr
M5,4 (3d5/2,3/2)
M5,4 (3d5/2,3/2) MO
Rydberg transitions in HBr remain strong and a broad transition to molecular orbital emerges

14. π* IP σ* NEXAFS of Nitrogen N2 Vibronic structures in
the 1 s to * transition

15. Molecular orbital illustrationCO
C2H2
MO axis * * *
C 1s
C 1s

17. E = (E*-Eo)  1/r Why would this correlation exist?
Multiple scattering theory
Simple picture- particle in a box- the shorter the bond the farther the energy separation
Scattering amplitude  Z

19. NEXAFS of molecules oriented on a surface
(angular dependence of resonance intensity)
When molecules adsorb on a surface, their molecular axis is defined by the axis of the substrate. Therefore, angle dependent experiments can be made by rotating the substrate with respect to the polarization of the photon beam. Using selection rules, the orientation of the molecule on a surface can be determined.
H H
C=C
C-C

20. Carbon Allotropes Hexagonal Diamond Diamond Graphene Graphite Nanotube
(Lonsdaleite)
Diamond
Graphene
Graphite
Nanotube

21. XANES of Benzene
LUMO + 1
LUMO
core

22. XANES systematic Increasing no. of benzene rings *1 C-H* *2
Tom Regier unpublished

23. Chemical systematic
C60
CNT

25. d - electron in transition metals
The dipole selection rule allows for the probing of the unoccupied densities of states (DOS) of d character from the 2p and 3p levels –
M3,2 and L3,2-edge XANES analysis
(relevance: catalysis)

26. Transition metal systematic
Filling of the d band across the period
Decreasing whiteline intensity across the period as the d band gets filled 4d 5d 3d

27. L3,2/M3,2 Whiteline and unoccupied densities of d states
3d metal: the L-edge WL for early 3d metals is most complex due to the proximity of the L3,L2 edges as well as crystal field effect; the 3d spin-orbit is negligible
4d metals: the L3,L2 edges are further apart, WL intensity is a good measure of 4d hole population
5 d metals: The L3 and L2 are well separated but the spin orbit splitting of the 5d orbital becomes important, j is a better quantum number than l, WL intensity is a good measure of d5/3 and d3/2 hole populations

28. 3d metals
Hsieh et al Phys. Rev. B 57, (1998)

29. d- band filling across 5d row
EFermi Ta W Pt Au
5d metal d band
Intensity (area under the curve) of the sharp peak at threshold (white line) probes the DOS
3p3/2-5d
2p3/2-5d
T.K. Sham et al. J. Appl. Phys. 79, 7134(1996)

30. d - charge redistribution in 4d metals
Ag Pd 4d charge transfer upon alloying
I. Coulthard and T.K. Sham, Phys. Rev. Lett., 77, 4824(1996)

31. Analysis of d hole in Au and Pt metal
In 5 d metals with a nearly filled/full d band
such as Pt and Au, respectively, spin orbit coupling is large so d5/2 and d3/2 holes population is not the same
Selection rule: dipole,
l = ± 1, j = ± 1, 0
L3, 2p3/2  5d5/2.3/2
L2, 2p1/2  5d3/2 EF
DOS
5d5/2 Pt
5d3/2
T.K. Sham et al. J. Appl. Phys. 79, 7134(1996)

32. Au L3
2p3/2 - 5d 5/2,3/2
Au L2
2p1/2 - 5d3/2
M. Kuhn and T.K. Sham Phys. Rev. 49, 1647 (1994)

33. *
* Phys. Rev. B22, 1663 (1980)

34. Probing depth profile with light: applications
photon e
ion
(photon)
surface
0.1-1 nm
near surface
nm-10 nm
Interface
Light-matter
Interaction:
Absorption
& Scattering
soft x-ray nm
bulk-like
The scientific case is probing matter with light, synchrotron light
Here is a schematic of the interaction of x-rays with a specimen; there are many regions of interest (surface, interface, bulk)
The interaction of light with matter takes place through absorption and scattering
both processes can be used as a probe to provide information on the morphology, structure and electronic properties of the specimen.
And both can be monitored with photons resulting from the interaction. To do this well the light source has to meet some contingent requirements
X-ray probes:
Electronic
properties
hard x-ray
Morphology
Structure

35. Soft X-ray vs. hard X-ray
Soft x-ray usually cannot penetrate the entire sample
 measurements cannot be made in the transmission mode.
Yield spectroscopy is normally used !
One absorption length (t1 = 1 or t1 = 1/ ) is a good
measure of the penetration depth of the photon
Example of one absorption lengths
Element density(g/cm3) hv(eV) mass abs (cm2/g) t1(m)
Si x x
Graphite x

36. Soft x-ray spectroscopy: unique features
• XAFS measurement in the soft x-ray (short absorption
length) region is often made using yield spectroscopy
Electron yield (total, partial, Auger)
X-ray fluorescence yield (total, selected wavelength)
Photoluminescence yield (visible, light emitting materials)
XEOL (X-ray excited optical luminescence) and TRXEOL (Time-resolved X-ray Excited Optical Luminescence)
• Since soft X - ray associates with the excitation of shallow core levels, the near-edge region (XANES/ NEXAFS) is the focus of interest for low z elements and shallow cores

37. E.G.: the Interplay of TEY and FLYhv
TEY
FLY
Diamond-like
c-BN
h-BN
Graphite-like
80 nm
Amorphous
a-BN
Si(100)
Cubic BN film grown on Si(100) wafer
? Preferred orientation of the h-BN film?

38. Boron K-edge normal hv TEY TEY * FLY  c-BN h-BN a-BN glancing
I (1s - *) minimum
a-BN
glancing
Si(100) * 
FLY
I (1s - *) maximum
X.T. Zhou et.al. JMR 2, 147 (2006)

39. XAFS and XEOL (Optical XAFS)
PLY
XAFS CB
Mono
Edge
hvop
Wavelength (nm)
Abs. EF
hvex VB

40. XEOL - Energy Domain • X-ray photons in, optical photons out
4/21/2017
XEOL - Energy Domain
• X-ray photons in, optical photons out
Illustrations
BN nanowires
Si nanowires
ZnS nanoribbons
Start with a brief introduction on the interaction of light with matter in the context of my talk
After that I’ll say a few words about the light source that I am working with followed by
My definition of photon in photon out spectroscopy,
Then I ‘ll describe the emerging capabilities with examples and summarize my talk by looking towards the future. 40
Test 40

41. Boron nitride nanotube (BNNT)
B-O bond
Oxygen
BNNT
W.-Q. Han et al. Nanoletter. 8, (2008)
h-BN
Liu et. al. (UWO) unpublished

42. Si K-edge XEOL of silicon nanowires
T.K. Sham et al. Phys. Rev. B 70, (2004) 42

43. Photoluminescence: Si K-edge43

44. ZnS hetero-crystalline nano-ribbon
520 nm
X.-T. Zhou et al., J. Appl. Phys. 98, (2005)
332 nm
ZnS nw
(zinc blend)
ZnS nw
(wurtzite)
R.A. Rosenberg et al., Appl. Phys. Lett. 87, (2005)

45. XEOL - Time Domain • Timing - resolved XEOL (TRXEOL) • Illustrations
4/21/2017
XEOL - Time Domain
• Timing - resolved XEOL (TRXEOL)
• Illustrations
ZnO: Nanodeedle vs Nanowire
CdSe-Si: Hetero nanostructures
Start with a brief introduction on the interaction of light with matter in the context of my talk
After that I’ll say a few words about the light source that I am working with followed by
My definition of photon in photon out spectroscopy,
Then I ‘ll describe the emerging capabilities with examples and summarize my talk by looking towards the future. 45
Test 45

46. TRXEOL at APS
T.K. Sham & R.A. Rosenberg, ChemPhysChem 8, (2007) 46

47. Time-gated spectroscopy
153 ns
Select time window(s)
Obtain spectra /yields of photons arriving within that window
Slow time window
Fast time window 47

48. CdSe -Si hetero nanoribbons hv = 1100 eV
X.H. Sun et al. J. Phys. Chem., C, 111, 8475(2007)
total optical yield ns 48
R.A. Rosenberg et. al. Appl. Phys. Lett., 89, (2006)

49. CdSe-Si Heterostructure: Se L3,2 - edge49

50. STXM: Scanning Transmission X-ray Microscopy
the SM
Courtesy of A.P. Hitchcock, McMaster
Fresnel Zone Plates
J.G. Zhou, J. Wang, H.T. Fang, C.X. Wu, J.N. Cutler, T.K. Sham, Chem. Comm. 46, 2778 (2010) 50
30nm

51. Mico/nano spectroscopy of N-CNT
TEM
STXM S7 S3 S8 S2 S9 S1
______500 nm
(S2)
J. Zhou, J. Wang et al. J. Phys. Chem. Lett. (2010) DOI: /jz100376v