Post on 18-Jan-2021
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The Three Principle Soft X-ray Spectroscopies
X-ray Absorption Fine Structure (XAFS)
X-ray Fluorescence Spectroscopy (XRF)
X-ray Photoemission Spectroscopy (XPS)
Photoemission spectroscopy: experimental system
Terminology of Photoemission Spectroscopy
XPS: x-ray photoelectron spectroscopy. Used to determine binding energies of core-levels. These binding energies shift with chemical state (chemical-shift).
UPS: ultra-violet photoelectron spectroscopy. Used to measure binding energies of valence orbitals.
ARPES: angle-resolved photoemission spectroscopy. Measures the intensity of valence band features as a function of emissionangle, which can be used to determine band-structure.
XPD: x-ray photoelectron diffraction. Measures the intensity of core-level features as a function of emission angle, which can be used to determine surface structure.
Basic model of photoemission physics
Energy conservation: measure binding energies
A typical X-ray Photoelectron Spectrum
0
50000
100000
150000
200000
250000
300000
350000
020040060080010001200
Ga
2pG
a 2p
3
Si 2
p
Ga
3d
Ga
3p
Si 2
s
Ga (LMM)Auger
Valence Band
Core-level electrons
Secondary electrons
The inelastic mean-free-path for an electron: Physical origin of surface sensitivity in electron spectroscopy
Primary electrons(elastic)
Secondary electrons(inelastic)
MFPλ
The minimum in the mean-free-path is as small as a single atomic layer.
Variable surface sensitivity by tunable photon energy: Case of pyrite (FeS2)
[100]side view, (100) surface plane
hν (eV)
170.00 165.00 160.00Binding Energy (eV)
Phot
oemi
ssio
nIn t
ensit
y (ar
bun
i ts) S 2p
720
510
380
310
280
250
210
bulk
surface I
surface II
X-ray Photoelectron Diffraction (XPD)
• A powerful tool for determining the atomic structure of surfaces• Precision of bond-length measurement is about 0.02 Angstrom• Source of electrons is known: determined by XPS binding energy• Theory is a multiple-scattering theory; certain experimental conditions permit a single-scattering interpretation• Has been used as a form of quantum holography: direct data inversion
Experimental XPD Patterns
X-ray Beamhνν=60-600 eV
Polar Angle θθ
Azimuth Angle φφ
Electron Analyzer
Diffraction Pattern
θ
φ
Linearly polarized light
Sample surface
XPD of Mn/Ni, ESCA (MgKαα, hνν=1253.6 eV)
Ni Auger, Ni substate Mn 2p, 3 ML MnNi
Mn 2p, 2 ML MnNi Mn 2p, 1 ML MnNi
Mn 2p, 4 ML MnNi
Map of the rowsABOVE the emitter
[110]
[111][112][114][001][103][101][211]
An XPD Diffraction “Volume”Cu(100)
77.3 eV 99.5 eV 124.4 eV
152.1 eV 182.6 eV 215.8 eV
251.8 eV 290.6 eV 332.1 eV
φ
Ele
ctro
n M
omen
tum
θ
φ
Ele
ctro
ns
Photons
Sample
XPD for surface structure determination: The case of Si(100)c4x2
-101.5 -101.0 -100.5 -100.0 -99.5 -99.0
Binding Energy [eV]
5
12
B4
3
θ=60°φ=15°
hν = 144 eV
S1-XPD (left) and MS simulations (right) at hv=140 and 160 eV. Dimer atoms only (Atom 1)
Si(100)c4x2: Dimers and Substrate
Fig.5 S4-XPD (left) and MS simulations (right) at hv=145 and 160 eV. Emitters are those silicon atoms in the second layer substrate labeled as ATOM 4.
Fig.4 S1-XPD (left) and MS simulations (right) at hv=140 and 160 eV. Emitters aredimer atoms at upper positions labeled as ATOM 1. In this figure, we only show the comparison at two energies.
Photoelectron Holography: Analogy to Optical Holography
Objects
Laser Beam
Reference Wave
Object Waves
Detector
Optical Holography
Photo-electron Holography
Method of Photoelectron Holography Inversions
•Barton Algorithm
•Synchrotron Data
•Improvements
A k k k i ikr dk dk dkx yk k k
x y
x y
( ) ( , , ) exp( ), ,
r k.r= −∫∫∫ χ
Energy
Dependent
Photo-e
Diffraction
k
kx
ky
A k i ik r Eoutout
out
k
( ) ( , , )exp( . ) sin( )cos( )cos( ), ,
r k.r= −∑χ θ ϕ θθ
θθθ ϕ
1
•Geometric corrections(Data, angular integration)
•Refraction effects(Low energy)
χ θ ϕχ θ ϕ
θ ϕ α θ ϕ θ ϕ( , , )
( , , )( , , )exp( ( , , )) ( , , )
kk
f k i k S k→
•Scattering factor, phase (SWIFT)
• s/d ratio (shift)• Polarization (asymmetry)
XPH example: Mn on Ni(100)
6.6 Å-1
4.0 Å-1φ θ
k
167 eV
60 eV
Kin
etic
Ene
rgy
Wav
eV
ecto
r
Direct inversion of XPD volumes, using a model called “photelectron holography.” The case of Mn atoms on a Ni(100) substrate is shown.
MODEL EXPERIMENTAL HOLOGRAM
Energy bands and Fermi Surfaces in solids
From Eli Rotenberg, Berkeley-Stanford Summer School 2001
Angle-Resolved Photoemission Spectroscopy (ARPES):A way to directly measure band-structure
• Conservation of energy: determine energy position of bands• Conservation of parallel momentum: determine momentum position• Symmetry selection rules: determine parity of band
From Eli Rotenberg, Berkeley-Stanford Summer School 2001
Measurement of energy bands by scanned-angle ARPES
From Eli Rotenberg, Berkeley-Stanford Summer School 2001
Example: Cu(100)
From Eli Rotenberg, Berkeley-Stanford Summer School 2001
ARPES of a single crystal of PbS (galena)
Resonant PhotoemissionValence Band
Int
ensi
ty
-30 -20 -10 0
Binding Energy
hν=250 eV
Poly(αα-methylstyrene) Resonant Photoemission
bb
dd
a - Valence Band Photoemissionb - Auger Emissionc - C 1s photoemission
(2nd harmonic radiation)d - Resonant Photoemission
LUMO / HOMOtransition
Mechanism of resonant photoemission
LUMO
Energy Levels
HOMO
C 1s
Eg
Con
duct
ion
Ban
dV
alen
ce B
and
Cor
eLe
vel
4 eV
Absorption Profile
Tot
al y
ield
300295290285
Photon Energy
C 1s edge
Valence Band
Int
ensi
ty
-30 -20 -10 0
Binding Energy
hν=250 eV
XAS
XPS
Two paths to the same final state
Transitions in Resonant Photoemission
hνν
IpUnoccupied state
Valenceband
C 1s
IpUnoccupied state
Valenceband
C 1s
hννIp
Valenceband
C 1s
Unoccupied stateBand gap (Eg)
Final state energy
Two (or more) paths to the same final state
X-ray Absorption Spectroscopy
Note the increases in absorption at characteristic energies.
2s,2p
1s
3p2p (2s)
1s