1 Core-level spectroscopy XPS, NEXAFS Martin Weinelt Max-Born-Institut und Freie Universität Berlin
1
Core-level spectroscopy
XPS, NEXAFS
Martin Weinelt
Max-Born-Institut und Freie Universität Berlin
2Electron spectroscopy
Inelastic mean free path
M. Henzler and W. Göpel, Oberflächenphysik des Festkörpers S. 100, Teubner Studienbücher, Stuttgart 1991
3
EF
Evac
XPS hν
X-ray absorption
Core-level spectroscopy
EF
Evac
XPS hν
X-ray photoemission
unoccupied valence statescore-level
Auger decay
4
Kinetic energy Ekinrefers to vakuum level of sample ΦProbe
Binding energy EB refers to the fermi level EF
Ekin = hν – EB - ΦProbe
4Energy scale
measurement
EkinEkin
EF
ΦanalyzerΦsample
sample analyzer
Measured:kinetic energy of electronswith respect to the analyzer
Ekin = hν – EB - ΦAnalyzer
Ekin
5
5X-ray photoemission spectroscopy - XPS
JF Moulder, WF Stickle, PE Sobol - 1995 - Physical Electronics, Handbook of X-ray Photoelectron Spectroscopy
6Übergangsrate |i> |f > für lange Zeiten, nach Einschalten der periodischen Störung HI
Elektronen im elektromagnetischen Feld
somit ist die Störung HI
kinetische Impuls
Coulomb-Eichung
Näherung
elektrischer Dipol magnetischer Dipol elektrischer Quadrupol
elektrische Dipolübergänge
Näherung
6Fermi´s Golden Rule
7
~ elektr. Dipolmoment
7
8
Atomorbitale charakterisiert durch Drehimpulsquantenzahlen l, m (Quantisierungsachse z)
Man zeigt leicht:
und damit:
Man zeigt:
und müht sich:
z.B. sind dipolerlaubte Übergänge: s p, p s,dund für rechts / linkszirkular polarisiertes Licht mit Wellenvektor kz gilt Δm = + / - 1
für atomare Wellenfunktionen, z.B. Rumpfniveaus
8Dipol selection rule
9
9Cross section
10
10X-ray photoemission spectroscopy - XPS
JF Moulder, WF Stickle, PE Sobol - 1995 - Physical Electronics, Handbook of X-ray Photoelectron Spectroscopy
11
B field from the proton in the electron's rest frame is
perturbation Hamiltonian
Spin-orbit interaction (classical)
12
12X-ray photoemission spectroscopy - XPS
JF Moulder, WF Stickle, PE Sobol - 1995 - Physical Electronics, Handbook of X-ray Photoelectron Spectroscopy
13
13XPS gas-phase Neon
N. Mårtensson and A. Nilsson, "High Resolution Core Level Photoelectron Spectroscopy of Surfaces and Adsorbates"
14
14Sum rule (Manne & Åberg)
Limit of „sudden approximation“G. Ertl, J. KüppersLow Energy Electrons and Surface Chemistry, S. 71VCH Verlagsgesellschaft, Weinheim, 1985
15Chemical shift
Kai Siegbahn, Chemie NP 1981
Electron spectroscopy for chemical analysis (ESCA) with X-rays
16
Koo
pman
‘sE
nerg
ie (e
V)
Binding energy (eV)
16Chemical shift
D.P. Woodruff, T.A. Delchar,Modern Techniques of Surface ScienceCambridge University Press 1986, S. 104
20
20Chemical shift
S. Dreiner et al., Phys. Rev. Lett. 86 (2001) 4068.
18X-ray tube
Monochromatische Röntgenstrahlung (X-rays)Charakteristische Linien:Al Kα-line hν = 1486,6 eV, Δhν = 0.85 eVMg Kα-line hν = 1253,6 eV, Δhν = 0.7 eVmit Kristallmonochromator ΔE ~ 0.3 eVunpolarisiert
21Azobenzene - alkanethiols
C1s – XPShν = 400 eV
π π*
22Azobenzene - alkanethiols
284.2 eV - LC1s
Erik McNellis, AG Reuter
0.2
286.0 eV
0.6
0.8
0.10.0
0.0
0.5
290.7 eV
397.7
0.1
0.20.0
23Line shape - molecules
23
Initial state
Final state
Initial state
Final state
24Line shape - molecule vs. solid
LebensdauerverbreiterungLorenzkurve
diskret, Monopolübergänge zwischen mol. Orbitalen
quasikontinuierliche Anregung um EFdiskrete Interbandübergänge diskete Plasmonenanregung
asymmetrische Linienformstufenartiger Untergrund
Kinetische Energie
meist als Gauß angenommen
meist als Gauß angenommen
Verluste beim Verlassendes Festkörpers diskrete Plasmonenanregungkontinuierlich
stufenartiger Untergrund
Franck-Condon Faktoren
24
25
Fa ltung von Loren tz + Gaus s = V oigt
Ga uss: exp -(x/ ) FW HM: 2ln( 2)2 2Γ ΓLo rentz : / (x + ) FW HM: 2Γ Γ Γ2 2 2Do niac h-Su njic: co s[ /2 + ( 1-) arcta n(E/ )] / (E + ) FW HM: 2πα Γ Γ αα 2 2(1- )/2
1 .00 .80 .6
0 .40 .20 .0 0. 0 0 .5 1.0
Ga ussLo rentzDo niac- Sunjic
Intensität
ΔE
25
1.0
0.8
0.6
0.4
0.2
0.0
-1.0 -0.5 0.0 0.5 1.0
Lorentz Gauß Doniach-Sunjic
ΔE
Line shape
26
EF
Evac
XPS hν
X-ray absorption
XPS vs. NEXAFS (near edge X-ray absorption fine strucutre)
EF
Evac
XPS hν
X-ray photoemission
27
Absorption
2p3/2
2p1/2
27Metallic screening
28
ΔE
EF
VB
Rumpfniveau
Evac
EB
X-ray Absorption (XAS) E*tot(N) XPS, Etot(N-1)
28Metallic screening
29
EF
Evac
XPS hν
X-ray absorption
Core-level spectroscopy
unoccupied valence states
Auger decay
30Molecular resonances
J. Stöhr, NEXAFS Spectroscopy, Springer, Berlin 1998
31Initial and final state rules in XAS
Intensity of a NEXAFS spectrum is governed bythe initial DOS (empty states)
Energetic positions of NEXAFS resonances are governed by the final, core-excited state (core exciton)
32Molecular orientation
33
EF
Evac
XPS hν
X-ray absorption
NEXAFS spectroscopy
1s
2p
ExEy
Ez π - orbital
σ - orbital
dipole-selection rules
34Molecular orientation
J. Stöhr, NEXAFS Spectroscopy, Springer, Berlin 1998
35Observation of geometrical changes
0.8
0.6
0.4
0.2
0.0
Inte
nsity
[a.u
.]
80 60 40 20 0orientation of orbital axis α [°]
I(p) I(s)
trans
switching to ‚torsioned‘ cis-AzBshould lead to change of resonance intensities
36Signature of photoisomerization
2
1
0
C1s
-π*-
inte
nsity
[arb
. uni
ts]
600040002000Illumination time [s]
dark
dark
dark
350 nm
>420 nm
white light
P~1015 photons/cm2 s
spectral changes as expected forlight induced switching
observed only under permanentSR irradiation
defect mediated switching?
37C1s - NEXAFS
38C1s - NEXAFS
Erik McNellis
39Azobenzene - alkanethiols
284.2 eV - LC1s
Erik McNellis, AG Reuther @ Scheffler (FHI)
0.2
286.0 eV
0.6
0.8
0.10.0
0.0
0.5
290.7 eV
397.7
0.1
0.20.0
40C1s - NEXAFS
41C1s - NEXAFS
σ
42NEXAFS
43
EF
Evac
XPS hν
X-ray absorption
Autoionization spectroscopy
Auger decay
44
Kinetic energy (eV)210 215 220
Inte
nsity
hν
5.5 fs
spectatorshift
210 220Kinetic energy (eV)
Inte
nsity
(arb
. uni
ts)
3s
244.8
- 0.2
- 0.5
- 1.0
0.2
0.5
Excitationenergy
Ar / Pt(111) ~ K / Pt(111)
Phys. Rev. Lett. 76 (1996) 1380
4 fs
4 fs
Time-scale of charge transfer
45Resonant Raman Auger (Resonant Photoemission)
Spectator Photoemission & shake up
hν
Kinetic energy of outgoing electron ∝ hν
46Resonant Raman Auger (Resonant Photoemission)
Participator Photoemission
Kinetic energy of outgoing electron ∝ hν
47
210 220Kinetic energy (eV)
Inte
nsity
(arb
. uni
ts)
3s
244.8
- 0.2
- 0.5
- 1.0
0.2
0.5
Excitationenergy
Ar / Pt(111) ~ K / Pt(111)
Phys. Rev. Lett. 76 (1996) 1380
Resonant Raman Auger
b
a
48Resonant Raman Auger
Phys. Rev. Lett. 76 (1996) 1380
49Interference Phenomena – Fano profile
PRL 78 (1997) 967.
50Large molecules