XPS, NEXAFS - Fachbereich Physik: Startseite · Core-level spectroscopy XPS, NEXAFS ... Physical...

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