Angle -Scanned X-ray Photoelectron Diffraction (XPD)Angle -Scanned X-ray Photoelectron Diffraction (XPD)

22 plot plot

IntensityIntensityminmin maxmax

€

anisotropy χ =I max − I min( )

I max

€

modulations as a function of ˆ k ; r k = const.

Energy -Scanned X-ray Photoelectron Diffraction:Energy -Scanned X-ray Photoelectron Diffraction:Angle Resolved Photoemission Fine Structure (ARPEFS)Angle Resolved Photoemission Fine Structure (ARPEFS)

36034032030028026024022020018016014012010080Kinetic Energy (eV)

p4g-N/Ni(100), N 1s, Normal emission

320280240200160120Kinetic Energy (eV)

N 1s, integral intensity

€

modulations as a function of r k ; ˆ k = const.

(2x2) p4g-N/NiO (100)(2x2) p4g-N/NiO (100)

Synchrotron radiation sourceSynchrotron radiation source

can be varied continuouslycan be varied continuously€

KE = hν − BE −φ

The Physical Origin of Intensity ModulationsThe Physical Origin of Intensity Modulations

k

rj

emitter scatterer

j

Ψ0

( k )

Ψj ( r

j→ k )

k

primary wave

scattered wave

scattering angle

h ν

€

Ir k ( )∝ Ψ final ˆ ε ⋅

r r Ψinitial

2

Ψ final

r k ( ) = Ψ0

r k ( ) + Ψ j

j∑

r r j →

r k ( )

Ψ jr r j →

r k ( ) = f θ j( )exp

ir k ⋅

r r j

r

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

The Shape of the Primary WaveThe Shape of the Primary Wave

€

Ir k ( )∝ Ψ final,p ˆ ε ⋅

r r Ψinitial,s

2

Ψ0 final,p ∝r ε ⋅

r k ⇒ I 0

r k ( )∝

r ε ⋅

r k ( )

2

dipole selection rules: dipole selection rules: l=±1l=±1

polarised light (plane xy)polarised light (plane xy) unpolarised lightunpolarised light

s initial state:s initial state: l=+1l=+1

The Amplitude of Scattered Waves:The Amplitude of Scattered Waves:Scattering Factors and their Energy DependenceScattering Factors and their Energy Dependence

δ=0 δ=1λ δ=2r

r

a) Forward scattering c) backscattering

emitter

scatterer

b) first orderconstructive interference

€

f θ( ) = f θ( ) exp iψ θ( )[ ]

f θ( ) = scattering amplitude

ψ θ( ) = scattering phase shift

FS vs BSFS vs BS

FS dominates for KE≥500 eV - direct information about interatomicFS dominates for KE≥500 eV - direct information about interatomic directions with no need of theoreticaldirections with no need of theoretical simulations;simulations; - if simulations are needed, Single Scattering- if simulations are needed, Single Scattering is often OK.is often OK.

BS is substantial for KE≤500 eV - precise information on bond distances;BS is substantial for KE≤500 eV - precise information on bond distances; - Multiple Scattering simulations needed to- Multiple Scattering simulations needed to

extract it;extract it; - works well for surface adsorbates.- works well for surface adsorbates.

320280240200160120Kinetic Energy (eV)

N 1s, integral intensity

SimulationsSimulations

-cluster based (from a few to several hundred atoms)-cluster based (from a few to several hundred atoms)

-real space (long range order not explicitly needed)real space (long range order not explicitly needed)

-electron waves (plane or curved) scattered off muffin-tin atomic potentialselectron waves (plane or curved) scattered off muffin-tin atomic potentials

- calculations of I(k) repeated as a function of (- calculations of I(k) repeated as a function of ( ) or |k|.) or |k|.

€

I k( )∝ ˆ ε ⋅ ˆ k +ˆ ε ⋅ ˆ r jr j

f j θ j( )j

∑ exp i kr j 1− cosθ j( ) +ψ j θ j( )[ ]{ }∫

2

dˆ ε

- the simplest possible model: Single Scattering Cluster - Plane Wave- the simplest possible model: Single Scattering Cluster - Plane Wave

-SSC-SW-SSC-SW

-MSC-SW (MSCD by Chen and van Hove; TXPD by Fadley and coworkers…)-MSC-SW (MSCD by Chen and van Hove; TXPD by Fadley and coworkers…)

primaryprimarywavewave

amplitude of pwamplitude of pwat the scatterer in rat the scatterer in rjj

scatteringscatteringamplitudeamplitude

phase shiftphase shiftdue to pathlengthdue to pathlength

differencedifference

phase shift duephase shift dueto the scatteringto the scattering

potentialpotential

A Note on “Short-Range Order”A Note on “Short-Range Order”

XPDXPD

EXAFSEXAFS

LEEDLEED

OKOK

OKOK

OKOK

OKOK

OKOK

OKOK

OKOK

OKOK

NONO

OKOK

NONO

NONO

EXAFSEXAFS

XPDXPD

LEEDLEED

Experimental - Home LabExperimental - Home Lab

0.8

0.7

0.6

0.5

0.4

0.3

γ

12108642time (min.)

25

20

15

10

z (Å)

XPS

ARXPS

XPD LEED

e-beamevaporator Gas line

e-beam heater

346344342340338336334332Binding energy (eV)

Pd3d, (√5x√5)-R27° - O/Pd (100) Pd 3d, 6.1 MLeq NiO/Pd, III growth protocol

=60°

Experimental - ELETTRA SRSExperimental - ELETTRA SRS

Chemisorption: Formate on Cu (100)Chemisorption: Formate on Cu (100)

a)a) Cross Bridge (CB)Cross Bridge (CB)b)b) Diagonal Atop (DA)Diagonal Atop (DA)c)c) Short Bridge (SB)Short Bridge (SB)

M. Sambi, G. Granozzi, M. Casarin, G. A. Rizzi, A. Vittadini, L. S. Caputi and G. Chiarello: Surf. Sci., 315 (1994) 309.

O-C FSO-C FS

Quantitative determination: SSC-SW simulationsQuantitative determination: SSC-SW simulations

dd(Cu-O)(Cu-O) = 1.95±0.05 Å = 1.95±0.05 Å

<<(OCO)(OCO) = 129°±5° = 129°±5°

V depositions in OV depositions in O22 atmosphere, p=5x10 atmosphere, p=5x10-8-8 mbar mbar

Reactive deposition of VOReactive deposition of VO2-x2-x multilayers on TiO multilayers on TiO22 (110) (110)

M. Sambi, M. Della Negra and G. Granozzi, Surf. Sci. 470 (2000) L116.

-pseudomorphic growthpseudomorphic growth

-Kikuchi bands developed: medium-to-long range orderKikuchi bands developed: medium-to-long range order

Ultrathin VOUltrathin VOxx (x≈1) film grown epitaxially on TiO (x≈1) film grown epitaxially on TiO22 (110) (110)

M. Della Negra, M. Sambi and G. Granozzi, Surf. Sci. 461 (2000) 118.

V stepwise depositionV stepwise deposition + UHV ann. 130-230°C+ UHV ann. 130-230°Cup to 4 MLup to 4 ML

- NaCl-like structure- NaCl-like structure

- epitaxial, SRO- epitaxial, SRO

directly from expt. data:directly from expt. data:

- in-plane orthorhombic in-plane orthorhombic distortiondistortion

- interlayer contraction- interlayer contraction

- interfacial buckling- interfacial buckling

from MSC-SWfrom MSC-SWsimulations:simulations:

XPSXPS

Ni 2p3/2, KE=632 eV

O KLL, KE= 510 eV

Epitaxial growth - NiO/Pd (100)Epitaxial growth - NiO/Pd (100)S. Agnoli, T. Orzali, M. Sambi and G. Granozzi, Surf. Sci. 569 (2004) 105.

Azimuthal PDAzimuthal PD

3 ML3 ML 5 ML5 ML

LEEDLEED(88 eV)(88 eV)

Polar PDPolar PD

DFT modelDFT modelSTMSTM

Soft-mode frequencies of surface overlayersSoft-mode frequencies of surface overlayers(2x2) surface-V(2x2) surface-V22OO33/Pd(111) - structural determination/Pd(111) - structural determination

M. Sambi, M. Petukhov, B. Domenichini, G. A. Rizzi, S. Surnev, G. Kresse, F. P. Netzer and G. GranozziSurf. Sci. 534 (2003) L234.

XPD FSXPD FS

V 2p, SSC-SW based on DFTV 2p, SSC-SW based on DFT V 2p, expt. KE=972 eVV 2p, expt. KE=972 eV

zzV-OV-O(expt.)=0.72±0.07 Å(expt.)=0.72±0.07 Å

zzV-OV-O(DFT)=0.723 Å(DFT)=0.723 Å

M. Sambi, S. Surnev, G. Kresse, F. P. Netzer and G. Granozzi, Phys. Rev. B68 (2003) 1554XX

(2x2) surface-V(2x2) surface-V22OO33/Pd(111) - vibrational study/Pd(111) - vibrational study

Soft phonon mode involving in-plane displacementsSoft phonon mode involving in-plane displacementsof O scatterers with respect to V emitters?of O scatterers with respect to V emitters?

DFT prediction:DFT prediction:

=14.7 cm=14.7 cm-1-1!!

€

Ψn γ ,ω( ) = N n ⋅H n γ /α( ) ⋅e−

γ /α( )2

2

€

α 2 =h

Iω

γγ - angular displ. from equilibrium- angular displ. from equilibrium - frequency- frequencyNNnn - normalisation constant - normalisation constant

HHnn - n - nthth order Hermite polynomial order Hermite polynomial

HARMONIC OSCILLATOR MODELHARMONIC OSCILLATOR MODEL

30

25

20

15

10

10080604020Frequency (cm-1)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-60 -40 -20 0 20 40 60

Angular displ. from equilibrium (deg)

Parameter: frequency 20-80 cm-1, =5cm-1 90 cm-1 100 cm-1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-40 -20 0 20 40 . ( )Angular displ from equilibrium deg

=45 cm-1

120100806040200Azimuthal angle (deg)

experiment, =68° - , , SSC SW best fit=40 cm-1

- , . SSC SW no vibr included

0.11

0.10

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

10080604020 (Frequency cm-1)

R1 R2 R2'

30 10x-3

25

20

15

10

5

0

806040 (Frequency cm-1)

706050403020100Polar angle (deg)

V 2p, s-V2O3; experiment V 2p, s-V2O3, SSC-SW, vibr. not included V 2p, s-V2O3, SSC-SW, vibr. included

=40±25 cm=40±25 cm-1-1

ALOISA - experimental setupALOISA - experimental setup

Variable Polarisation Photoelectron DiffractionVariable Polarisation Photoelectron Diffraction- investigating the surface relaxation of bulk crystals - - investigating the surface relaxation of bulk crystals -

M. Sambi and G. Granozzi, Surf. Sci 415 (1998) L1007.

M. Sambi, M. Casarin, A. Verdini, D. Cvetko, L. Floreano, A. Morgante, in preparation.

ZnO (0001)ZnO (0001)

top viewtop view

side viewside view

O 1s and Zn 3s, KE~300 eVO 1s and Zn 3s, KE~300 eV

Data ProcessingData Processing

Why does it work?Why does it work?

Near Node Photoelectron HolographyNear Node Photoelectron Holography

- a 2a 2 plot in the form of the modulation function plot in the form of the modulation function is a hologram of the atoms surrounding the emitteris a hologram of the atoms surrounding the emitter

€

χ r

k ( ) =I

r k ( )− I 0

r k ( )( )

I 0

r k ( )

- a Fourier transform of the modulation function of the type:- a Fourier transform of the modulation function of the type:

€

φ rr ( ) = χ

r k ( )∫∫ exp i

r k ⋅

r r ( )dkxdky

allows us - in principle - to obtain the positions of atoms surroundingallows us - in principle - to obtain the positions of atoms surrounding the emitter directly from angular distributions.the emitter directly from angular distributions. (within one De Broglie wavelength of the photoelectron wave).(within one De Broglie wavelength of the photoelectron wave).

- problems due to the strong anisotropy of electron-atom scattering problems due to the strong anisotropy of electron-atom scattering both in amplitude and in phaseboth in amplitude and in phase

image distortionsimage distortions shifts in atomic positionsshifts in atomic positions

€

PLD = kr j 1− cosθ j( ) + Ψ j θ j( )

Near Node Photoelectron Holography - the ConceptsNear Node Photoelectron Holography - the Concepts

Th. Greber and J. Osterwalder, Chem. Phys. Lett. 256 (1996) 653; Prog. Surf. Sci. 53 (1996) 163.Th. Greber and J. Osterwalder, Chem. Phys. Lett. 256 (1996) 653; Prog. Surf. Sci. 53 (1996) 163.

Near Node Photoelectron Holography - the ExperimentNear Node Photoelectron Holography - the Experiment

J. Wider, F. Baumberger, M. Sambi, R. Gotter, A. Verdini, F. Bruno, D. Cvetko, A. Morgante,T. Greber and J. Osterwalder,Phys. Rev. Lett. 86 (2001) 2337.

J. Spence, Nature 410 (2001) 1037.

The role of the Si-suboxide structure at the interface:an angle scanned photoelectron diffraction study

C. Westphal, S. Dreiner, M. Schürman, F. Senf, H. Zacharias, Thin Solid Films 400 (2001) 101.

BESSY II

Surface core level shift PD: clean W (110),(1x1) Fe/W(110) and (7x14) Gd/W(110)

C.S. Fadley & M. Van Hove Group, Berkeley

ALS

Surf. Sci. 441 (1999) 301.

Z12 =2.076±0.05 ÅZ23 =2.286±0.05 Å.

PRL 79 (1997) 2085.

Epitaxial growth - Vanadium and Vanadium OxidesEpitaxial growth - Vanadium and Vanadium Oxideson TiOon TiO22 (110) (110)

-0.2 ML V on TiO-0.2 ML V on TiO22 (110) (110)

-annealing at 473 K in UHV-annealing at 473 K in UHV

M. Sambi, G. Sangiovanni G. Granozzi and F. Parmigiani, Phys. Rev. B. 54 (1996),13464.

Initial stages of epitaxyInitial stages of epitaxy

Bridging oxygen relaxationBridging oxygen relaxation

1 ML V on TiO1 ML V on TiO22 (110) + annealing at 473 K in O (110) + annealing at 473 K in O22

CHEMICAL SHIFT PDCHEMICAL SHIFT PD

ARPEFSARPEFSM. Sambi, M. Della Negra, G. Granozzi, Z. S. Li, J. Hoffmann Jørgensen and P. J. Møller, Appl. Surf. Sci. 142 (1999) 135.

ASTRID