Energy -Scanned X-ray Photoelectron Diffraction: Angle Resolved Photoemission Fine Structure (ARPEFS) (2x2) p4g-N/NiO (100) Synchrotron radiation source can be varied continuously
The Physical Origin of Intensity Modulations
The Shape of the Primary Wave dipole selection rules: l=±1 polarised light (plane xy) unpolarised light s initial state: l=+1
The Amplitude of Scattered Waves: Scattering Factors and their Energy Dependence
FS vs BS FS dominates for KE500 eV - direct information about interatomic directions with no need of theoretical directions 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 KE500 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.
Simulations -cluster based (from a few to several hundred atoms) -real space (long range order not explicitly needed) -electron waves (plane or curved) scattered off muffin-tin atomic potentials - calculations of I(k) repeated as a function of ( ) or |k|. - the simplest possible model: Single Scattering Cluster - Plane Wave -SSC-SW -MSC-SW (MSCD by Chen and van Hove; TXPD by Fadley and coworkers…) primarywave amplitude of pw at the scatterer in r j scatteringamplitude phase shift due to pathlength difference phase shift due to the scattering potential
A Note on Short-Range Order XPDEXAFSLEED OKOK OKOKOK OKOKOK NOOKNO NO EXAFS XPDLEED
Experimental - Home Lab XPS ARXPS XPDLEED e-beam evaporator Gas line e-beam heater
Experimental - ELETTRA SRS
Chemisorption: Formate on Cu (100) a)Cross Bridge (CB) b)Diagonal Atop (DA) c)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 FS
Quantitative determination: SSC-SW simulations d (Cu-O) = 1.95±0.05 Å < (OCO) = 129°±5°
V depositions in O 2 atmosphere, p=5x10 -8 mbar Reactive deposition of VO 2-x multilayers on TiO 2 (110) M. Sambi, M. Della Negra and G. Granozzi, Surf. Sci. 470 (2000) L116. -pseudomorphic growth -Kikuchi bands developed: medium-to-long range order
Ultrathin VO x (x1) film grown epitaxially on TiO 2 (110) M. Della Negra, M. Sambi and G. Granozzi, Surf. Sci. 461 (2000) 118. V stepwise deposition + UHV ann °C + UHV ann °C up to 4 ML - NaCl-like structure - epitaxial, SRO directly from expt. data: - in-plane orthorhombic distortion distortion - interlayer contraction - interfacial buckling from MSC-SW simulations:
XPS Ni 2p 3/2, KE=632 eV O KLL, KE= 510 eV Epitaxial growth - NiO/Pd (100) S. Agnoli, T. Orzali, M. Sambi and G. Granozzi, Surf. Sci. 569 (2004) 105. Azimuthal PD 3 ML 5 ML LEED (88 eV) Polar PD
DFT model STM Soft-mode frequencies of surface overlayers (2x2) surface-V 2 O 3 /Pd(111) - structural determination M. Sambi, M. Petukhov, B. Domenichini, G. A. Rizzi, S. Surnev, G. Kresse, F. P. Netzer and G. Granozzi Surf. Sci. 534 (2003) L234. XPD FS V 2p, SSC-SW based on DFT V 2p, expt. KE=972 eV z V-O (expt.)=0.72±0.07 Å z V-O (DFT)=0.723 Å
M. Sambi, S. Surnev, G. Kresse, F. P. Netzer and G. Granozzi, Phys. Rev. B68 (2003) 1554XX (2x2) surface-V 2 O 3 /Pd(111) - vibrational study Soft phonon mode involving in-plane displacements of O scatterers with respect to V emitters? DFT prediction: =14.7 cm -1 ! =14.7 cm -1 !
- angular displ. from equilibrium - angular displ. from equilibrium - frequency - frequency N n - normalisation constant H n - n th order Hermite polynomial HARMONIC OSCILLATOR MODEL
=40±25 cm -1 =40±25 cm -1
ALOISA - experimental setup Variable Polarisation Photoelectron Diffraction - 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) top view side view O 1s and Zn 3s, KE~300 eV Data Processing
Why does it work?
Near Node Photoelectron Holography
- a 2 plot in the form of the modulation function is a hologram of the atoms surrounding the emitter is a hologram of the atoms surrounding the emitter - a Fourier transform of the modulation function of the type: allows 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 both in amplitude and in phase both in amplitude and in phase image distortions shifts in atomic positions
Near Node Photoelectron Holography - the Concepts Th. Greber and J. Osterwalder, Chem. Phys. Lett. 256 (1996) 653; Prog. Surf. Sci. 53 (1996) 163.
Near 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) 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.
Z 12 =2.076±0.05 Å Z 23 =2.286±0.05 Å. PRL 79 (1997) 2085.
Epitaxial growth - Vanadium and Vanadium Oxides on TiO 2 (110) -0.2 ML V on TiO 2 (110) -annealing at 473 K in UHV M. Sambi, G. Sangiovanni G. Granozzi and F. Parmigiani, Phys. Rev. B. 54 (1996), Initial stages of epitaxy
Bridging oxygen relaxation
1 ML V on TiO 2 (110) + annealing at 473 K in O 2 CHEMICAL SHIFT PD ARPEFS M. Sambi, M. Della Negra, G. Granozzi, Z. S. Li, J. Hoffmann Jørgensen and P. J. Møller, Appl. Surf. Sci. 142 (1999) 135. ASTRID