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Department of Physics

PhD - Scanned-energy mode photoelectron diffraction

Photoelectron diffraction is the phenomenon in which the directly emitted component of a photoelectron wavefield (from a core level of an atom) interferes coherently with components of the same wavefield which are elastically scattered by surrounding atoms. This interference leads to a modulation of the measured photoelectron intensity as a function of emission angle or photoelectron energy (and thus wavelength) because of the changes in the scattering pathlengths produced. These modulations thus provide information on these pathlengths and thus the relative positions of emitter and near-neighbour scatterers - i.e. the local geometrical structure.

molecular adsorbate

In order to obtain structural information on the position of adsorbate atoms on a surface the photoemission from an adsorbate core level can be monitored in this way; the fact that substrate backscattering is required for an adsorbate above the surface means photoelectron kinetic energies should be below about 400-500 eV. A brief history of the method can be found in reference 1. In scanned-energy mode photoelectron diffraction, photoemission modulation spectra are recorded at a range of emission angles and the full analysis is conducted with the aid of multiple scattering simulations. In a well-established collaboration [e.g. 2-4] with the group of Alex Bradshaw in the Fritz Haber Institute in Berlin we have developed the experimental and analysis resources, and the methodology for structure determination for adsorbates (especially molecules containing C, N and O atoms) and analysed more than 80 adsorbate structures [see bibliography link below], including many molecular adsorbates not only on metal surfaces, but also on semiconductors [e.g. 13, 15] and oxides [16]. The most recent review of the method and its achievements can be found in ref. [17]. The figure on the right shows the structure deduced for one of the more complex adsorbates studied, that of glycine on Cu(110). The experiments up to 2000 were conducted on a purpose-built beamline on the BESSY I synchrotron radiation facility in Berlin, and are now pursued on an undulator beamline at the BESSY II facility (see BESSY). Analysis of the data is based on an initial estimate using a direct inversion of experimental PhD spectra via either a Fourier transform approach or using a 'projection method' [5-8, 18]. The programme has been consistently supported by EPSRC and links to final reports of earlier grants may be found for 1996-1999 and 1999-2002 and 2002-2005 (see link on left). Following the departure of Alex Bradshaw to become scientific director of the Max Planck Institute for Plasma Physics near Munich, the Berlin end of the collaboration is now funded by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich (Sfb) 546 with a strong focus on structural properties of transition metal (especially vanadium) oxide surfaces, a small Berlin-based group retaining its base within the Fritz Haber Institute. In addition, however, a continuing parallel theme is the study of molecular adsorbates on metal surfaces, including biologically-relevant 'small' model molecules, such as amino acids.

An important technical development which we were able to exploit routinely with the combination of enhanced spectral resolution and high photon flux at BESSY II is chemical shift photoelectron diffraction. Early model studies at BESSY I showed that chemical shifts in photoelectron binding energies can be used to obtain chemical as well as elemental specificity in local structural information, either from the same atom in different coordinations within a single molecule (e.g. C in COO- and CH3- in the acetate species, or acetylenic and thrifluoromethyl species in trifluoropropyne [14]) or in different coadsorbed molecular fragments (e.g. PF, PF2 and PF3 coadsorbed on Ni(111)) [9,10]. But BESSY-II allowed us to pursue this method, to explore the potential for studying weak absorbers and low adsorbate coverages [11,12], and to fully exploit this potential (e.g. [19-22]). This work has now been transferred to the I09 beamline at the Diamond Light Source facility in the UK (only about 50 miles from Warwick) which offers some important advantages over BESSY-II, not only in its more convenient location, but also a wide-angle analyser with parallel detection providing much more rapid data accumulation.

References

  1. D.P.Woodruff Photoelectron and Auger electron diffraction Surface Sci. 299/300 (1994) 183-198
  2. D.P.Woodruff and A.M.Bradshaw Adsorbate structure determination on surfaces using photoelectron diffraction Rep.Prog.Phys. 57 (1994) 1029-1080
  3. A.M.Bradshaw and D.P.Woodruff, Structure determination of molecular adsorbates using photoelectron diffraction in Applications of Synchrotron Radiation: High-Resolution Studies of Molecules and Molecular Adsorbates on Surfaces, ed. W.Eberhardt (Springer-Verlag, Berlin, 1995) pp127-169
  4. D.P.Woodruff, R.Davis, N.A.Booth, A.M.Bradshaw, C.J.Hirschmugl, K-M.Schindler, O.Schaff, V.Fernandez, A.Theobald, Ph.Hofmann and V.Fritzsche An integrated approach to adsorbate structure determination using photoelectron diffraction: direct imaging and quantitative simulation Surface Sci. 357/358 (1996) 19-27
  5. V.Fritzsche and D.P.Woodruff A direct photoelectron diffraction method for adsorbate structure determination Phys.Rev.B 46 (1992) 16128-16134
  6. K.-M.Schindler, Ph.Hofmann, V.Fritzsche, S.Bao, S.Kulkarni, A.M.Bradshaw and D.P.Woodruff Experimental demonstrations of direct adsorbate site identification using photoelectron diffraction Phys.Rev.Letters 71 (1993) 2054-7
  7. Ph.Hofmann, K-M.Schindler, S.Bao, A.M.Bradshaw and D.P.Woodruff, Direct identification of atomic and molecular adsorption sites using photoelectron diffraction Nature 368 (1994) 131-2
  8. Ph.Hofmann, K-M.Schindler, V.Fritzsche, S.Bao, A.M.Bradshaw and D.P.Woodruff, Experimental tests of new direct methods for adsorbate structure determination using photoelectron diffraction J.Vac.Sci.Technol. A 12 (1994) 2045-2050
  9. K-U.Weiss, R.Dippel, K-M.Schindler, P.Gardner, V.Fritzsche, A.M.Bradshaw, A.L.D.Kilcoyne and D.P.Woodruff, Chemical shift photoelectron diffraction from molecular adsorbates Phys.Rev.Letters 69 (1992) 3196-3199
  10. K.-U.Weiss, R.Dippel, K.-M.Schindler, P.Gardner, V.Fritzsche, A.M.Bradshaw, D.P.Woodruff, M.C.Asensio and A.R.Gonzalez-Elipe Structure determination of coadsorbed molecular fragments using chemical shift photoelectron diffraction Phys.Rev.Letters 71 (1993) 581-4
  11. P.Baumgartel, J.J.Paggel, M.Hasselblatt, K.Horn, V.Fernandez, O.Schaff, J.H.Weaver, A.M.Bradshaw, D.P.Woodruff, E.Rotenberg and J.Denlinger, 'Structure determination of the (Ö 3xÖ 3)R30° boron phase on Si{111} using photoelectron diffraction', Phys.Rev.B. 59 (1999) 13014-13019
  12. R.Terborg, J.T.Hoeft, M.Polcik, R.Linsay, O.Schaff, A.M.Bradshaw, R.Toomes, N.A.Booth, D.P.Woodruff, E.Rotenberg and J.Denlinger, 'Structural precursor to adsorbate-induced reconstruction: C on Ni(100)' Phys.Rev.B 60 (1999) 10715-8
  13. P.Baumgärtel, R.Lindsay, O.Schaff, T.Giessel, R.Terborg, J.T.Hoeft, M.Polcik and A..M.Bradshaw, M.Carbone, M.N.Piancastelli, R.Zanoni , R.L.Toomes and D.P.Woodruff, 'The dimers stay intact: a quantitative photoelectron study of the adsorption system Si{100}(2x1)-C2H4' New.J.Phys 1 (1999) 20.1-20.15
  14. R.L.Toomes, R.Lindsay, P.Baumgärtel, R.Terborg, J.-T.Hoeft, A.Koebbel, O.Schaff, M.Polcik, J.Robinson, D.P.Woodruff, A.M.Bradshaw and R.M.Lambert 'Structure determination of propyne and 3,3,3-trifluoropropyne on Cu(111)' J.Chem.Phys. 112 (2000) 7591-7599
  15. R.Terborg, P.Baumgärtel, R.Lindsay, O.Schaff, T.Gieb el, J.T.Hoeft, M.Polcik, R.L.Toomes, S.Kulkarni A.M.Bradshaw and D.P.Woodruff, 'The local adsorption geometry of acetylene on Si(100)(2x1)', Phys.Rev.B. 61 (2000) 16697-16703
  16. J.-T.Hoeft, M.Kittel, M.Polcik , S.Bao, R.L.Toomes, J.-H.Kang, D.P.Woodruff, M.Pascal and C.L.A.Lamont 'Molecular Adsorption Bondlengths at Metal Oxide Surfaces: Failure of Current Theoretical Methods' Phys.Rev.Lett. 87 (2001) 086101-(1-4)
  17. D. P. Woodruff, ‘Adsorbate structure determination using photoelectron diffraction’, Surf. Sci. Rep. 62 (2007) 1-38
  18. D.P.Woodruff, P.Baumgärtel, J.T.Hoeft, M.Kittel and M.Polcik 'Direct methods in photoelectron diffraction; experiences and lessons learnt based on the use of the projection method' J.Phys.:Condens.Matter 13 (2001) 10625-10645
  19. D.I.Sayago, J.T.Hoeft, M.Polcik, M.Kittel, R.L.Toomes, J.Robinson, D.P.Woodruff, M.Pascal, C.L.A.Lamont, G. Nisbet 'Bond Lengths and Bond Strengths in Weak Chemisorption: N2, CO and CO/H on Nickel Surfaces' Phys.Rev.Lett. 90 (2003) 116104-1-4
  20. D.I.Sayago, M.Kittel, J.T.Hoeft, M.Polcik, M.Pascal, C.L.A.Lamont, R.L.Toomes, J.Robinson and D.P.Woodruff, 'The structure of the Ni(100)c(2x2)-N2 surface: a chemical-state-specific scanned-energy mode photoelectron diffraction determination' Surf.Sci.538 (2003) 59-75
  21. D.I. Sayago, M. Polcik, R. Lindsay, J.T. Hoeft, M. Kittel, R.L.Toomes and D.P.Woodruff, 'Structure determination of formic acid reaction products on TiO2(110)' J. Phys. Chem. B 108 (2004) 14316-14323
  22. Click here for a full bibliography.
Prof Phil Woodruff <D.P.Woodruff@warwick.ac.uk>

Last modified: 24 February 2008