When an X-ray Bragg reflection is established in a crystal the incident and scattered waves interfere to produce an X-ray standing wavefield in the crystal which has a periodicity in intensity equal to the periodicity of the associated scatterer planes of the crystal. A full dynamical (multiple scattering) formulation of the X-ray scattering under these conditions shows that the range - in X-ray incidence angle or wavelength - over which the nominal total reflectivity of the Bragg condition is maintained is finite, and the standing wave thus exists throughout this range. However, the juxtaposition of the nodal planes of the standing wave relative to the scatterer planes - i.e. the phase of the standing wave - shifts in a systematic and predictable fashion within this range by one half of the scatterer plane spacing. This can be used to locate specific atomic species within the solid by monitoring the X-ray absorption of these species as a function of scattering condition within the total reflectivity range. If the absorber lies on the nodal planes of the standing wave no adsorption is detected, while if they lie on the antinodes, enhanced absorption is found. In this way XSW can be used to locate specific atoms, relative to the bulk scatterer planes, not only within a crystal, but also at the surface of a crystal, because the standing wavefield extends far above the surface. A key limitation of this general XSW method, however, is that under general conditions the angular range of the total reflectivity condition - the 'rocking curve width - can be very narrow (seconds of arc) so the experiment is only possible on samples which have a very high degree of crystalline perfection. Typically this restricts the method to semiconductor samples (especially Si) and a few materials grown with especial care.

In experiments in the 1980s the Warwick group, in collaboration with Rob Jones at Nottingham University, showed that this restriction can be overcome by working at Bragg conditions close to normal incidence to the scatterer planes [1]; in this case the Bragg condition goes through a turning point and the effective rocking curve width can be of order 1 degree or more. The resulting normal incidence XSW (NIXSW) method has now been applied successfully to many adsorbate systems (Cl, S, CH3S-, CH2ClCH2Cl, Na, O, Rb, CH3O-, PF3 etc.) on standard metal surfaces (especially Cu, Ni, Al) [2] using the Daresbury Synchrotron Radiation Source (SRS), sadly now closed down). We have published two general reviews of the method [3,4] This is a continuing programme in which we are seeking to push the method to problems of increasing complexity and potential technical relevance. Even before the closure of the the SRS we were making increasing use of the European Synchrotron Radiation Facility (ESRF) in Grenoble, but since 2015 we have been ujsing the greatly superior I09 beamline at the DIAMOND Light Source, only about 50 miles from the university. The very significant improvement in both flux and resolution at the ESRF undulator stations an this third-generation X-ray source first allowed us to exploit 'chemical shifts' in photoelectron binding energies to determine the structure of coadsorbed molecular fragments and reaction products, and our first such results were published in 1999 and 2000 [2,5]. It is now possible to map the local adsorption structure through adsorbate phase diagrams and to make further progress on the NIXSW technique in the study of low atomic number adsorbate species - especially C, N and O. An important aspect of the proper application of this methodology, however, is to take proper account of the backward-forward asymmetry in the photoemission signal due to non-dipole (quadrupole) transitions, and having first demonstrated the importance of this problem [6] we have recently shown that by the use of proper calibration experiments [7,8] it is straight-forward to apply these corrections. A particularly important aspect of the new I09 beamline at DIAMOND is the ability to perform NIXSW structural studies, but also to perform high-resolution soft-Xray photoelectron spectroscopy (SXPS) to obtain much higher-resolution chemical shift information that can aid the interpretation of the chemical-state specificic NIXSW data. Our current work is focussed on investigations of larger essentially planar pi-bonded molecules on metal surfaces, in collaboration with Giovanni Costantini in the Chemistry Deparetment at Warwick, with the aim of gaining a better understanding of metal-organic interfaces that underpin the properties of a wide range of organic electronic devices.

References

 

1. D.P.Woodruff, D.L.Seymour, C.F.McConville, C.E.Riley, M.D.Crapper, N.P.Prince and R.G.Jones Phys.Rev.Lett. 58 (1987) 1460: Surface Sci. 195 (1988) 237
2. G.J.Jackson, J.Lüdecke, D.P.Woodruff, A.S.Y.Chan, N.K.Singh, J.McCombie, Robert G.Jones, B.C.C.Cowie, V.Formoso, Chemical-Shift X-ray Standing Wave Studies: Coadsorption Site Determination of PF_x Fragments on Ni(111) Surf.Sci. 441 (1999) 515-528
3. D.P.Woodruff, Normal incidence X-ray standing wave determination of adsorbate structures, Prog. Surf. Sci. 57 (1998) 1
4. D.P.Woodruff, Surface structure determination using X-ray standing waves, Rep.Prog.Phys. 68 (2005) 743
5. G.J.Jackson, D.P.Woodruff, R.G.Jones, N.K.Singh, A.S.Y.Chan, B.C.C.Cowie, V.Formoso, Following Local Adsorption Sites through a Surface Chemical Reaction: CH₃SH on Cu(111) Phys.Rev.Lett. 84 (2000) 119-122
6. C.J.Fisher, R.Ithin, R.G.Jones, G.J.Jackson, D.P.Woodruff, B.C.C.Cowie and M.F.Kadodwala, 'Non-dipole photoemission effects in X-ray standing wavefield determination of surface structure' J.Phys.:Condens.Matter 10 (1998) L623-9
7. G.J.Jackson, B.C.C.Cowie, D.P.Woodruff, R.G.Jones, M.S.Kariapper, C.J.Fisher, A.S.Y.Chan and M.Butterfield, ‘Atomic quadrupolar photoemission asymmetry parameters from a solid state measurement’, Phys.Rev.Lett. 84 (2000) 2346-2349
8. J.Lee, C.Fisher, D.P.Woodruff, M.G.Roper, R.G.Jones, B.C.C.Cowie ‘Non-dipole effects in photoemission-monitored X-ray standing wave experiments: characterisation and calibration’ Surf.Sci..494 (2001) 166-182
9. click here for complete bibliography.

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Prof Phil Woodruff <D.P.Woodruff@warwick.ac.uk>