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Synchrotron radiation-based methods

A strong theme of the group's research is the development and application of quantitative structural methods to studies of adsorbate structures, especially of molecular species (including reaction intermediates) on transition and noble metal surfaces. The two main techniques used by Professor Phil Woodruff in this part of the programme are (backscattering) photoelectron diffraction (PhD) and normal incidence X-ray standing wavefield absorption (NIXSW), and both employ synchrotron radiation.

The photoelectron diffraction technique was developed in collaboration with Prof. Alex Bradshaw at the Fritz Haber Institute (FHI) in Berlin using the BESSY and BESSY-II storage rings in Berlin. As a result of Alex Bradshaw's transfer to Scientific Director of the Institute for Plasma Physics in Munich in 1999, Phil spent about one week each month in Berlin until the end of 2011 at the FHI, managing this project and its associated collaborations within a 'Sonderforschungsbereich' funded by the German research council (Deutsche Forschungsgemeinschaft), and supervising PhD students in both Warwick and Berlin. More recently, however, this effort has moved to the more convenient and now superior I09 beamline at the DIAMOND Light Source.

The photoelectron diffraction programme has been particularly fruitful in the study of molecular species containing C, N and O atoms and has led to quantitative structural analyses of intermediates such as the methoxy, formate and acetate species and to increasing understanding of the structural aspects of simple hydrocarbon species and even more complex molecules such as glycine (the simplest amino acid) and methyl pyridine with Ni and Cu surfaces. The availability of new third-generation soft X-ray synchrotron radiation sources offers important new opportunities for a 'chemical-shift' variant of this technique (the figure shows the structure determined from such an experiment on coadsorbed PF3, PF2 and PF), and this work is now proceeding on the BESSY II source in Berlin. Somewhat later we devoted increasing attention to the generally ill-understood problem of oxide surface structure.

NIXSW is a modification of the more general XSW method developed by the Warwick group which is particularly applicable to metal surfaces, and has been used to study atomic (e.g. Cl, S, O, Na, Rb) and molecular (e.g. methyl thiolate and longer-chain alkyl thiolates) on Cu, Ni and Al surfaces. All the early development was conducted at the Daresbury SRS facility in Cheshire, but subsequently some of this work transferred to the third-generation ESRF X-ray source in Grenoble in France where it proved possible to exploit chemical state sensitivity and extend studies even to light atoms like C, N and O. This work is also now being pursued at the same I09 beamline at DIAMOND which uniquely (worldwide) allows the use of both techniques on the same sample. The NIXSW technique is proving particularly valuable as a probe of the the geometry of model metal-organic interfaces of relevance to inorganic semiconductor devices, such as phthalocyanines and TCNQ on coinage metal (111) surfaces.