Normal Incidence X-ray Standing Wave

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 on station 6.3 at Daresbury Professor D.P.Woodruff of Warwick University, in collaboration with Dr 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° 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) using the Synchrotron Radiation Source (SRS).

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Last updated 16th March 1999 Please send any comments or suggestions to Author Bruce Cowie or Web Editor Ian Kirkman