Science, Technology and Engineering

Centre for Materials and Surface Science

Toroidal Spectrometer Project - Photoelectron Diffraction

Photoelectron Diffraction is a well establised technique for investigating details of the atomic structure of crystalline materials in the immediate surface region. Photoexcitation of a core electron within the surface region causes the generation of an electron wave which interacts with neighbouring atoms introducing energy dependent phase shifts. The interference of all scattered waves generates a complex intensity pattern visible over the entire emission hemisphere.

Structural information is obtained by comparing the experimental interference pattern with the results of a multiple scattering calculation generated using an assumed atomic structure which is subsequently improved by iteration until the best match is obtained. A number of computer codes have been published which facilitate this process [ 1,2 ]. An alternative method, designed to provide structural information directly and usually referred to as photoelectron holography, has also been published in a number of variants [3 ,4 ] but has yet to reach a satisfactory level of accuracy for general use.

The La Trobe toroidal spectrometer is particularly well suited to perform Photoelectron Diffraction experiments since the pass energy may be chosen large enough to enclose the entire profile of a selected core energy level. Integrating the intensity from the CCD image radially provides a significant decrease in acquisition time compared to point-by-point intensity acquisition: background subtraction is also facilitated since the line profile is known. Since emission covering 180 degrees of polar angle is routinely detected, a rotation of the sample of 180 degrees in the azimuthal sense is all that is required to obtain the intensity distribution over the hemisphere.

We have found that intensity maps such as the examples shown below may be acquired within 1 hour on U56 at BESSY. These patterns are at least as detailed as any we have seen in the literature.

Full hemisphere diffraction pattern of graphite in stereo projection. Carbon 2p core level emission using normally incident p-polarised photons of 310eV energy. No background subtraction has been applied.

Stereo projection of Cu(111) photoelectron diffraction taken with 520eV linearly polarized photons using the Cu 3p energy level.

[1] B.P.Tonner et al. Phys Rev B43,14423 (1991)
[2] F.J.Garcia de Abajo et al. Phys Rev B 63, 075404-1-16 (2001)
[3] S.Omori et al. Phys Rev Lets.88, 055504-1 (2002)
[4] T.Matsushita et al J.Elect. Spectrosc. 144-147,1175 (2005)

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