ROD: a program for surface X-ray crystallography

The determination of surface structures using X-ray diffraction has become increasingly popular with the emergence of ever more powerful synchrotron radiation sources. Most experiments have been performed on surfaces in vacuum, but the technique has also found applications in the determination of interfaces buried beneath a solid (Robinson et al., 1988) or liquid (De Vries, Goedtkindt et al., 1998). Few other techniques are available for this latter type of studies. The technique closely resembles bulk X-ray crystallography; the main differences are the weakness of the diffracted signal and the twodimensional nature of the diffraction features. Reviews of the technique have been presented by Feidenhans'l (1989), Robinson (1991), Vlieg & Robinson (1992) and Robinson & Tweet (1992). Existing bulk-crystallographic programs are less suitable for surface crystallography, mainly because with such programs it is dif®cult to describe the continuous intensity distribution along rods perpendicular to the surface and because it is dif®cult to take the surface roughness into account properly. This is the reason why a program was written especially for surface crystallography. Its name, ROD, points to the two-dimensional nature of the diffraction features. The present paper describes the basics of ROD; for a more comprehensive description the reader is referred to the manual (Vlieg, 1999).