The adsorbate layer is in general ordered, due to adsorbate-substrate forces and adsorbate-adsorbate forces. Ordering has been shown by extensive studies of the dynamics of photofragment recoil from the surface, as well as by polarised Fourier Transform Infrared (FTIR) spectroscopy.
Photofragments (e.g., atomic H recoiling from a sub-monolayer of HCl adsorbed on LiF(001), photolysed at 193 nm) directed downward at an inert crystal surface recoil at non-specular angles, indicative of `Localised Atomic Scattering' (LAS).
This is evidence that the scattered H, coming from ordered HCl(ad), rebounds from selected atomic locations on the surface. In more recent work using a reactive rather than an inert substrate (Si(111)) it has been found that the downward-directed photofragments (e.g., Cl atoms from chlorobenzene) react closely-adjacent to the `parent' molecule (the chlorobenzene, in the example cited). This is clear evidence of `Localised Atomic Reaction', (LAR).
These and other systems involving the photodissociation and electron-impact dissociation of individual molecules, studied by Scanning Tunneling Microscopy (STM), will be described. The successive application of (a) adsorbate ordering and (b) adsorbate imprinting, should provide a versatile method for the marking of molecular-scale (1 - 100 nm) patterns on reactive surfaces such as semiconductors.