Molecular Order at the Surface of an Organic Monolayer Studied by Low Energy Helium Diffraction.

We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. With this uniquely surface-sensitive and non- destructive technique, we show that the monolayer of the alkane thiol CH sub 3 (CH sub 2) sub (21) SH on Au (111) is composed of small, ordered domains (lattice constant 5.01 +- 0.02angstroms), a large fraction of which share a common orientation. The helium diffraction intensities decrease smoothly with increasing temperature and vanish around 100K, presumably due to thermally induced displacements of the ends of the molecular chains.