The adsorption of hydrocarbons on platinum studied by low-energy electron diffraction intensities. The ordered (2x2) overlayers of acetylene and ethylene on the (111) crystal face of platinum

The intensities of the low-energy electron diffraction beams back-scattered from the ordered (2x2) overlayers of acetylene and ethylene on the platinum (111) crystal face are measured by a photographic technique. The intensities are presented in an appendix in the form of normalized diffraction beam intensity versus incident electron energy ( I-V curves). Acetylene spontaneously forms the ordered (2X2) adsorbate on platinum at 300 K and low exposure under ultrahigh vacuum conditions. The intensity profiles reveal that this structure is metastable; and upon heating to 350-400 K for 1 hr it undergoes a transformation to a stable structure with the same (2x2) unit cell size. The acetylene transformation is thought to involve a strengthening of the carbon-platinum bond accompanied by an expansion of the carbon-carbon bond length. Ethylene adsorbes on the platinum (111) surface and with electron beam exposure forms an ordered (2x2) surface structure that is identical to the stable acetylene structure as shown by the intensity profiles.