The adsorption of hydrocarbons on platinum studied by low-energy electron diffraction intensities. The ordered (2 x 2) 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 (2 x 2) 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 (2 x 2) 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 (2 x 2) 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 (2 x 2) surface structure that is identical to the stable acetylene structure as shown by the intensity profiles