SPECTROSCOPY AND PHOTOCHEMISTRY OF ACETYLENE ADSORBED ON NACL(100)

Polarized infrared absorption spectroscopy is used to determine molecular tilt and orientation of acetylene physisorbed to NaCl(100). The spectra are qualitatively explained by an ordered structure similar to the low-temperature crystalline phase of acetylene. We also find evidence for a high-temperature, disordered lattice-gas phase of this system. Upon exposure to 184.9 nm light the lattice gas undergoes a hydrogen exchange reaction, while the structured phase is unreactive. In contrast to gas-phase photolysis, which yields a polymer, molecular hydrogen, and a collection of small hydrocarbons, no, products other than acetylene isotopomers are observed in the surface-phase reaction. The rate of the reaction shows an. exponential temperature dependence and is interpreted as being surface diffusion limited. We propose that this photochemical hydrogen exchange occurs by a radical chain reaction.