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Results are presented for the photolysis of acetylene physisorbed to polycrystalline films of NaC1. We find evidence for two adsorbed phases of this system: a low-temperature structured phase and a high-temperature lattice gas. Upon exposure to 184.9-nm light, the high-temperature phase undergoes the hydrogen exchange C2H2 + C2D2 e 2C2HD, while the low-temperature 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 exchange occurs by a radical chain reaction.

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