Generation of Ketene With High Quantum Yield by a KrF Laser

Acetone was photolyzed around 770 K by pulsed laser irradiation at 248 nm. The methyl radicals, generated in the primary step, trigger a chain reaction, producing ketene (CH2CO) and methane. Long chains (high quantum yields) result from low radical concentrations. Using a collimated laser beam of low intensity, quantum yields up to 300 have been demonstrated. Approximate rate constants have been derived and used for extrapolation to higher temperatures and different densities. Compared to the thermal process, the photochemical formation of ketene is faster. Therefore its thermal decomposition can be avoided to some extent. But the improvement is probably too small to be of technical interest. We also found indications that the thermal decomposition of acetone above about 500 to 600 K yields three fragments in a single step.