Flash photolysis under non-isothermal conditions

The temperature rise which accompanies every flash photolytic reaction interferes with, and often makes impractical, measurements of the reaction rate constants. This difficulty may be partly overcome if the whole reaction vessel is uniformly irradiated by both the photolytic and the analyzing flash lamps.A flash photolysis apparatus with these characteristics was used to study bromine atom recombination. A 10 to 15 fold gain in atomic concentration, which corresponds to a 100 to 225 fold increase in three-body recombination rate, compared with the work of previous authors, was achieved with this apparatus. The reaction rate constants were determined from the changes in absorption of Br2 at either 4 035 Å or at 4 980 Å. The recombination rate constant of bromine in an excess of helium at 90 ± 20 °C was found to be equal to (0.8 ± 0.3)109 l2 mole−2 s−1 (measured at 4 980 Å) and (0.5 ± 0.1)109 l2 mole−2 s−1 (measured at 4 035 Å). The results suggest that the technique herein described can yield meaningful data, even though the reaction was accompanied by a 105 °C temperature rise. There was little heat exchanged between the reacting gas and the walls of the reaction vessel. Consequently the reaction vessel behaved as an effective calorimeter throughout the reaction.