Comparison of the thermal and infrared laser induced unimolecular decompositions of allylmethylether, ethylacetate, and isopropylbromide

Abstract

The thermal and the infrared laser-induced unimolecular decompositions of three molecules, allylmethylether, ethylacetate, and isoproplybromide, have been studied. The thermal decompositions were studied using a single-pulse shock tube and the laser-induced reactions using a 0.63 cm i.d. gas cell with NaCl windows with radiation at 9.28 μ from a Lumonics 203 TEA laser and modest energy fluences (4.5J/cm2). In both studies the molecules were used in pairs (allylmethylether with ethylacetate and isopropylbromide with ethylacetate) so that relative unimolecular rate constants could be accurately determined. Although the products produced in the thermal [where equilibrium kinetics holds] and laser-induced reactions were identical, the relative rate constants showed variations. In the system in which only one reactant absorbed the laser light (ethylacetate in the mixture with isopropylbromide) the ratio of unimolecular rate constants almost reached the thermal value of this ratio at the highest pressures studied (50 torr), but the decomposition of the pumped reactant became dominant in the laser experiments at the lowest pressures (2 torr). A closer adherence to the ’’thermal’’ rate constants were observed in the analogous study of the decomposition of the two molecules which both absorbed the laser light (allylmethylether with ethylacetate). Details of the experiment, the data obtained, and the extent to which multiphoton decomposition of large molecules occur under these ’’mild’’ conditions are discussed.