Laser-induced infrared multiphoton isomerization reactions of 2,4- and 1,3-hexadienes

Efficient and clean isomerization has been observed in the system consisting of the conjugated 2,4- and 1,3-hexadienes following infrared multiphoton excitation of these species. Variations of both laser fluence and pressure of added inert gas are seen to significantly effect branching ratios and yields. Where a competition exists between a low activation energy, low preexponential factor pathway and one with a high activation energy, high preexponential factor, an increase in fluence is seen to favor the latter pathway. An estimate of the degree of excitation in the molecule is obtainable from product branching ratios and inert gas quenching behavior. Photoacoustic measurements of energy input coupled with observation of product ratios indicate that the production of multiple products in a single laser pulse is compatible with a sequential isomerization mechanism which takes place in an almost vibrationally adiabatic fashion. Thermal and CW laser studies of hexadiene isomerization have been performed. Thermal studies yield rate constants and ΔH and ΔS for the various isomers. Thermal studies coupled with multiphoton studies have helped establish the isomerization pathway connecting the cis,trans-2,4- and trans-1,3-hexadiene isomers and have confirmed the other isomerization pathways in the system. The pathways for CW cw laser-induced isomerization have been studied.