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 AH and AS 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.
Clean isomerization between a number of cyclobutenes and their corresponding butadienes has been induced via multiple photon infrared absorption. Appropriate choices of laser frequency allow the reaction to be pushed in either direction, even though the cyclization reaction is typically 10 kcal and 4 e.u. thermodynamically disfavored. A significant dependence of yields on fluence and added inert gas pressure is observed. A simple rate equation model is developed which reproduces the major experimental observations regarding the system. Absorption cross sections, a necessary parameter for the rate equation model, are measured both calorimetrically and photoacoutically. The absorption cross section does not vary significantly as a function of laser fluence implying that absorption cross sections for successive photon absorption processes are essentially constant. The measured absorption cross section is also very similar in magnitude to the absorption cross section taken from the infrared spectrum of the compound at the laser frequency. No pressure effects on the absorption cross section are observed over the pressure range studied.
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