The dynamics of laser-induced infrared multiphoton isomerization in substituted cyclobutenes-butadienes

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.