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.
-The tunability of the CO2 molecular gas laser permits selective multiphoton infrared excitation of one isomer in a mixture of isomers. This capability can be exploited to drive isomerization reactions in a contrathermodynamic direction. Examples of such reactions are the trans + cis isomerization of alkenes and the electrocyclic isomerization of butadienes to cyclobutenes. It is also possible to achieve some selectivity in consecutive isomerization reactions (A + B + C) and in competing reactions (A + B ÷ C) via multiphoton infrared excitation. For example, consecutive reactions can be stopped at the intermediate product B in cases where the activation energy for B + C is lower than that for A + B. The product ratio B/C in competing reactions is, in some cases, dependent upon the average vibrational temperature of the reactant A and hence can be altered by changing laser fluence or collisional frequency. In addition, pulsed infrared lasers can be used to create high concentrations of vibrationally excited fragmentation products. We are currently seeking to correlate product vibrational energy distributions with reactant structure and decomposition mechanism.
INTRODUCT IONReactions of electronically excited polyatomic organic molecules have occurred on earth since the prebiotic era and have been the subject of scholarly publications since the emergence of the modern chemical literature. All photochemists are familiar with the Stark-Einstein law: electronic excitation occurs upon absorption of a single quantum of light. Under the best of circumstances, the electronically excited molecule can undergo chemical transformation with unit efficiency. Absorption of a single infrared photon produces a vibrationally excited molecule with insuffficient energy to undergo chemical transformation. With the advent of high powered infrared lasers, it has become possible to effect chemical reactions via multiphoton absorption in an intense laser field (1-4). Thus laser technology has spawned a new field of chemistry, infrared photochemistry. From a historical perspective it is interesting to note that the initial investigations of photochemical mechanisms were conducted in the vapor phase (5). Chemical applications of infrared lasers have thus far been limited almost exclusively to the vapor phase.
THE CO2 LASERThe commonly used source for infrared laser chemistry is the pulsed CO2 molecular gas laser. The important characteristics of the CO2 laser are (a) high power (ca. 1020 photons/pulse or several gigawatts/cm2 in a focused beam), (b) moderately short pulse duration (< 1 is), and (c) limited tunability from ca. 907 to 992 cm1 and from 1016 to 1092 cm1. Its high power makes the CO2 laser well suited for many technological (welding, cutting, etc.) as well as chemical applications. Thesimplest chemical application of CO2 lasers is the uniform heating of the irradiated volume either by direct absorption 1683
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