Multiphoton Infrared Laser-Induced Absorption and Reaction of Organic Esters

Article abstract of DOI:10.1021/ja00384a032

Date are reported for the multiphoton, pulsed, infrared laser-induced unimolecular reactions of a series of organic esters.The dependence of the reaction probability and absorption cross sections on the laser fluence, laser frequency, reactant pressure, and bath gas pressure was determined for ethyl acetate, ethyl fluoroacetate, ethyl acrylate, n-butyl acetate, sec-butyl acetate, and ethyl 2-bromopropionate.The extent of collisional energy transfer from excited ester molecules to cold molecules was characterized by a thermal monitor molecule; collisional energy transfer leading to reaction of the monitor was negligible at pressures <0.10 torr.The laser absorption cross sections were independent of pressure but mildly dependent on fluence; all cross sections extrapolated to the single-photon, broad-band cross section at low laser fluence.Direct and sensitized excitation of the bifunctional esters (sec-butyl acetate and ethyl 2-bromopropionate) gave similar results, demonstrating that the absorbed laser energy was randomized before chemical reaction.Log-log plots of reaction probability, P(Φ), vs. fluence showed approximately linear behavior for P(Φ) = 10^-2-10^-4 with slopes of 4-6.At high fluences P(Φ) values asymptotically approach constant values between 0.6 and 1.0.For ethyl acetate and ethyl fluoroacetate the reacion probability for the same amount of absorbed energy was dependent on laser frequency and fluence; this is evidence that only a certain, fluence-dependent, fraction of molecules absorb the laser energy.Addition of bath gas reduced the reaction probabilities, especially at low laser fluence, indicating that postpulse reaction was very important for P(Φ) <0.1.