COMPETITION AMONG COLLISIONAL DEACTIVATION, IONIZATION, AND DISSOCIATION IN THE MULTIPHOTON EXCITATION OF OCTAFLUOROCYCLOOCTATETRAENE

Article abstract of DOI:10.1021/j100320a018

The gas-phase UV multiphoton induced chemistry and multiphoton ionization of octafluorocyclooctatetraene (OFCOT) have been examined as a function of laser pulse energy, laser wavelength, and background gas pressure.The production of hexafluorobenzene (HFB), tetrafluoroethylene (TFE), and tetrafluoromethane (TFM) as well as carbon has been observed.An intense fluorescence has also been recorded and attributed to the Swan bands of C₂.The laser intensity dependent bulk kinetics of the OFCOT decomposition were found to be first order with a rate constant of 5.7 (+/-0.2)E3 s^-1 at a (<*>15 ns) pulse energy of 25 mJ and a sample pressure of 750 mTorr.Qualitative aspects of the photochemistry of HFB and TFE, under identical conditions, are also reported.The addition of N2 to the sample is shown to increase the rate of photolysis and decrease the MPI current.This observation is attributed to the competition between ionization and dissociation from the initially created valence state of the target molecule.The increase in the number of collisions results in a larger fraction of excited-state molecules undergoing relaxation to states which are either geometrically or energetically unfavorable with respect to ionization.The result, therefore, is an increase in the yield of the dissociation products.A simple kinetic rate equation model is in agreement with these observations.OFCOT appears to be a member of a unique group of large, organic molecules exhibiting this type of competition.