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| - Gas-Phase Collisional Relaxation of the CH2I Radical after UV Photolysis of CH2I2
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| - Transient UV absorption spectra and kinetics of the CH2I radical in the gas phase have been investigated at313 K. Following laser photolysis of 1−3 mbar CH2I2 at 308 nm, transient spectra in the wavelength range330−390 nm were measured at delay times between 60 ns and a few microseconds. The change of theabsorption spectra at early times was attributed to vibrational cooling of highly excited CH2I radicals bycollisional energy transfer to CH2I2 molecules. From transient absorption decays measured at specificwavelengths, time-dependent concentrations of vibrationally “hot” and “cold” CH2I and CH2I2 were extractedby kinetic modeling. In addition, the transient absorption spectrum of CH2I* radicals between 330 and 400nm was reconstructed from the simulated concentration−time profiles. The evolution of the absorption spectraof CH2I* radicals and CH2I2* due to collisional energy transfer was simulated in the framework of a modifiedSulzer−Wieland model. Additional master equation simulations for the collisional deactivation of CH2I* byCH2I2 yield 〈ΔE〉 values in reasonable agreement with earlier direct studies on the collisional relaxation ofother systems. In addition, the simulations show that the shape of the vibrational population distribution ofthe hot CH2I* radicals has no influence on the measured UV absorption signals. The implications of ourresults with respect to spectral assignments in recent ultrafast spectrokinetic studies of the photolysis of CH2I2in dense fluids are discussed.
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