Gas-Phase Collisional Relaxation of the CH2I Radical after UV Photolysis of CH2I2.

Transient UV absorption spectra and kinetics of the CH2I radical in the gas phase have been investigated at 313 K. Following laser photolysis of 1−3 mbar CH2I2 at 308 nm, transient spectra in the wavelength range 330−390 nm were measured at delay times between 60 ns and a few microseconds. The change of the absorption spectra at early times was attributed to vibrational cooling of highly excited CH2I radicals by collisional energy transfer to CH2I2 molecules. From transient absorption decays measured at specific wavelengths, time-dependent concentrations of vibrationally “hot” and “cold” CH2I and CH2I2 were extracted by kinetic modeling. In addition, the transient absorption spectrum of CH2I* radicals between 330 and 400 nm was reconstructed from the simulated concentration−time profiles. The evolution of the absorption spectra of CH2I* radicals and CH2I2* due to collisional energy transfer was simulated in the framework of a modified Sulzer−Wieland model. Additional master equation simulations for the collisional deactivation of CH2I* by CH2I2 yield 〈ΔE〉 values in reasonable agreement with earlier direct studies on the collisional relaxation of other systems. In addition, the simulations show that the shape of the vibrational population distribution of the hot CH2I* radicals has no influence on the measured UV absorption signals. The implications of our results with respect to spectral assignments in recent ultrafast spectrokinetic studies of the photolysis of CH2I2 in dense fluids are discussed. [ABSTRACT FROM AUTHOR]