title: Gas-phase collisional relaxation of the CH2I radical after UV photolysis of CH2I2.
abstract: Transient UV absorption spectra and kinetics of the CH(2)I radical in the gas phase have been investigated at 313 K. Following laser photolysis of 1-3 mbar CH(2)I(2) 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 CH(2)I radicals by collisional energy transfer to CH(2)I(2) molecules. From transient absorption decays measured at specific wavelengths, time-dependent concentrations of vibrationally "hot" and "cold" CH(2)I and CH(2)I(2) were extracted by kinetic modeling. In addition, the transient absorption spectrum of CH(2)I radicals between 330 and 400 nm was reconstructed from the simulated concentration-time profiles. The evolution of the absorption spectra of CH(2)I radicals and CH(2)I(2) 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 CH(2)I by CH(2)I(2) yield DeltaE 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 CH(2)I 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 CH(2)I(2) in dense fluids are discussed.
authors.affiliation: Institut für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Lenzer
authors.forename: Kawon
authors.initials: K
authors.lastname: Oum
authors.forename: Jörg
authors.initials: J
authors.lastname: Schroeder
authors.forename: Kentaro
authors.initials: K
authors.lastname: Sekiguchi