Photofragmentation infrared emission studies of vibrationally excited free radicals CH3 and CH2I

Abstract

Time and wavelength‐resolved infrared fluorescence techniques are used to study the photofragmentation dynamics of CH₂I₂ and CH₃I at the excimer laser wavelengths of 248 and 308 nm. Emission is detected from vibrationally excited CH₂I and CH₃ radicals as well as from the excited iodine atoms [I*(²P_1/2→²P_3/2)] produced in the photolysis. A complete infrared fluorescence spectrum of the highly excited CH₂I radical is obtained as a function of time after the 248 nm dissociating laser pulse, providing both spectroscopic and vibrational deactivation data for the radical. Significant CH₂I emission is observed at all wavelengths, indicating that excitation occurs into a very high density of states, nearing the vibrational quasicontinuum. Stronger emission features are observed in the region of the C–H stretching vibrations, the CH₂ bending motion, and a combination band of these two modes. Deactivation rates for various spectral features of the highly excited CH₂I radical with CH₂I₂ and argon are presented, along with a discussion concerning the relaxation of the highly excited radical spectrum. In the photolysis of CH₃I at 248 nm, infrared fluorescence is observed directly from the out‐of‐plane bend of the CH₃ radical. In addition, at 248 nm, the quantum yield (Φ_I*) for I*(²P_1/2) production is measured to be 0.81±0.03 for CH₃I and 0.46±0.04 for CH₂I₂, while dissociation of CH₂I₂ at 308 nm yields Φ_I*=0.25±0.02. The I* quenching rates with CH₃I and CH₂I₂ are measured to be (2.5±0.4)10⁻¹³ and (3.6±0.3)10⁻¹³ cm³ molecule⁻¹ sec⁻¹, respectively.