Polyhalide photofragment spectra. I. Two-photon two-step photodissociation of methylene iodide

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  • Peter M. Kroger
    Department of Chemistry, Yale University, New Haven, Connecticut 06520
  • Peter C. Demou
    Department of Chemistry, Yale University, New Haven, Connecticut 06520
  • Stephen J. Riley
    Department of Chemistry, Yale University, New Haven, Connecticut 06520

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<jats:p>Photofragment spectroscopy of CH2I2 at a wavelength of 266 nm shows evidence for a two-photon, two-step dissociation process. The first photon absorption causes transition to a repulsive excited state of B1 symmetry that breaks up rapidly to yield highly internally excited CH2I radicals and an I atom. On average, between 80% and 90% of the effective available energy for this process appears in CH2I rovibrational excitation. Once formed, a sizable fraction of the CH2I fragments may absorb a second photon, dissociating to CH2 radicals and another I atom. Energy distribution analysis indicates that the CH2 fragments are both translationally and internally excited, although quantitative estimation of their rovibrational excitation is complicated by the possibility of electronic excitation of one or more fragments in the two-step process. An approximate analysis of the CH2 fragment recoil angular distribution allows a crude resolution of the CH2I fragment internal excitation into its rotational and vibrational components. In the absence of electronic excitation, 40% of the CH2I internal energy is found to be rotational and 60% vibrational, qualitatively consistent with the picture of impulsive recoil of a semirigid radical. Kinetic analysis of the dependence of CH2I and CH2 fragment production on photolyzing light intensity permits extraction of the cross section for second photon absorption. From this, it is estimated that the extinction coefficient for CH2I radical absorption at 266 nm is 30 l mole−1⋅cm−1.</jats:p>

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