Energy partitioning in the collision-free multiphoton dissociation of molecules: Energy of ? CF2 from CF2HCl, CF2Br2, and CF2Cl2

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  • John C. Stephenson
    Laser Chemistry Program, Institute for Materials Research, National Bureau of Standards, Washington, DC 20234
  • David S. King
    Laser Chemistry Program, Institute for Materials Research, National Bureau of Standards, Washington, DC 20234

Abstract

<jats:p>We have developed a simple laser-excited fluorescence method to determine the translational energy of the nascent products of infrared multiphoton dissociations, and have applied this method to the measurement of the average translational energy ET of the ? 1A1 CF2 radicals formed from the collision-free dissociation of CF2HCl, CF2Br2, and CF2Cl2 by CO2 TEA laser pulses. The initially formed CF2 (v,J,K) is distributed in many vibrational (v) and rotational (J,K) states, and we have obtained ET (v,J,K) specifically for different values of these internal quantum numbers. ET of the CF2 is different for each parent molecule, and is independent of the intensity or wavelength of the CO2 laser for the range of values investigated. For the CF2 produced from CF2HCl, ET was the same for fragments formed with no vibrational energy and for those formed in the ν2=5 level with 3320 cm−1 of vibrational excitation, and ET was also the same for the products formed with little rotational excitation (ER?40 cm−1) and for those born with substantially higher rotational excitation (ER=240 cm−1). Assuming a Boltzmann velocity distribution in the photoproducts, we deduce an average kinetic energy ET=6.9±2, 1.5±0.5, and 1.7±0.5 kcal/mol for the nascent CF2 fragments formed from CF2HCl, CF2Cl2, and CF2Br2, respectively. The method we used previously to measure the collision-free distribution of vibrational energy Ev in the CF2 formed from CF2Br2 and CF2Cl2 was applied to the CF2 from CF2HCl. The distribution was found to follow the relation P (Ev) =exp(−Ev/kTv), where P (Ev) is the probability that a nascent CF2 fragment contains vibrational energy Ev, k is Boltzmann’s constant, and Tv, the single parameter necessary to characterize the distribution, is the vibrational temperature. For the CF2 formed from CF2HCl, Tv =1160±100 K. These measurements complete our determination of the complete distribution of energy (electronic, vibrational, rotational, and translational) in the CF2 product of these reactions.</jats:p>

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