Photodissociation of CF2BrCH2I at 248, 266, and 308 nm

Abstract

The technique of photofragmentation translational spectroscopy has been used to study the photodissociation of CF2BrCH2I at excitation wavelengths of 248, 266, and 308 nm. The primary photofragments are CF2BrCH2 and either I(2P1/2) or I(2P3/2), although some C–Br bond fission does occur at 248 and 266 nm. A large fraction of the CF2BrCH2 radical product contains enough internal excitation after the primary process to undergo secondary dissociation into CF2CH2 and Br. Secondary dissociation is also observed to take place at 248 and 266 nm via absorption of a photon by the CF2BrCH2 photofragment. By observing the threshold for the spontaneous secondary dissociation process, the reaction enthalpy for CF2BrCH2I→CF2CH2+Br+I, was determined to be 67.5±2 kcal/mol, which leads to: Δ H○f,0 (CF2BrCH2I)=−92.6±2 kcal/mol. The c.m. translational energy distributions were derived for both the I(2P1/2) and I(2P3/2) dissociation channels resulting from primary C–I bond breakage. The I(2P1/2)/I(2P3/2) branching ratios are 3.3, 9.0, and 0.5 for excitation wavelengths of 248, 266, and 308 nm, respectively. The translational energy distributions also reveal that a major fraction of the CF2BrCH2 product radicals are formed with high internal energies, averaging around 50% of the excess energy. The angular distributions of dissociation products with respect to the laser polarization indicate that the primary photodissociation process for the ground and excited state channels at both wavelengths proceeds via a parallel transition—i.e., the transition moment must be nearly parallel to the C–I bond.