PHOTODISSOCIATION OF BROMOCARBONS AT 193 NM, 222 NM, AND 248 NM - QUANTUM YIELDS OF BR ATOM AT 298-K

The primary quantum yields, PHI(Br), for the formation of Br atom in the photodissociation of CF2Br2 and CH3Br at 248, 222, and 193 nm, and of CF3Br at 222 and 193 nm were measured at 298 K. The bromine atoms were directly detected via resonance fluorescence following pulsed laser photolysis of the molecules of interest. The Br atom quantum yields in CF2Br2 photolysis increased with decreasing wavelengths: 1.01 +/- 0.15, 1.63 +/- 0.19, and 1.96 +/- 0.27 at 248, 222, and 193 nm, respectively. The PHI(Br) values in CH3Br and CF3Br were close to unity at all the wavelengths: 0.92 +/- 0.15 and 1.12 +/- 0.16 at 222 and 193 nm, respectively, for CF3Br; 1.01 +/- 0.16, 1.10 +/- 0.20, and 1.05 +/- 0.11 at 248, 222, and 193 nm, respectively, for CH3Br. Quantum yield of H atom formation in the photolysis of CH3Br at 193 nm was measured to be 0.002 +/- 0.001. H atom could not be detected in the photolysis at 248 and 222 nm. In all cases the PHI(Br) values were found to be independent of buffer gas pressure or the photolysis laser fluence. Our results suggest that the quantum yields for dissociation of all the molecules considered here are unity; therefore, atmospheric lifetime calculations carried out by assuming a unit dissociation quantum yield are correct. The nature of the photodissociation process is discussed in the light of previous and present results.