Photodissociation of alkyl halides with two chromophores near 234nm

Abstract

Bond cleavages of alkyl halides with two chromophores progress competitively and produce a number of fragments via various paths. We select $CF_2ClBr$ and $CH_2ClBr$ to see the effect of two absorption parts. In the photodissociations of $CF_2ClBr$ and $CH_2ClBr$ near 234nm, the branching ratios are obtained using REMPI-TOF-MS and velocity distributions are measured using REMPI-imaging technique. The relative quantum yields of $CF_2ClBr$ are Φ(Br)=0.53, $\phi(Br^*)=0.22$, Φ(Cl)=0.17, and $\phi(Cl^*)=0.08$ respectively, and the branching ratio of C-Br to C-Cl is about 3. The $CF_2Cl+Br$ channel reveals a direct mode of dissociation and the $CF_2Br+Cl$ channel does an indirect mode. The relative quantum yields of $CH_2ClBr$ are Φ(Br)=0.56, $\phi(Br^*)=0.26$, Φ(Cl)=0.12, and $\phi(Cl^*)=0.07$, respectively, and the branching ratio of C-Br to C-Cl is about 4.6. The dissociation of $CH_2ClBr$ shows similar to that of $CF_2ClBr$. The $CH_2Cl+Br$ channel is a direct dissociation, whereas $CH_2Br+Cl$ is an indirect dissociation. The fission of the weakest bond C-Br is the dominant process for both molecules. Ground state halogen atoms are about 2 times more generated than spin-orbit excited state atoms. On the contrary, the branching ratio is somewhat different. The secondary reaction of $CF_2Cl$ and $CH_2Br$ radicals made an effect on the ratio.