Experimental and theoretical studies of the adsorption and photochemistry of dibromodifluoromethane (halon-1202) on a model carbonaceous aerosol surface

The adsorption and photochemistry of dibromodifluoromethane, CF₂Br₂, on highly ordered pyrolytic graphite (HOPG) was studied by theoretical and experimental methods as a model for heterogeneous photochemical reactions occurring on carbonaceous aerosols in the upper troposphere and lower stratosphere. The monolayer-covered HOPG surface was characterized using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) both before and after broadband ultraviolet (UV) irradiation from ∼225–350 nm. In the absence of UV radiation, molecular adsorption and desorption of CF₂Br ₂ was observed (∼140 K) while irradiation of the monolayer resulted in photodissociation of the adsorbate to yield CF₂Br and Br atoms. The most prevalent reaction observed in these experiments was the recombination of a large fraction of the photolyzed molecules (∼70%) to reform CF ₂Br₂ during TPD measurements. Minor channels revealing the formation of Br₂ (∼6%) and C₂F₄Br ₂ (∼1.4%) were present as well. Comparison of the estimated integrated photodissociation cross section (1.9 × 10−19 cm 2) to the cross section calculated from the TPD data (4.5 × 10−19 cm2) suggested that photodissociation of CF₂Br₂ occurred by direct photoabsorption by the adsorbate and not by dissociative electron attachment mediated by the surface. Differences in the distributions of Br₂ and C₂F ₄Br₂ when compared to results of gas phase and matrix isolation experiments were attributed to possible structural constraints or reduced surface diffusion for one or more of the adsorbates due to the high density of photogenerated species trapped in the surface layer. Theoretical studies of the preferred adsorption sites and orientations of CF₂Br ₂ and CF₂Br on the HOPG surface were examined using a cluster model approach to gain additional insight into the experimental data. Second-order Møller-Plesset perturbation theory (MP2) calculations were performed using 6-31G* and 6-31G(2d) basis sets and a single layer slab approximation consisting of either a four (pyrene) or seven (coronene) ring polycyclic aromatic hydrocarbon to model the HOPG surface. The preferred adsorption site for CF ₂Br₂ was found to be located at the hollow site of a six-membered ring for each combination of surfaces and basis sets examined. The interaction between the molecule and the surface was dipolar in nature with a small amount of charge transfer occurring from the surface to the adsorbate as determined from a Mulliken population analysis. For the CF₂Br radical, different preferred adsorption sites were found for the four and seven ring substrates independent of the basis set used. The atop site was the preferred location on the four ring system while the bridge site was preferred on the seven ring system. The radical favored these sites because it contains a singly occupied molecular orbital to which electron density can be added stabilizing it on the surface. Rotation of both species on the seven ring surface revealed binding energies similar to those obtained for the starting geometries suggesting that free rotation occurs at each adsorption site. These results were used to rationalize the product distributions obtained from the TPD experiments and the decrease in background intensity of the EELS data after UV irradiation.