Absolute cross sections for dissociative electron attachment to condensed CH3Cl and CH3Br: Effects of potential energy curve crossing and capture probability

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Abstract
We report cross sections for the trapping of 0–10 eV electrons by CH3Cl and CH3Br physisorbed onto a Kr covered Pt substrate, measured as a function of Kr film thickness and methyl halide concentration. The molecules stabilize electrons incident at the surface by the dissociation of transient CH3Cl− and CH3Br− ions into an atomic anion and a neutral fragment [dissociative electron attachment DEA]. For CH3Cl, the condensed phase absolute DEA cross section at ≈0.5 eV, reaches 13×10−18 cm2±50%, which is 104–106 times larger than the gas phase cross section. At higher energies (5–10 eV) for CH3Cl, our measurements provide a lower limit for the DEA cross section. For CH3Br, the maximum DEA cross section occurs below the vacuum level; we measure an absolute magnitude of 3.0×10−16 cm2±50% near 0 eV, which is 100 times larger than the corresponding gas phase value. These enhancements in cross section arise from the lowering of the potential energy surfaces of intermediate anions due to polarization induced in the Kr layer and metal substrate. An increase in DEA cross section with a reduction in the distance of transient anions from the metal surface, is explained by the effect of image charges on the energy at which anion and neutral ground state potential energy curves cross. Below thicknesses of 5 ML of Kr, a decrease in DEA cross section is observed and attributed to a reduction in the electron capture probability of the halide due to competition with transfer to the metal substrate.