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

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.

Photoinduced Fragmentation of Multilayer CH3Br on Cu/Ru(001)

“…(iii) Dissociation of caged negative ion complexes, (CH 3 Br) n -, n g 1 that are created as a solvationlike process that stabilizes the ionic species. 6 Ethane (mass 30) and ethylene (mass 27, the choice of this mass was to avoid the large background at mass 28) TPD peaks are observed near 95 K. These molecules are absent from the TPD spectra in the case of the nonirradiated system. The peak near 95 K, shown in Figure 8c, practically coincides with the minimum of the d∆ /dT spectrum, shown in Figure 8b.…”

“…For our system of CH 3 Br on Cu(2ML)/Ru(001) at 82 K, three assumptions can be made based on earlier studies: 2,3,6,7 (i) No significant direct photodissociation channel takes place. (ii) The maximum penetration depth of photoelectrons is ∼20 ML.…”

“…2,6,25 These ions generate electric field around them and are attracted to their image charge inside the bulk metal underneath. To caracterize the dissociation products embedded inside the multilayer structure after irradiation and to probe the multilayer restructure during the diffusion of the Br -ions, a set of ∆ -TPD spectra were recorded under similar condition as for the ∆p-TPD.…”

“…However, if a low energy photoelectron is ejected from the surface to be attached to a parent molecule within the multilayer, an externally assisted anionic bound state, (CH 3 Br) n -, n g 1, may be formed. 6 Such a "solvation" process should stabilize the anion via polarization of the surrounding medium. A recent study has reported a DEA cross section of ∼1 × 10 -16 cm 2 for low energy electrons near and below 1 eV, for CH 3 Cl molecules embedded inside a Kr solid matrix.…”

“…As directly measured by Sanche and co-workers, electrons at kinetic energy of about 0.5 eV are sufficient to attach and cause DEA of methyl halides. 6 Upon the formation of molecular multilayers, the direct dissociation route (CH 3 X (ad) + hν f CH 3(gas/ad) +X (gas/ad) ) becomes increasingly dominant, 2,3 whereas the DEA channel is quenched, because of limited penetration into the multilayers of the ejected photoelectrons. The dependence of the photofragmentation yield on the number of deposited methyl halide layers was previously established for CH 3 Cl/Pt(111) 7 by monitoring the dependence of the photolysis rates and photoelectron yields on the multilayer thickness, where mean free path was obtained for low energy (>1 eV) photoelectrons.…”