Electron-stimulated chemistry of CF3I adsorbed on Ag(111) C—F bond cleavage and C—C coupling

Abstract

For CF₃I adsorbed on Ag(111), reactions stimulated by 100 eV electrons are characterized by temperature programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). In the absence of electron irradiation, multilayers desorb at 100 K, two orientations in the first layer desorb at 110 and 128 K, and CF₃ radical desorption occurs at 300 K. After electron irradiation of coverages below one layer [θ < 0.30 monolayer (ML)], the remaining parent desorption is broadly distributed between 110 and 225 K, there is a C₂F₅ radical desorption peak at 340 K and the 300 K CF₃ radical desorption is replaced by a peak at 240 K. At coverages greater than 0.30 ML, TPD after electron irradiation reveals two new products (CF₂I₂ and C₂F₃I) formed in roughly equal amounts. RAIRS suggests that adsorbed CF₂I₂ has C_2v symmetry with both I atoms bound to Ag. We propose that C—C coupling to form C₂F₅ occurs by insertion of CF₂⁺ or CF₂, formed by impact ionization of the parent, into adsorbed CF₃, previously formed during either adsorption or the earlier stages of electron irradiation. CF₂I₂ formation is described in terms of two possible processes: (i) formation of I^–, which reacts with neighbouring CF₃I, and (ii) C—F fragmentation of CF₃I⁺, formed in multilayers, into CF₂I⁺, which reacts with previously formed atomic iodine. At 0.75 ML, the cross-section for loss of CF₃I by all removal processes is 1.1 ± 0.2 × 10^–16 cm², whereas that for CF₂I₂ formation is ca. 10^–17 cm².