ELECTRON-STIMULATED CHEMISTRY OF CF3I ADSORBED ON AG(111) C-F BOND-CLEAVAGE AND C-C COUPLING

For CF3I adsorbed on Ag(111), reactions stimulated by 100 eV electrons are characterized by temperature programmed desorption (TPD) and refl ection-absorption infrared spectroscopy (RAIRS). In the absence of ele ctron irradiation, multilayers desorb at 100 K, two orientations in th e first layer desorb at 110 and 128 K, and CF3 radical desorption occu rs at 300 K. After electron irradiation of coverages below one layer [ theta < 0.30 monolayer (ML)], the remaining parent desorption is broad ly distributed between 110 and 225 K, there is a C2F5 radical desorpti on 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 rough ly 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 C 2F5 occurs by insertion of CF2+ or CF2, formed by impact ionization of the parent, into adsorbed CF3, previously formed during either adsorp tion or the earlier stages of electron irradiation. CF2I2 formation is described in terms of two possible processes: (i) formation of I-, wh ich reacts with neighbouring CF3I, and (ii) C-F fragmentation of CF3I, formed in multilayers, into CF2I+, which reacts with previously form ed atomic iodine. At 0.75 ML, the cross-section for loss of CF3I by al l removal processes is 1.1 +/- 0.2 x 10(-16) cm(2), whereas that for C F2I2 formation is ca. 10(-17) cm(2).