Thermal, electron, and photon induced chemistry of acetone on Ag(111)

Acetone, (CH3)(2)CO and (CD3)(2)CO, adsorbed on Ag(111) at 95 K was studied using using thermal, photon, and electron activation. Adsorption and desorption involve no dissociation. The temperature-programmed desorption (TPD) spectra exhibit three resolvable peaks, two of which (146 and 134 K) are assigned to the first layer and the third (127-134 K) to multilayers. TPD, after sequentially dosing 1 ML of (CD3)(2)CO followed by 1 ML of (CH3)(2)CO, shows extensive mixing of the two adsorbates throughout the full width of the desorption peaks. This suggests rapid motion, on the TPD time scale, below the onset of desorption (120 K). RAIRS analysis at 95 K indicates that the orientation of adsorbed acetone is coverage-dependent, but the C=O bond remains nearly parallel to the Ag(111) surface at all coverages. At the lowest coverages the average C-C-C plane position is 50 degrees from the surface normal; at higher coverages (up to monolayer) this plane tilts toward the surface normal (22 degrees). Dissociation and desorption of adsorbed (CH3)(2)CO are initiated by 100 eV electrons; TPD products include ketene, methane, and high-temperature (CH3)(2)CO derived from acetone enolate. RAIRS after electron irradiation provides evidence for electron-induced reorientation in which the C=O bond moves away from the surface plane. The cross section for loss of parent was of order 10(-16) cm(2). Photon irradiation at 248 and 193 nm produced no effects with cross sections higher than 10(-21) cm(2).