ELECTRON-INDUCED CHEMISTRY OF METHANOL ON AG(111)

On Ag(111) at 120 K, there is no thermal dissociation of methanal, but electron irradiation induces fragmentation. After irradiation of mono layers, temperature-programmed desorption (TPD) contains water (at 172 K), methane (190 K), hydrogen (200 K), and formaldehyde (225 and 255 K). X-ray photoelectron spectroscopy (XPS) intensities before and afte r irradiation are comparable, indicating little product ejection durin g irradiation. For multilayers there is an additional product in TPD-g lycolaldehyde or methyl formate. During irradiation, the work function rises, reflecting adsorbate structural changes. The XPS, TPD, and wor k function measurements return to the clean surface values upon heatin g from 120 to 450 K. Evidently, impact ionization initiates the chemis try, and the resulting cations fragment, reneutralize, and chemisorb t o Ag. Dissociation of a C-H bond to form H2C(a)O-(a)H or a C-O bond to form C-(a)H-3 and O-(a)H accounts for the products found after relati vely short irradiations. Evidence for O-(a)H bond breaking by electron irradiation appears only after long irradiation times, Formaldehyde o ligomers, stabilized by interactions with Ag, are proposed to account for the H2CO desorption at 255 K.