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.