Adsorption of formic acid on Si(111)7×7 at room temperature: a valence band photoemission and Si2p photodesorption study

Formic acid is the simplest of the carboxylic acids and a model adsorption system for several surfaces. In spite of the simple structure, formic acid reactivity and photoreactivitymay be quite complex. In this paper, a study is presented on the deuterated formic acid adsorption on Si(111)7× 7 at roomtemperature. The study is performed both by valence band photoemission and by photonstimulated desorption as a function of time and of photon energy in the 90–120 eV range. A primarily adsorption on rest atoms is found. This is verified by monitoring rest atoms and adatom intensity as a function of formic acid exposure. Further checks were made to control that surface adatoms were still free to react after the adsorption of formic acid. The photon stimulated desorption produces 5 single positively charged fragments: D+, O+, OD+ CO+ and CDO+. Possible fragmentation mechanisms are discussed.