Low Energy Electron Induced Processes in Dielectrics,
Icy Satellites, Stratosperic Clouds, Nanolithography and Radiotherapy.

Léon Sanche

Groupe en Sciences des Radiations, Faculté de médecine,
Université de Sherbrooke, Sherbrooke (Québec) Canada J1H 5N4.

Electrons with energies in the range 0-30 eV can induce at interfaces and within condensed matter specific reactions which are of relevance to applied fields such as nanolithography, dielectric aging, radiation waste management, radiation processing, astrobiology, planetary and atmospheric chemistry, surface photochemistry, radiobiology, radiotherapy and ballistic electronics. The action of low energy electrons (LEEs) in materials of relevance to five of these fields has been investigated with model systems consisting of pure or doped thin molecular films. The target film is deposited on a metal or semi-conductor substrate and bombarded by a LEE beam (0-30 eV) under ultra high vacuum (UHV) conditions. Neutral fragments and ions emanating from these films are analysed by mass spectrometry. The products remaining in the films are analyzed in situ by X-ray photoelectron and electron energy loss spectroscopies; they can also be removed from the UHV system and analyzed by HPLC and LC/MS.

By comparing the results of the theory and different experiments, it is possible to determine fundamental mechanisms that are involved in the chemical reactions induced by LEEs. Such mechanisms involve (1) the formation of transient anions which play a dominant role in the fragmentation of all molecules investigated; (2) dipolar dissociation which produces an anion and a cation and (3) reactive scattering, which induces non-thermal reactions. The transient anions fragment the parent molecules by decaying into dissociative electronically excited states or by dissociating into a stable anion and a neutral radical. These fragments usually initiate other reactions with nearby molecules, causing further chemical damage. The damage caused by transient anions is dependent on the molecular environment. This research is financed by the CIHR, NSERC and the NCIC.