Invited Paper SS2-TuA2
Electronic Excitation and Bond Dissociation via Transient Anion Formation at the Surfaces of Molecular and Biomolecular Solids

Tuesday, October 16, 2007, 2:00 pm, Room 611

Electrons with energies in the range 0-30 eV can induce at interfaces and surfaces specific reactions which are relevant 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 (LEE) at the surface of molecular and biomolecular solids has been investigated with model systems consisting of pure or doped thin molecular films. Some of the results will be presented at the conference. In these experiments a target film is deposited on a metal or semi-conductor substrate and bombarded by a LEE beam under ultra high vacuum (UHV) conditions. Backscattered electrons are energy analysed to determine the energy and characteristics of electronic excited states at the surface of the molecular solid. Bond dissociation is analysed by mass spectrometry (MS); i.e., we determine the energy, mass and intensity of the neutral fragments and ions emanating from the films vs electron energy. 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 processes induced by LEE. Such mechanisms involve the formation of transient anions which play a dominant role in the fragmentation of all molecules investigated and dipolar dissociation which produces an anion and a cation. 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.