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Research Assistant, CMRC-Radiation Sciences Group, University of Sherbrooke, Quebec, Canada. B.Sc. (Hons) Physics 1984 (Manchester UK), Ph.D. Physics 1988 (Manchester UK) contact me at abass@courrier.usherb.ca

Research interests: Electron impact studies of condensed molecules:

My research (and that of this laboratory) focuses on the interactions of low energy electrons (LEE) in condensed media. Such electrons constitute an important secondary product of irradiation since a single high energy particle can liberate many hundreds of electrons with typical energies less than 20 eV. It is likely that LEE are responsible for significant radiation damage to living tissue and in particular to DNA. One way to understand the effects of LEE (and the approach used in this laboratory), is to bombard simple condensed phase systems (i.e., vacuum deposited molecular solids) with electrons and to observe the induced changes and energy loss processes with a variety of surface science techniques. By studying materials of increasing molecular complexity, it is anticipated that a comprehensive understanding of the effects of LEE will be acquired and that this will in-turn reveal the role played by electrons in radiation damage to the cell. In fact  research undertaken in this laboratory has numerous other applications, from the aging of insulators in high voltage power cables to meterology and planetary science. Of particular personel interest is the relationship of our reseach to the physics of nano-scale structures and devices, since the materials we study are typically of nano-meter thicknesses. In particular, we are able observe how the presence of a substrate (or co-adsorbate) alters the electronic properties of a thin molecular solid, often in surprising ways.

Underlying much of this work is an appreciation that for electrons of low energy, many of their interactions are dominated by temporary negative ions (TNI) or resonances, which form when an electron of a specific energy is captured by an atom or molecule. The formation of a TNI can radically increase electron scattering cross-sections at the energy of the resonance and can thus enhance energy transfer to the film. In addition to enhancing both vibrational and electronic exitation within a thin film, a TNI can effect chemical change on a molecular target.  Probably the best understood mechanism for electron mediated chemistry is dissociative electron attachment, (DEA) where an electron of well-defined energy is captured by a molecule to form a TNI which then dissociates to form an anionic and neutral fragment.   Electrons of higher energy (e.g., >12 eV) may create simultaneously both anions and cations via dipolar dissociation (DD). Recent experiments have identified a new way in which molecular damage may occur when resonantly-enhanced vibrational excitaion of a large polyatomic molecule, brings into close proximity two atomic centres which react and break free of the parent molecule.

I am principally involved in three classes of experiment:

1. The electron stimulated desorption (ESD) of metastable particles: Neutral atoms or molecules in easily-detectable, long-lived metastable states can also be expelled into vacuum following the dissociation of the original molecular target, by the repulsive electrostatic interaction of the excited particle with its neighbours (cavity expulsion) or via the transfer of vibrational energy. Desorbed metastables provide unique information on excitation processes occuring within a thin molecular solid while their (typically) low kinetic energies allow the film porosity and and morphology to be investigated.
2. The ESD of anionic particles. The desorption of anions following DEA and  DD can be observed in vacuum using standard mass spectrometric techniques. The anion yield measured as a function of incident electron energy has allowed numerous TNI to be identified. Moreover, a molecular target that allows DEA can be considered as a source of low kinetic energy anions which may react further with other molecules within a film. Recent work has included anion ESD studies of reactive scattering and investigations into effects of film morphology on the desorption/reaction processes.
3. Charge trapping and accumulation in thin films. By measuring how quickly a rare gas film charges when doped with a  molecular impurity and subjected to electron bombardment, one can obtain a cross section for electron trapping. Comparisons between condensed- and gas-phase measurements for the same molecule allow the effects of condensation on the electron capture process to be understood. Charge accumulation in macroscopic samples (e.g., industrial polymers) which charge very rapidly can be measured using electron bombardment from a pulsed electron gun together with a  Kelvin probe.