Relation between plasmons and the valence-band density of states in polymethylmethacrylate: Influence of ion irradiation on damage selectivity

Abstract

A physical model is presented that aims at rationalizing the selectivity of bond breakage observed when polymethylmethacrylate is irradiated by ions in the 10–500 Kev energy range. This model, previously proposed by Brandt and Ritchie, is based on electronic collective effects. The coupling between the pure plasma oscillation at ω_p and the oscillation of free electrons at (ω² _k0)^½ makes the whole electronic population resonant at the frequency ω_rp = (ω² _p+ (ω² _k0))^½. By computing the valence-band density of states, we are able to calculate (ω² _k0) and then to deduce the theoretical value of ω_rp. On the other hand, we provide an experimental measurement of ω_rp and study its dependence on ion fluence by electron-energy-loss spectroscopy. The validity of the model of Brandt and Ritchie is then discussed in the light of both theoretical and experimental data.