The influence of fast secondary electrons on the aromatic structure of polystyrene

The influence of 200 keV electrons on the characteristic 7 eV pi-pi* peak in polystyrene was investigated using electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope. pi bond degradation is proportional to the total radiative dose. There is an apparent dependence on dose rate such that at higher rates pi bonding seems less susceptible to damage. This result is an artefact explained in terms of a probe size effect associated with fast secondary electrons (FSEs). FSEs are sufficiently energetic to degrade the pi bond, and they can travel laterally within the sample to specimen regions not illuminated by the incident electron beam. Thus the seemingly high resistance of the polystyrene pi-pi* peak to damage under focused probed scanning transmission electron micrscopy (STEM) irradiation is attributed to the fact that damage due to FSEs occurs predominantly outside the area interrogated by the incident electron probe. The extent of delocalized FSE pi bond damage is measured using spatially resolved STEM measurements. The damage behaviour is the same at both -134 degrees C and room temperature. Experimentally determined pi bond damage cross-sections for polystyrene are consistent with energy dependent carbon K ionization cross-sections. This finding supports the idea that damage to valence bonds in organic macromolecules is at least in part due to core excitations.