VIBRATIONAL STRUCTURE OF POLYETHYLENE USING HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY - ENERGY-DEPENDENCE AND CHARGE NEUTRALIZATION EFFECTS

The vibrational structure of polyethylene surfaces has been studied using high-resolution electron energy loss spectroscopy (HREELS). We investigated the effects that primary electron beam energy and auxiliary charge neutralization have on HREELS vibrational selection rules. For thin polymer films on conducting supports, requiring no charge neutralization, a simple dipole-allowed spectrum results for low primary beam energies less than or equal to 3 eV. Under these conditions we have clearly observed end group terminations at polyethylene surfaces. At higher primary beam energies, resonance and impact scattering enhancement of non-dipole-active modes were observed to complicate the vibrational spectrum. Correlations are made with infrared- and Raman active modes. Secondary electron beam neutralization was used to acquire HREELS data of thick insulating films, and it was observed to additionally influence the vibrational spectra. The lowest usable neutralization conditions allowed acquisition of dipole-allowed spectra for low beam energies. With increasing neutralization conditions, intense non-dipole-active modes were observed at all primary beam energies. We show that surface charging and primary electron beam energy affect the scattering behavior and that the near-surface volume of polymer which is accessible to dipole scattering and not impact scattering contributes greatly to the dominance of dipole-allowed modes at low primary beam energy.