Electron energy loss spectroscopy and the optical properties of polymethylmethacrylate from 1 to 300 eV

Abstract

Energy loss spectra of 80 keV electrons transmitted through thin films polymethylmethacrylate (PMMA) were measured with a resolution of 0.1 eV for energy losses from 1 to 300 eV. From the loss spectra, the dielectric response function of PMMA was obtained from 1 to 100 eV and compared with recent synchrotron radiation results. The spectrum of valence excitations from 5 to 13 eV is shown to be characteristic of the pendant group and is compared to experimental gas phase spectra and molecular orbital (CNDO/S) calculations of model molecules. The spectrum of core electron excitations above 285 eV provides a measure of the distribution of empty molecular orbitals and, when the carbon 1s binding energies are taken into account, a qualitatively accurate description of the observed spectrum is made from the CNDO ground state calculation. The energy loss spectra of 20, 40, and 100 eV electrons reflected from the surface of PMMA exhibit a triplet excitation at 4.2 eV and indicate the extreme sensitivity of this material to radiation damage. Finally, the spectrum of electron damage is shown to be similar to that reported for uv photolysis but with a strong previously unreported peak at 6.3 eV assigned here to excitation of radiation induced isolated unsaturated bonds.