Methyl group dynamics in poly(methyl methacrylate): From quantum tunnelingto classical hopping

We present an extensive neutron scattering study on the ester methyl dynami cs of poly(methyl methacrylate) (PMMA) in the temperature range from 1 to 2 00 K. The crossover from quantum tunneling to classical hopping has been in vestigated in an energy window from 0.5 mu eV to 2 meV. Inclastic measureme nts of the librational levels have been performed in the millielectronvolt range. The results have been analyzed in terms of a model that considers a Gaussian distribution of potential barriers for methyl group rotation. A 6- fold correction to the main 3-fold term of the potential has been added to get a good description of the librational peak. This distribution of mixed- symmetry barriers gives an excellent description of the spectra in the whol e temperature range.