Inelastic neutron scattering study of low energy excitations in polymer thin films.

We report inelastic neutron scattering measurements on polystyrene thin films in a glassy state in the meV region. We found in elastic scattering that the mean square displacement <u2> decreased with film thickness, and hence the corresponding force constant f increased. In inelastic and quasielastic scattering, we observed the so-called boson peak at around 1.5 meV and the picosecond fast process for the first time in thin films, both of which decreased in intensity with film thickness. These results were discussed in terms of the potential hardening due to the confinement of polymer chains and/or the interfacial dead layer.