Neutron inelastic scattering from ethylene: An unusual spectrum

Neutron inelastic scattering spectra of crystalline ethylene have been measured at two temperatures, 4 and 80 K, between 80 and 250 meV. The spectra are found to consist of weak and shifted peaks superimposed on a very broad background. This is accounted for by multi-phonon effects and Doppler broadening due to the small molecular mass of ethylene. A generalization to neutron scattering from adsorbed molecules is made.