Inelastic neutron scattering study of the ``rotator'' phase transition in <i>n</i> -nonadecane

Abstract

A simple kinematic model for rotational jump diffusion of a normal alkane about its long axis (circular random walk model) is developed. Inelastic neutron scattering data obtained on the Fermi chopper time-of-flight instrument at the National Bureau of Standards reactor using an incident neutron wavelength of 2.47 Å (Δ λ/λ ≃ 3.8 %) are compared with the predictions of the model. Data taken below the temperature of the ``rotator'' phase transition in n -nonadecane (295 °K) show no quasielastic scattering due to diffusive motions. Data taken in n -nonadecane in its disordered solid phase show quasielastic scattering consistent with the circular random walk model. Estimates for values of the model parameters of 3.5 psec. for τ1 and N ≥ 8 are obtained.