Incoherent neutron scattering study of molecular motions in liquid cyclopropane

Liquid cyclopropane has been studied by incoherent neutron scattering at four temperatures: 155, 175, 194 and 250K. Self-diffusion coefficients have been extracted from the analysis of the quasi-elastic profile at low momentum transfer and good resolution (1 and 37μeV). They have been checked by n.m.r. spin-echo measurements between 182 and 297K. The excellent agreement between the two kinds of results shows that in the investigated Q-range (0.12 to 0.39A-1) the translational diffusion satisfies the conditions of a hydrodynamic regime. At higher momentum transfer the reorientational contributions to the quasi-elastic profile have been analysed with a time-of-flight spectrometer using resolutions of 37 and 225μeV. To limit the number of parameters to be introduced in the rotational model, we have used the complementary information of Raman spectroscopy. A polarized line of A1 symmetry (nusub(cc)) and a depolarized one of E'' symmetry (nusub(CH)) have been analysed and have allowed the diffusion coefficients about the two main axes of the molecule to be evaluated. These values have been introduced in a model of rotational diffusion to interpret the quasi-elastic profile up to Q values of the order of 2.35A-1. The authors present a critical analysis of the agreement between the experimental and calculated profiles, taking into account the uncertainty in the values of the various parameters used. (author)